Body Fat Calculator

What Is Body Fat?

Body fat is adipose tissue — the fat stored under your skin, around your organs and inside your bone marrow. Body fat percentage divides that mass by your total weight. Everything else, meaning muscle, bone, organs, connective tissue and the water in all of them, is grouped together as lean mass or fat-free mass. The two halves always add back to the number on your scale: an 85 kg man estimated at 21.2% is carrying 18.0 kg of fat and 67.0 kg of lean tissue.

That split is the whole reason to reach for a tape. Weight tells you how much of you there is; the percentage tells you what kind. It is the distinction a bathroom scale is physically incapable of making, and it is why two people with identical figures on paper can be in visibly different condition.

The mass-to-height screen that clinics and charts still start from, and why it disagrees with this number for anyone who trains, is the BMI Calculator.

The Fat You Cannot Lose

Not all of it is surplus. A structural fraction sits in cell membranes, bone marrow, the brain and spinal cord and around the organs, and it is called essential fat. Exercise-physiology texts put that fraction at roughly 2 to 5 percent of body weight in men and 10 to 13 percent in women, the difference being sex-specific fat tied to reproductive function. Those are conventional figures from laboratory and cadaver work, not thresholds a tape measure can locate.

The remainder is storage fat: the reserve under the skin and packed around the abdominal organs. That is the portion that grows and shrinks with intake and activity, and the portion this calculator is genuinely tracking, because it is the portion that changes the circumference of a waist.

The practical consequence is a floor beneath the scale. Enter a 180 cm man with a 74 cm waist and a 41 cm neck and the tool returns 3.6% under the label Essential fat — a reading far more likely to mean a badly measured neck than a genuinely 3.6% body.

Body Fat vs Body Weight

Two men, both 180 cm and both exactly 85 kg, put through this calculator.

Same height, same weight, 12.7 points apart. Man A's panel splits his 85 kg into 11.7 kg of fat and 73.3 kg of lean tissue; Man B's splits the same 85 kg into 22.4 kg and 62.6 kg. A bathroom scale reads 85 kg for both of them. That gap is the one the tape is there to fill.

It cuts the other way too. If Man B lost six kilograms of muscle and no fat, he would weigh 79 kg while still carrying the same 22.4 kg of fat, which is 28.4% of him rather than 26.4%. The scale would call that progress, and this page would only register it if the tape moved with him.

What This Calculator Asks For

Seven controls sit on the form. Five of them feed the percentage, one sets the unit for four of those five, and one changes only the kilograms.

• Gender — picks between two equations with different constants. It moves the answer further than anything else on the form.
• Select Unit — Centimeters or Inches. It governs height, waist, neck and hip together. It does not touch weight.
• Height — appears in both equations, pulling against the girth term. A taller body gets a lower estimate from the same tape figures.
• Waist — the measurement carrying most of the signal.
• Neck — subtracted from the waist, so a thicker neck lowers the estimate.
• Hip — read by the women's equation only. With Male selected the hip box is ignored outright: 180 cm, 94 cm waist and 40 cm neck return 21.2% whether that box holds 105, 999 or nothing at all.
• Weight — always kilograms, whatever the unit picker says, because it has no picker of its own. It never moves the percentage. It only converts it into fat mass and lean mass, and those two rows vanish if you leave it blank.

There is no age field, and neither equation contains an age term, so nothing here adjusts for the drift from lean tissue toward fat that accompanies aging at an unchanged shape.

How Do You Calculate Body Fat?

The method behind this page is the U.S. Navy circumference equation, published in 1984 by J.A. Hodgdon and M.B. Beckett at the Naval Health Research Center. It became the screening tool for military body composition standards because it needs a tape measure and a wall, and it spread far outside the services for exactly that reason.

Whatever you type is converted to centimeters first, and then the centimeter form of the equation runs. That detail matters more than it sounds. The method has two published forms, one in centimeters and one in inches, and they are not interchangeable — feeding one the other's units is the classic way to land six points off in either direction.

