What Is Body Fat Percentage?
Divide the weight of the fat on your body by your total body weight, multiply by a hundred, and that figure is your body fat percentage. Everything that is not fat — muscle, bone, organs, connective tissue and the water in all of it — is grouped together as lean mass, or fat-free mass. An 80 kg man carrying 16.5 kg of fat is 20.7% fat and 63.5 kg lean, and those two figures always add back to the weight he started with.
The appeal over a scale reading is that it separates two things weight cannot. Two people at the same height and the same weight can differ by ten points of body fat, and the scale reports them as identical. What the percentage cannot do is measure anything directly — this page infers it from the shape of your torso, which is why the sections below spend as much time on where the tape goes as on the equation itself.
For the mass-to-height ratio that health services screen with, and why it disagrees with this number for anyone who trains, see the BMI Calculator.
Essential Fat and Storage Fat
Body fat is not one substance with one job. Part of it is structural: fat in bone marrow, in the membrane of every cell, around the organs, in the brain and the rest of the central nervous system. That fraction is called essential fat, and the conventional figures in exercise-physiology texts put it near 2 to 5 percent of body weight in men and 10 to 13 percent in women, the gap being sex-specific fat associated with reproductive function. It is a convention drawn from cadaver and laboratory work, not a threshold any tape measure can find.
The rest is storage fat — the reserve under the skin and around the abdominal organs. That is the portion that changes when your intake and activity change, and the portion this calculator is really tracking, because it is the portion that changes the circumference of your waist.
The practical consequence is a floor. Nobody is 0% fat, and a reading in the low single digits for a man or the low teens for a woman is not a target to chase — it is the region where the calculator starts reporting essential tissue as though it were surplus.
What the Calculator Asks For, and What It Uses
Nine controls sit on the form. Five of them feed the percentage, two are unit pickers, one produces the kilogram split, and one does nothing at all.
- Gender — chooses between two equations with different constants. It changes the answer more than any other control.
- Height — appears in both equations. Taller people get a lower estimate from the same waist and neck.
- Waist circumference — the measurement carrying most of the signal.
- Neck circumference — subtracted from the waist, so a thicker neck lowers the estimate.
- Hip circumference — used for women only. Entering a hip value with Male selected changes nothing at all: 175 cm, 90 cm waist, 38 cm neck returns 20.7% whether the hip box holds 100, 999, or nothing.
- Weight — does not affect the percentage. It only converts the percentage into a fat-mass and lean-mass split in kilograms, and those two rows disappear if you leave it blank.
The Age box is the control that does nothing. Neither equation has an age term, and running the same measurements at 18, 30, 45 and 70 returns 20.7% every time. One option is also inert: the Feet.Inches setting on the height picker is not decoded by the calculation, which reads the figure as plain centimeters, and every realistic combination tested with it selected came back as the amber "Check your inputs" notice rather than a number. Type your height in centimeters or in total inches instead.
The form runs when you press Calculate, and again each time you press it. It does not recalculate as you type, and because every number box starts empty, the panel opens on its empty state rather than a worked example. Reset returns it to that state.
Body Fat Percentage vs BMI
BMI and body fat percentage answer different questions with different inputs, and neither is a version of the other. BMI needs your weight and your height; it has no way to distinguish a kilogram of muscle from a kilogram of fat. This page needs a tape measure and no scale at all; it has no way to distinguish a heavy frame from a light one.
That difference shows up plainly. Weight moves BMI and leaves this reading untouched: a 175 cm man with a 90 cm waist and a 38 cm neck reads 20.7% at 60 kg, at 80 kg and at 100 kg, while his BMI across that range travels from 19.6 to 32.7. The tape is describing his shape; the scale is describing his mass. Run both and you have described the body twice over, which is more than either does alone.
Where they agree is that both are screening figures. Neither is a measurement of health, and neither is precise enough to argue about a single point.
How Do You Calculate Body Fat Percentage?
This calculator uses the U.S. Navy circumference method, published in 1984 by J.A. Hodgdon and M.B. Beckett of the Naval Health Research Center as reports 84-11 for men and 84-29 for women. It is the equation behind military body composition screening, and it is popular outside the military because it needs a tape measure and nothing else.
Every measurement is normalized to centimeters first, then the centimeter form of the equation is applied. The two published forms of this method — one in centimeters, one in inches — are not interchangeable, and mixing them is the classic way to produce an answer that is wrong by several points in either direction.
The Formula, Written Out
Two equations, one per sex. Both take base-10 logarithms of a girth term and of height in centimeters.
Men: %BF = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
Women: %BF = 495 / (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450
The shape of both is the same: a girth term is compared against height, the result is inverted, and the constants 495 and 450 convert an estimated body density into a percentage. That last step is the Siri equation, the standard bridge from density to percent fat.
The most useful thing to notice is what the girth term contains. For men the equation never sees the waist and the neck separately — only the difference between them. A 100 cm waist with a 48 cm neck and a 90 cm waist with a 38 cm neck both give a girth of 52 cm, and both return exactly 20.6567% at 175 cm. For women it never sees the three measurements separately either, only waist plus hip minus neck, which is why a centimeter added to the waist and a centimeter added to the hip move the answer by precisely the same amount.
Height works in the opposite direction and much more weakly. At 175 cm with a 52 cm girth the answer is 20.7%; at 190 cm with the same girth it is 18.2%.
Step by Step
By hand, in centimeters, for a man of 175 cm with a 90 cm waist and a 38 cm neck. The calculator performs the same sequence after converting whatever units you chose.
- Subtract the neck from the waist. 90 − 38 = 52 cm. Women add the hip first, so the term is waist + hip − neck.
- Take the base-10 logarithm of that girth. log₁₀(52) = 1.71600.
- Take the base-10 logarithm of your height in centimeters. log₁₀(175) = 2.24304.
- Build the denominator: 1.0324 − 0.19077 × 1.71600 + 0.15456 × 2.24304 = 1.05172.
- Divide 495 by it. 495 ÷ 1.05172 = 470.6567.
- Subtract 450. 470.6567 − 450 = 20.6567, which the panel rounds to 20.7%.
Two steps trip people up. The logarithm is base 10, not natural — using ln instead of log₁₀ produces a number that is not close. And the subtraction in step one happens before the logarithm, not after: log₁₀(90) − log₁₀(38) is not log₁₀(52).
If you supplied a weight, one more step follows: multiply the weight by the percentage for fat mass, then subtract that from the weight for lean mass. At 80 kg, 80 × 0.206567 gives 16.5 kg of fat and 63.5 kg of lean tissue.
Where to Put the Tape
The equation cannot tell where you measured. It reads three or four numbers and returns a percentage, so every bit of this method's accuracy lives in the tape work.
- Waist — men measure at the navel, women at the narrowest part of the torso, which usually sits above the navel. The NHS gives a simpler rule for the general waist measurement: find the midpoint between your lowest rib and your hip bone, roughly level with the belly button.
- Neck — just below the larynx, with the tape sloping very slightly downward at the front. Shoulders relaxed, not shrugged.
- Hips — women only, around the widest part of the buttocks, tape level all the way round.
- Height — standing, without shoes, heels together against a wall.
Breathe out normally and read the tape at the end of the breath, without pulling it into the skin. It should be snug enough to stay put and loose enough to slide a finger under. Measure twice and take the second reading, since the first is usually the tighter one.
How much this matters is easy to quantify. For a 175 cm man with a 38 cm neck, moving the waist reading from 88 cm to 92 cm walks the estimate from 19.2% to 22.1% — nearly three points across four centimeters of tape placement. For a 165 cm woman with a 32 cm neck and 98 cm hips, waists of 73, 75 and 77 cm return 27.9%, 28.9% and 29.9%.
| Waist reading | Man, 175 cm, 38 cm neck | Woman, 165 cm, 32 cm neck, 98 cm hips |
|---|---|---|
| 2 cm under | 19.2% | 27.9% |
| 1 cm under | 19.9% | 28.4% |
| As measured | 20.7% | 28.9% |
| 1 cm over | 21.4% | 29.4% |
| 2 cm over | 22.1% | 29.9% |
Consistency beats precision here. A tape placed two centimeters too low every single time still produces a usable trend; a tape placed somewhere different each month produces noise that looks like progress.
One Picker Sets the Units for All Four Measurements
The height picker is not only a height picker. Whichever unit it holds — cm or inch — is applied to the waist, neck and hip figures as well, even though those three boxes are labeled in centimeters. The weight picker is separate and genuinely independent, so kilograms with inches is a valid combination.