The Body Fat Formula

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 two round constants are not part of the circumference work at all. The logarithmic middle estimates body density; 495 and 450 are the Siri equation, the standard conversion from density into percent fat.

What deserves your attention is what each equation can and cannot see. The men's version never sees the waist and the neck separately, only the gap between them: a 94 cm waist with a 40 cm neck and a 100 cm waist with a 46 cm neck both give 54 cm, and both return 21.2% at 180 cm. The women's version sees only waist plus hip minus neck, so 78 + 100 − 33, 80 + 98 − 33 and 78 + 101 − 34 all reduce to 145 cm and all return 31.5% at 163 cm.

Height pushes the opposite way, and weakly. Hold the men's girth term at 54 cm and the estimate falls from 23.8% at 165 cm to 19.6% at 190 cm — a quarter of a metre of stature buys about four points.

Calculating Body Fat Step by Step

By hand, in centimeters, for a man of 180 cm with a 94 cm waist and a 40 cm neck. The calculator runs this same sequence once it has converted whichever units you chose.

• Subtract the neck from the waist: 94 − 40 = 54 cm. Women add the hip first, making the term waist + hip − neck.
• Take the base-10 logarithm of that girth: log₁₀(54) = 1.73239.
• Take the base-10 logarithm of height in centimeters: log₁₀(180) = 2.25527.
• Assemble the denominator: 1.0324 − 0.19077 × 1.73239 + 0.15456 × 2.25527 = 1.05049.
• Divide: 495 ÷ 1.05049 = 471.21040.
• Subtract 450: 471.21040 − 450 = 21.21040, which the panel prints as 21.2%.

Two of those steps catch people out. The logarithm is base 10, not natural — swapping in ln does not give a near miss, it gives nonsense. And the subtraction happens before the logarithm, never after: log₁₀(94) − log₁₀(40) is a different number from log₁₀(54).

With a weight supplied, two more steps follow. Multiply the weight by the percentage for fat mass, then take that away from the weight for lean mass: 85 × 0.2121040 gives 18.0 kg of fat and 67.0 kg of lean tissue.

Where to Put the Tape

Nothing in the equation knows where the tape went. It receives three or four numbers and hands back a percentage, so the accuracy of this method is very largely a question of technique. The sites below are the ones the Navy protocol specifies, and the same ones recorded in the Marine Corps validation survey cited further down this page.

• Waist — men at the level of the navel; women at the thinnest part of the torso, which usually sits a little above the navel.
• Neck — just below the larynx, tape sloping slightly downward at the front, shoulders relaxed rather than shrugged.
• Hips — women only, around the greatest protrusion of the buttocks, tape level the whole way round.
• Height — standing, no shoes, heels together against a wall.

Breathe out normally and read the tape at the end of that breath, without pulling it into the skin. Snug enough to stay in place, loose enough to slide a finger under. Measure twice and use the second reading, since the first is usually the tighter of the two.

How much this matters is easy to put numbers on. The columns below hold the neck, hip and height steady and move only the waist.

Four centimeters of tape placement walk the man from 19.8% to 22.6% and the woman from 30.5% to 32.4% — and on the way they carry her past the line where her label changes from Average to Above average.

Repeatability matters more than getting the site exactly right. A tape that sits two centimeters too low every single month still produces a trend you can act on. One that lands somewhere new each time produces noise dressed up as progress.

Using This Body Fat Calculator Online

The form is deliberately short, and it behaves in a few ways worth knowing before you trust a number that came out of it.

• It runs when you press Calculate, and again every time you press it. It does not recalculate as you type.
• Every number box starts empty, so the panel opens on a dash and a prompt rather than on a worked example.
• Gender starts on Male and the unit picker starts on Centimeters. Both are two-button controls rather than dropdowns.
• Reset clears the boxes and returns the panel to that empty state.
• Copy result puts the readout and every breakdown row on your clipboard, one per line — the percentage, the category, the Method line, and the two mass rows when they are present.
• The arithmetic runs in your browser rather than on a server. That is not the same thing as privacy: this page carries analytics and advertising tags like every other page here.