Used consistently, the two routes agree exactly. A man at 70 inches, 36-inch waist, 15-inch neck and 180 lb returns "21.1% | Category: Average | Fat mass: 17.2 kg | Lean mass: 64.4 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)"; the same body entered as 177.8 cm, 91.4 cm, 38.1 cm and 81.65 kg returns the identical line.
Used inconsistently, the error is large and it never announces itself:
- Picker on inch, centimeter figures typed — a 175 cm man with a 90 cm waist and a 38 cm neck reads 27.3% and Above average instead of 20.7% and Average. A 165 cm woman with a 75 cm waist, 98 cm hips and a 32 cm neck reads 54.4% instead of 28.9%.
- Picker on cm, inch figures typed — the same man entering 69, 35.5 and 15 reads 14.2% instead of 20.7%, a flattering 6.5 points low.
Neither mistake produces an error message, because both produce arithmetically valid numbers. Check the picker before you trust a reading that surprised you.
What Each Measurement Is Worth
Because the equation is logarithmic, no measurement is worth a fixed number of points — but over the range most people occupy, these are the exchange rates.
| Change of 1 cm in… | Man (175 cm, 90 cm waist, 38 cm neck) | Woman (165 cm, 75 cm waist, 98 cm hips, 32 cm neck) |
|---|---|---|
| Waist | +0.71 points | +0.50 points |
| Hip | not used | +0.50 points |
| Neck | −0.72 points | −0.50 points |
| Height | −0.17 points | −0.27 points |
For men the waist and the neck pull on the same girth term in opposite directions, but not by quite the same amount: a centimeter of waist is worth +0.71 and a centimeter of neck −0.72. Subtracting from the girth term moves it into the range where each centimeter counts for slightly more, so the two are near-mirrors rather than exact ones. For women the waist and the hip are exact matches — both add to the same sum — and a centimeter on either is worth +0.50.
The waist figure shrinks as the waist grows. For the same 175 cm man, a centimeter is worth 0.85 points at an 80 cm waist, 0.71 at 90 cm and 0.61 at 100 cm. A leaner reader's number is therefore twitchier, and a heavier reader needs more centimeters to move it.
Height barely matters by comparison, which is worth knowing before you argue with the number. For the man in the table, a centimeter of height is worth 0.17 of a point against the waist's 0.71 — four times less; for the woman it is 0.27 against 0.50.
Body Fat Percentage Examples, Worked Through
Four entries, each one run through the calculator on this page. The panel output is quoted exactly as it appears.
Man, Metric
Male, 175 cm, 80 kg, waist 90 cm, neck 38 cm, height picker on cm.
The girth term is 90 − 38 = 52 cm, and the rest is the six steps above:
495 ÷ (1.0324 − 0.19077 × log₁₀(52) + 0.15456 × log₁₀(175)) − 450 = 495 ÷ 1.05172 − 450 = 470.6567 − 450 = 20.6567
- The panel returns
- 20.7% | Category: Average | Fat mass: 16.5 kg | Lean mass: 63.5 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)
Average is the fourth of the five labels for men, running from 18% up to just under 25%. The fat and lean figures add back to 80.0 kg exactly, because that is all they are — his weight cut in two by the percentage.
Woman, Metric
Female, 165 cm, 62 kg, waist 75 cm, hips 98 cm, neck 32 cm.
The girth term adds the hip before subtracting the neck: 75 + 98 − 32 = 141 cm.
495 ÷ (1.29579 − 0.35004 × log₁₀(141) + 0.22100 × log₁₀(165)) − 450 = 495 ÷ 1.03354 − 450 = 478.9359 − 450 = 28.9359
- The panel returns
- 28.9% | Category: Average | Fat mass: 17.9 kg | Lean mass: 44.1 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)
Widen the waist to 80 cm and the hips to 100 cm on the same 62 kg woman — seven centimeters added to the girth term — and the line becomes "32.4% | Category: Above average | Fat mass: 20.1 kg | Lean mass: 41.9 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)". That is 2.1 kg of the same body weight reassigned from lean to fat.
Note that the label moved and the weight did not. Nothing about her mass changed in that comparison; only the tape did.
Man, Inches and Pounds
Male, height picker on inch, 70 in, 180 lb, waist 36 in, neck 15 in.
Every length is converted at 2.54 cm per inch and the weight at 0.453592 kg per pound before the same equation runs. Nothing about the method changes.
- The panel returns
- 21.1% | Category: Average | Fat mass: 17.2 kg | Lean mass: 64.4 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)
Remember that the inch setting applies to the waist and neck boxes too, whatever their labels say. Leaving the picker on cm and typing the same four figures reads them as centimeters, and the panel comes back with "14.6% | Category: Fitness | Fat mass: 12.0 kg | Lean mass: 69.7 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)" — a 6.5-point error with no warning attached.
The Same Man, Three Months Later
This is the comparison the tool is actually built for — not one reading, but two.
| Start | After three months | |
|---|---|---|
| Height / neck | 180 cm / 40 cm | 180 cm / 40 cm |
| Weight | 88 kg | 84 kg |
| Waist | 95 cm | 90 cm |
| Body fat | 21.9% | 18.4% |
| Fat mass | 19.3 kg | 15.4 kg |
| Lean mass | 68.7 kg | 68.6 kg |
The scale moved 4.0 kg. The calculator attributes 3.84 kg of that to fat and 0.16 kg to lean tissue, which is the outcome a sensible diet is aiming for. The lean rows above are rounded to a tenth of a kilogram, so that 0.16 kg reaches the screen as 68.7 kg against 68.6 kg. The percentage fell 3.5 points, and five centimeters off the waist is where all of it came from.
Note what did not happen: the label stayed on Average for both readings, because the male Average band runs from 18% up to 25% and both numbers sit inside it. A category is a coarse instrument — it changes a handful of times in a lifetime, while the number changes every month.
The lean-mass figure is the one worth carrying forward, because protein targets are set from lean tissue rather than scale weight. Take 68.6 kg into the Protein Calculator.
Body Fat Percentage Chart for Men and Women
Four tables. The first is the classification the tool prints, the next two let you look up a percentage from a tape measure without entering anything, and the last marks where each label gives way to the next.
The labels follow the American Council on Exercise reference ranges, the five-band scheme behind the widely quoted figures that 18 to 24 percent for men and 25 to 31 percent for women is typical for a non-athlete, and that 25 percent and over for men or 32 percent and over for women is classed as obesity. ACE's own calculator page carrying that table is no longer published — the address now lands on a tools index that does not list it — so take the bands below as the convention this tool applies rather than a chart you can look up at the source. The bottom band is where the tool departs from the scheme most: Essential fat here is a catch-all for everything beneath the athlete floor at 6 and 14 percent, not a claim that 5.9 percent is essential tissue. The conventional essential figures are the 2 to 5 percent and 10 to 13 percent quoted earlier on this page.
| Label the tool prints | Men | Women |
|---|---|---|
| Essential fat | Under 6% | Under 14% |
| Athletic | 6% to under 14% | 14% to under 21% |
| Fitness | 14% to under 18% | 21% to under 25% |
| Average | 18% to under 25% | 25% to under 32% |
| Above average | 25% and over | 32% and over |
"Essential fat" as a label is a warning rather than a compliment. It marks a reading at or below the structural minimum, and for most readers who see it the likeliest explanation is a mis-measured neck or waist rather than a genuinely single-digit body.
Chart for Men: Waist Minus Neck, Against Height
Because the men's equation only ever sees the difference between waist and neck, one column covers every pair that produces it. Subtract your neck from your waist, find that figure on the left, and read across to your height.
| Waist − neck | 165 cm | 170 cm | 175 cm | 180 cm | 185 cm | 190 cm |
|---|---|---|---|---|---|---|
| 40 cm | 12.8% | 12.0% | 11.1% | 10.3% | 9.5% | 8.8% |
| 45 cm | 17.1% | 16.2% | 15.4% | 14.5% | 13.7% | 13.0% |
| 50 cm | 21.0% | 20.1% | 19.2% | 18.4% | 17.6% | 16.8% |
| 55 cm | 24.5% | 23.6% | 22.7% | 21.9% | 21.1% | 20.3% |
| 60 cm | 27.8% | 26.9% | 26.0% | 25.2% | 24.3% | 23.5% |
| 65 cm | 30.9% | 30.0% | 29.1% | 28.2% | 27.4% | 26.5% |
| 70 cm | 33.8% | 32.9% | 31.9% | 31.1% | 30.2% | 29.4% |
| 75 cm | 36.5% | 35.6% | 34.6% | 33.7% | 32.9% | 32.0% |
Read a row across and the whole effect of height is visible: 25 centimeters of extra stature is worth about four points at a fixed girth. Read a column down and the first five-centimeter step is worth 4.2 to 4.3 points while the last is close to 2.7, because the logarithm flattens as the girth grows.