Four guard messages can appear in the amber "Check your inputs" panel, and each of them names one specific problem.

• "Enter your height, waist and neck measurements" — one of those three boxes is empty, zero or not a number.
• "Enter your hip measurement" — Female is selected and the hip box is blank. Men may leave it empty.
• "Waist must be larger than neck — check the two measurements" — the girth term would come out at zero or below. Nearly always two figures typed into each other's boxes.
• "Those measurements don't produce a sensible result — please re-check them" — the equation returned a figure at or below zero, or above 75%.

An empty or zero weight box triggers none of those. It quietly drops the fat-mass and lean-mass rows and leaves the percentage standing on its own.

One Picker Sets the Units for Four Boxes

Select Unit governs height, waist, neck and hip at once, even though all four boxes carry a cm label. Weight is the exception: it has no picker and is always read as kilograms, so kilograms alongside inches is a perfectly valid combination here.

Used consistently, the two routes agree exactly. A man at 71 in, 37 in waist, 15.5 in neck and 85 kg gets back "21.6% | Category: Average | Fat mass: 18.3 kg | Lean mass: 66.7 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)", and the identical body typed as 180.34 cm, 93.98 cm and 39.37 cm returns the same line character for character.

Used inconsistently, the error is large and it arrives silently:

• Picker on Inches, centimeter figures typed — the 180 cm man with a 94 cm waist and a 40 cm neck reads 27.9% and Above average instead of 21.2% and Average, 6.7 points too high. The 163 cm woman reads 57.2% instead of 31.5%, which is 25.8 points adrift.
• Picker on Centimeters, inch figures typed — a man who is genuinely 71 in, 37 in and 15.5 in reads 15.1% and Fitness instead of 21.6% and Average, 6.5 points of flattery.

Neither slip raises a warning, because both produce arithmetically respectable numbers. When a reading surprises you, check the picker before you start questioning your body.

Body Fat Examples, Worked Through

Four entries, each run through the calculator on this page. Every panel line below is quoted exactly as the tool produces it.

Example 1: Man, Centimeters

Male, 180 cm, 85 kg, waist 94 cm, neck 40 cm, picker on Centimeters.

The girth term is 94 − 40 = 54 cm, and the rest is the six steps set out above.

495 ÷ (1.0324 − 0.19077 × log₁₀(54) + 0.15456 × log₁₀(180)) − 450
= 495 ÷ 1.05049 − 450
= 471.2104 − 450 = 21.2104

The panel returns

21.2% | Category: Average | Fat mass: 18.0 kg | Lean mass: 67.0 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)

Average is the fourth of the five male labels, running from 18% up to just under 25%. The two mass rows add back to 85.0 kg, because that is all they are: his own weight cut in two by the percentage.

Example 2: Woman, Centimeters

Female, 163 cm, 68 kg, waist 78 cm, hips 100 cm, neck 33 cm.

Here the girth term adds the hips before it subtracts the neck: 78 + 100 − 33 = 145 cm.

495 ÷ (1.29579 − 0.35004 × log₁₀(145) + 0.22100 × log₁₀(163)) − 450
= 495 ÷ 1.02812 − 450
= 481.4621 − 450 = 31.4621

The panel returns

31.5% | Category: Average | Fat mass: 21.4 kg | Lean mass: 46.6 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)

Leave the hip box blank with Female selected and nothing is calculated at all — the panel answers "Enter your hip measurement". The women's equation has no fallback that works without it.

Example 3: Man, Inches

Male, picker on Inches, 71 in tall, 37 in waist, 15.5 in neck, weight 85 kg.

Each length is multiplied by 2.54 before the same equation runs — 180.34 cm, 93.98 cm and 39.37 cm. The weight box stays in kilograms whatever the picker says, because the picker never reaches it.

The panel returns

21.6% | Category: Average | Fat mass: 18.3 kg | Lean mass: 66.7 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)

Leave the picker on Centimeters and type those same three figures and the tool answers "15.1% | Category: Fitness | Fat mass: 12.8 kg | Lean mass: 72.2 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)" — 6.5 points of flattery, with no warning attached to it.