Chart for Women: Waist Plus Hips Minus Neck, Against Height
The women's equation sees waist + hip − neck as a single figure, so the same one-column treatment works. For the 165 cm woman above — 75 cm waist, 98 cm hips, 32 cm neck — that combined figure is 141 cm.
| Waist + hip − neck | 155 cm | 160 cm | 165 cm | 170 cm | 175 cm | 180 cm |
|---|---|---|---|---|---|---|
| 115 cm | 17.6% | 16.3% | 15.0% | 13.7% | 12.5% | 11.4% |
| 120 cm | 20.5% | 19.1% | 17.8% | 16.6% | 15.4% | 14.2% |
| 125 cm | 23.3% | 21.9% | 20.6% | 19.3% | 18.1% | 16.9% |
| 130 cm | 26.0% | 24.6% | 23.3% | 22.0% | 20.7% | 19.5% |
| 135 cm | 28.7% | 27.2% | 25.9% | 24.6% | 23.3% | 22.1% |
| 140 cm | 31.2% | 29.8% | 28.4% | 27.1% | 25.8% | 24.6% |
| 145 cm | 33.7% | 32.3% | 30.9% | 29.6% | 28.3% | 27.0% |
| 150 cm | 36.2% | 34.7% | 33.3% | 32.0% | 30.7% | 29.4% |
| 155 cm | 38.6% | 37.1% | 35.7% | 34.3% | 33.0% | 31.8% |
| 160 cm | 40.9% | 39.4% | 38.0% | 36.6% | 35.3% | 34.0% |
Each five-centimeter step down this table is worth between 2.9 and 2.3 points, against 4.3 to 2.7 for a man. Read that as a property of the two equations rather than a claim about physiology: the women's constants simply respond less steeply to girth.
Where the Labels Change
The girth at which each label gives way to the next, to the nearest tenth of a centimeter. Same two girth definitions as the charts above.
| Height | Men: 6% | Men: 14% | Men: 18% | Men: 25% |
|---|---|---|---|---|
| 165 cm | 33.0 cm | 41.3 cm | 46.1 cm | 55.7 cm |
| 170 cm | 33.8 cm | 42.3 cm | 47.3 cm | 57.0 cm |
| 175 cm | 34.6 cm | 43.3 cm | 48.4 cm | 58.4 cm |
| 180 cm | 35.4 cm | 44.3 cm | 49.5 cm | 59.7 cm |
| 185 cm | 36.2 cm | 45.3 cm | 50.6 cm | 61.1 cm |
| 190 cm | 37.0 cm | 46.3 cm | 51.7 cm | 62.4 cm |
In plain waist terms, a 175 cm man with a 38 cm neck crosses out of Fitness into Average a shade under an 86.4 cm waist, and out of Average into Above average a shade over 96.4 cm — 96.40008 cm, which is why 96.4 itself still prints Average.
The women's boundaries, on waist + hip − neck:
- 155 cm tall — 14% at 108.9 cm, 21% at 120.9 cm, 25% at 128.1 cm, 32% at 141.5 cm
- 160 cm tall — 111.1 cm, 123.3 cm, 130.7 cm, 144.4 cm
- 165 cm tall — 113.3 cm, 125.7 cm, 133.3 cm, 147.2 cm
- 170 cm tall — 115.4 cm, 128.1 cm, 135.8 cm, 150.0 cm
- 175 cm tall — 117.6 cm, 130.5 cm, 138.3 cm, 152.8 cm
- 180 cm tall — 119.7 cm, 132.8 cm, 140.8 cm, 155.5 cm
For the 165 cm woman with 98 cm hips and a 32 cm neck, that puts the Fitness-to-Average line at a 67.3 cm waist and the Average-to-Above-average line at 81.2 cm.
How to Read Your Result
The panel returns one percentage, one label, and — if you filled in the weight box — two mass figures. Each of the four deserves a different amount of your attention.