Example 4: Twelve Weeks of Change

One reading tells you where you are. Two tell you what is happening. The same man measured three times, at 178 cm with a 39 cm neck throughout.

The scale moved 5.5 kg. Computed from the unrounded figures rather than from the rounded ones in the table, 4.8 kg of that came out of the fat compartment and 0.7 kg out of lean tissue — 88% of the loss from fat, which is roughly what a moderate deficit with training attached is aiming at. The percentage fell 4.0 points, and every bit of that came from six centimeters off the waist.

Watch what the label did. It moved once, at week six, and then sat still while the number kept falling. Categories are coarse instruments — they change a handful of times in a lifetime, while the number changes every month.

Turning a target percentage into a weekly rate of loss is a calorie question rather than a tape question. Work the rate out with the Calorie Deficit Calculator.

Body Fat Chart for Men and Women

Four tables. The first is the classification this tool prints. The next two let you look a percentage up from a tape measure without entering anything. The last shows what a percentage actually weighs.

The labels follow the American Council on Exercise reference ranges — the five-band scheme behind the widely quoted figures that 18 to 24 percent in men and 25 to 31 percent in women is typical for a non-athlete, and that 25 percent and over in men, or 32 percent and over in women, is classed as obesity. ACE's own page carrying that table is no longer published; the address now lands on a tools index that does not list it. Treat the bands below as the convention this tool applies rather than a chart you can look up at source. Note too that the bottom band is a catch-all: Essential fat here means anything beneath the athlete floor at 6% and 14%, not a claim that 5.9% is structural tissue.

The colored meter under the readout runs on a single gender-neutral scale that matches neither column above, so the two can disagree: a man at 26.4% sits in the meter's Average band while the Category line reads Above average. The Category line is the one that follows this table.

Body Fat Chart for Men

Because the men's equation only ever sees waist minus neck, one column covers every pair of measurements that produces the same difference. Subtract your neck from your waist, find that figure on the left, and read across to your height.

Read a row across and the whole effect of height is visible: ten inches of stature is worth a little over four points at a fixed girth. Read a column down and the first two-inch step costs 4.7 points while the last costs 2.9, because the logarithm flattens as the girth grows.

Body Fat Chart for Women

The women's equation treats waist plus hip minus neck as a single figure, so the same one-column treatment works. Add your waist to your hips, subtract your neck, and look the total up.

Each four-inch step down this table is worth between 5.9 and 4.6 points, and each two inches of height between 1.2 and 1.6. Read that as a property of the constants rather than a claim about physiology: the women's equation responds less steeply per centimeter of girth, and it has more centimeters to work with because the hips are inside the sum.

What a Percentage Weighs

A percentage says nothing about kilograms until a weight is entered. This is one single reading — the 180 cm man with a 94 cm waist and a 40 cm neck, at 21.2% — carried by bodies of different mass.

The percentage column never moves, because no weight term appears anywhere in the equation. Only the last two columns respond, and they respond to the number you typed rather than to anything that was measured on your body.

Where the Labels Change

The waist at which each label gives way to the next, for the two bodies used throughout this page. These are the smallest readings, to the nearest tenth of a centimeter, that the tool assigns to the new label.

Those waists belong to those necks, hips and heights and nobody else's, but the rule for moving them is easy to carry. For a man, add a centimeter to the neck and every boundary waist above moves up by a centimeter with it, since the equation only ever sees the difference. For a woman the hips work in reverse: a centimeter more hip pulls every boundary waist down by one.

Her 45.5 cm figure is a mathematical boundary rather than a realistic waist. It is worth showing because of what it says about the scale: at that height and hip measurement, no waist a person actually has would reach the Essential fat band.

How to Read Your Result

The panel hands back a percentage, a category, and — when the weight box was filled — two mass figures. Each of them deserves a different amount of your attention.