What the Panel Shows
For the 175 cm, 80 kg man with a 90 cm waist and a 38 cm neck, the tool returns "20.7% | Category: Average | Fat mass: 16.5 kg | Lean mass: 63.5 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)", which the page lays out as a large 20.7% with four rows beneath it. A Copy result button puts the whole thing on your clipboard.
Leave the weight box empty and the two mass rows simply do not appear: the same man gets "20.7% | Category: Average | Method: U.S. Navy circumference (Hodgdon & Beckett)". Nothing else is lost, because the percentage never depended on the weight in the first place.
What Your Weight Changes, and What It Does Not
This surprises most people the first time they notice it, and it is the most important thing to understand about a circumference estimate.
The percentage is produced entirely by the tape. Hold the tape figures still and vary the weight box, and the reading does not move at all — 60, 70, 80, 90 and 100 kg all return 20.7% for the same 175 cm man with a 90 cm waist and a 38 cm neck. What changes is the split: 12.4, 14.5, 16.5, 18.6 and 20.7 kg of fat respectively.
So the two mass rows are not a second measurement. They are the percentage multiplied by whatever you typed into the weight box, and they inherit every bit of the percentage's error plus whatever your scale contributes. If you entered pounds, the conversion happens first — 176 lb at those measurements gives 16.5 kg of fat and 63.3 kg of lean.
It also means a gain of ten kilograms with an unchanged waist reads as no change in body fat percentage at all, which is not physiologically plausible. If the scale has moved and the tape has not, one of the two measurements is wrong.
Boundary Readings and the Rounding Rule
The percentage on screen is rounded to one decimal place, but the label is chosen from the unrounded figure. Near a cutoff the two can appear to disagree, and both are behaving correctly.
For a 175 cm man with a 38 cm neck, a 96.4 cm waist computes to 24.9999% and displays as "25.0% | Category: Average". Move the tape half a millimeter to 96.45 cm and it computes to 25.0323%, still displays as 25.0%, and now reads "Category: Above average". The same happens at the Fitness line: 86.35 cm gives a displayed 18.0% labeled Fitness, and 86.4 cm gives a displayed 18.0% labeled Average.
Women see it at their own cutoffs. At 165 cm with 98 cm hips and a 32 cm neck, a 67.25 cm waist displays 25.0% under Fitness and a 67.3 cm waist displays 25.0% under Average.
The rule that follows is simple: within about a tenth of a point of 6, 14, 18 or 25 for men, or 14, 21, 25 or 32 for women, you are on a boundary and not in a category. Your tape work is not accurate to a tenth of a point anyway — one centimeter of waist is worth seven tenths.
Reading the Trend Instead of the Number
A single reading from this method carries a few points of uncertainty. A series of readings taken the same way carries much less, because a consistent tape error cancels out of the difference between two measurements. That is the whole case for measuring monthly rather than agonizing over one result.
Give the trend enough time to exceed the noise. One centimeter off the waist is about 0.7 of a point for a man and 0.5 for a woman, which is the same order as the gap between a snug tape and a very snug one. Three to five centimeters over three months is a signal; three millimeters over two weeks is not.
Measure at the same time of day and in the same state — first thing in the morning, before eating, after using the bathroom — and write down the raw centimeters as well as the percentage. The raw figures let you recheck an odd result later; the percentage on its own does not.
Why It Says "Check Your Inputs"
Four guard messages can appear in the amber panel, and each one names a specific problem.
- "Enter your height, waist and neck measurements" — one of those three boxes is empty, zero or non-numeric.
- "Enter your hip measurement" — Female is selected and the hip box is blank. Men can leave it empty.
- "Waist must be larger than neck — check the two measurements" — the girth term would be zero or negative. Almost always the two figures have been typed into the wrong boxes.
- "Those measurements don't produce a sensible result — please re-check them" — the equation returned something at or below zero, or above 75%. This is also what the Feet.Inches height option produces, since the calculation does not decode that format.
An empty or zero weight box never triggers any of these. It simply drops the fat-mass and lean-mass rows and shows the percentage on its own.
The same equation runs on a simpler form — one unit picker for every measurement, no age box — if you would rather not think about which control does what. That version is the Body Fat Calculator.
Limits: When This Estimate Does Not Apply
The honest summary is that this is a screening estimate built for a fit military population and generalized well beyond it. Here is where it stops being reliable.
How Accurate Is the Navy Method?
The best recent evidence comes from military populations, where these methods are used in earnest. A 2022 study in Frontiers in Physiology measured 430 male and 179 female US Marines aged 18 to 57 by DXA, by circumference and by bioelectrical impedance. Against DXA, the abdominal-circumference estimate it tested ran low for men by 2.6 ± 3.7 percentage points in the under-30 group and 2.5 ± 3.7 in the over-30 group, and high for women by 2.3 ± 4.3 and 1.3 ± 4.8 respectively. Its authors concluded the method is a field-expedient way to classify people, not research-grade body composition data.
A smaller 2017 study in the Journal of Strength and Conditioning Research taped 23 male ROTC cadets by the Army's 2002 circumference equation and compared them with DXA. Thirteen of the 23 fell outside a ±2 to 4 point zone of agreement, and taping generally read low: no cadet exceeded the 20–22% compliance threshold by tape, while seven of the 23 did by DXA.
Neither paper put the Hodgdon and Beckett equation itself on trial — the 2022 study named an abdominal-circumference method and the 2017 study the Army's, both tape-and-height estimates of the same type. Read the figures as evidence about circumference estimation as a class, and treat that as the closest thing to a direct answer that published work offers.
Two patterns run through both studies and are worth carrying into your own reading:
- The error is not random noise around your true value. It is a bias plus noise, and the direction of the bias depends on your sex.
- The bias also depends on where you sit. Lean people tend to be overestimated and high-body-fat people underestimated, so the method compresses the range at both ends.
For what the reference method measures that a tape cannot, and how wide the gap really is, see the DEXA Body Fat Calculator.
Body Shapes the Equation Reads Wrong
The method assumes a relationship between torso girths and total fat that held in the sample it was built from. Where your proportions depart from that sample, the estimate departs with them.
- A thick neck from heavy pressing, or a naturally broad build, subtracts from the girth term and flatters the result. For a man, every centimeter of neck is worth 0.72 points in the reassuring direction.
- A muscular midsection does the opposite. The equation reads abdominal circumference as fat, so a heavy squat and deadlift habit shows up as a higher estimate than it should.
- Fat carried mostly on the legs or arms is close to invisible here — a man's reading is built from the torso alone, and a woman's from the torso and hips.
- Very lean and very heavy readers sit at the ends where both validation studies found the largest bias.
- The reading is identical at 18 and at 70. Neither equation has an age term, so nothing adjusts for the shift from lean tissue toward fat that accompanies aging at a constant shape.
None of this makes the tool useless for those readers. It makes the absolute number unreliable while leaving the trend intact, because a bias that stays put cancels out between two readings.
Who Should Not Use This
The equations were fitted on adults and there is no version of them for a growing body, so this page is not appropriate for children or teenagers — body composition in under-18s is assessed against age- and sex-specific growth charts instead.
It is also not valid during pregnancy. Waist and hip circumferences change for reasons that have nothing to do with fat storage, and the equation has no way to know that. The NHS applies similar exclusions to its own waist-based assessment tool, which advises against use in pregnancy, in anyone with or suspected of having an eating disorder, in people with a growth condition affecting height, and in adults with a BMI over 35.
That last exclusion is worth noting here too: at very high body fat the circumference method carries its largest downward bias, so it is the group for whom the number is least trustworthy.
What to Measure Alongside It
You already have a tape in your hand, and two things you can do with it carry risk information this percentage does not.
- The raw waist measurement. The US National Heart, Lung, and Blood Institute puts the risk threshold at more than 40 inches for men and more than 35 inches for women. That is a threshold on the centimeters themselves, not on the percentage they feed.
- Waist-to-height ratio. Divide waist by height in the same units; the NHS rule is to keep your waist to less than half your height, and it needs no separate cutoff by sex.
- A DXA scan or hydrostatic weighing, if a real number matters — for a research question, a clinical decision, or a competitive weight class.
Where the tape method earns its place is frequency. A DXA scan is accurate and you will have perhaps two a year; this takes a few minutes and you can have one every month, and for watching a change unfold the frequent estimate usually tells you more than the rare measurement.
Whatever you use, keep the raw measurements. A percentage is a conclusion; the centimeters behind it are the evidence, and only the evidence can be rechecked.