The Lines the Panel Returns

For the 180 cm, 85 kg man with a 94 cm waist and a 40 cm neck, the tool returns "21.2% | Category: Average | Fat mass: 18.0 kg | Lean mass: 67.0 kg | Method: U.S. Navy circumference (Hodgdon & Beckett)". The page lays that out as a large 21.2% with four rows beneath it and a colored meter below those.

Empty the weight box and two of those rows simply disappear: the same man now gets "21.2% | Category: Average | Method: U.S. Navy circumference (Hodgdon & Beckett)". The estimate loses nothing, because the percentage never rested on his weight in the first place.

That last row is not a result but a provenance note. It names the equation, so a figure copied into a training log can later be compared with one taken the same way rather than with a scan or a smart scale.

Why Your Weight Does Not Move the Percentage

This catches people out, and it explains most of the complaints that circumference estimates attract.

Hold the tape figures still, change the weight box, and the reading does not budge: 65, 75, 85, 95 and 105 kg all return 21.2% for the same 180 cm man with a 94 cm waist and a 40 cm neck. What changes is the split — 13.8, 15.9, 18.0, 20.1 and 22.3 kg of fat respectively.

The two mass rows are therefore not a second measurement. They are one percentage multiplied by whatever you typed, and they carry the percentage's full error plus whatever your scale contributes on top.

It follows that a ten-kilogram gain with an unchanged waist reads here as no change in body fat whatsoever, which is not physiologically plausible. When the scale has moved and the tape has not, one of the two was measured badly.

Readings on a Boundary

The figure on screen is rounded to a single decimal place, but the label is picked from the unrounded one. Close to a cutoff the two can look as though they contradict each other, and both are behaving correctly.

At 180 cm with a 40 cm neck, a 99.7 cm waist computes to 24.9694% and prints 25.0% under the label Average. Shift the tape one millimeter to 99.8 cm and it computes to 25.0327%, still prints 25.0%, and now reads "Category: Above average". The same thing happens lower down, where 84.3 cm and 84.4 cm both display 14.0% under the labels Athletic and then Fitness.

Women meet it at their own cutoffs. At 163 cm with 100 cm hips and a 33 cm neck, a 79.0 cm waist computes to 31.9519% and a 79.1 cm waist to 32.0008%; both display as 32.0%, and the labels read Average and Above average.

So within roughly a tenth of a point of 6, 14, 18 or 25 for men, or 14, 21, 25 or 32 for women, you are standing on a boundary rather than sitting in a category. Tape work is not accurate to a tenth of a point anyway: for that man, one centimeter of waist is worth almost seven tenths.

How Much a Centimeter Is Worth

The equation is logarithmic, so no measurement is worth a fixed number of points anywhere on the scale. Across the range most readers occupy, these are the exchange rates.

For the man, waist and neck pull on the same term in opposite directions but not by identical amounts — +0.68 against −0.69 — because subtracting from the girth moves it into the range where each centimeter counts for slightly more. For the woman they are exact twins: waist and hip both add into the same sum, so a centimeter on either is worth the same +0.49.

The waist figure also shrinks as the waist grows. For that same man, a centimeter is worth 0.88 of a point at an 80 cm waist, 0.73 at 90 cm, 0.68 at 94 cm and 0.59 at 105 cm. A leaner reader's number is twitchier; a heavier reader needs more tape to shift it.

Height barely registers next to any of that — 0.17 of a point per centimeter for the man against the waist's 0.68, four times less — which is worth knowing before anyone re-measures their height hoping for a kinder answer.

Reading the Trend Instead of the Number

One reading from this method carries several points of uncertainty. A run of readings taken the same way carries far less, because a consistent tape error subtracts itself out of the gap between any two of them. That is the entire argument for measuring monthly instead of arguing with a single result.

Give the trend enough room to clear the noise. A centimeter off the waist is 0.68 of a point for the man above and 0.49 for the woman, which is the same order of magnitude as the difference between a snug tape and a very snug one. Three to six centimeters over three months is a signal; three millimeters over a fortnight is not.

Moving the number is a calorie question before it is a tape question, and the maintenance figure to work down from is your TDEE Calculator.

Measure at the same time of day and in the same state, and record the raw centimeters next to the percentage. Six months on, the centimeters are what let you work out whether an odd reading was your body or your tape.

Limits: When This Estimate Does Not Apply

The honest summary is that this is a screening estimate, fitted on a fit military population and applied far beyond it. Here is where it stops being reliable.

How Accurate Is It Against a DXA Scan?

The most directly relevant evidence is a 2022 US Marine Corps body composition survey published in Frontiers in Physiology, which measured 430 men and 179 women aged 18 to 57 by DXA and by the Hodgdon and Beckett 1984 circumference equations — the same method this page runs, taken there in its published inch form rather than the centimeter form used here. Against DXA the tape estimate ran low for men, by 2.6 ± 3.7 percentage points under 30 and 2.5 ± 3.7 over 30, and high for women, by 2.3 ± 4.3 under 30 and 1.3 ± 4.8 over 30. Its 609 volunteers were serving Marines meeting the Corps' annual fitness standard, so the sample was fitter than the readers this page serves.

Two features of those figures matter more than their size. The error is not random scatter around your true value: it is a bias whose direction depends on your sex, with scatter piled on top. And the same paper reports that on an individual basis lean men and women were overestimated while people with higher body fat were underestimated, so the method quietly compresses the range at both ends.

What the reference method actually measures, what it costs, and how wide the gap really runs is covered on the DEXA Body Fat Calculator.

For scale, that same survey put bioelectrical impedance — the barefoot smart-scale reading — much closer to DXA, at 0.4 ± 3.2 points for men under 30 and 1.4 ± 3.1 for women under 30. A tape is cheaper and more repeatable than either. It is not more accurate.

Builds the Equation Reads Wrong

Both equations assume a fixed relationship between torso girths and whole-body fat — the relationship that held in the sample they were fitted to. Depart from that sample's proportions and the estimate departs alongside you.

• A thick neck, whether from heavy pressing or simply a broad build, subtracts from the girth term and flatters the answer. Each centimeter of neck is worth 0.69 of a point in the reassuring direction for the man above.
• A muscular midsection does the opposite. The equation reads abdominal circumference as fat, so a heavy squat and deadlift habit comes back as a higher estimate than the body deserves.
• Fat carried mainly on the legs or arms is close to invisible here. A man's reading is built from the torso alone, a woman's from the torso and hips.
• Very lean and very heavy readers sit at exactly the ends where the Marine Corps survey found the largest individual errors.
• Nothing adjusts for age. The same tape figures return the same percentage at 18 and at 70.

That does not make the tool worthless for those readers. It makes the absolute figure untrustworthy while leaving the direction of travel intact, because a bias that stays put subtracts itself out of the gap between two readings.

Who Should Not Use This

Both equations were fitted on adults and neither has a version for a growing body, so this page is not appropriate for children or teenagers. A growing body is assessed against an age- and sex-specific reference instead: the World Health Organization classifies 5- to 19-year-olds by BMI-for-age against its growth reference, counting more than one standard deviation above the median as overweight and more than two as obesity.

Pregnancy rules it out as well. Waist and hip circumferences change then for reasons that have nothing to do with fat storage, and the equation has no way of knowing. The NHS applies the same kind of exclusions to its own waist-based tool, advising against use in pregnancy, in anyone diagnosed with or suspecting an eating disorder, in people with a growth disorder or another condition affecting height, and in adults whose BMI is over 35.

That last exclusion carries over here for a particular reason. At high body fat the circumference method runs its largest downward bias, so the group whose number matters most is also the group whose number can be trusted least.

What to Measure Alongside It

Two other things can be done with the tape already in your hand, and both carry risk information this percentage does not.

• The raw waist figure. The US National Heart, Lung, and Blood Institute states it plainly: "A waist circumference of more than 35 inches for women or more than 40 inches for men increases your risk." 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 unit; 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, when a real number matters — a clinical decision, a research question, a competitive weight class.

Frequency is what a tape has over a scan. You might have two DXA scans in a year; you can take this measurement every month, and for watching a change unfold a frequent estimate usually beats a rare measurement.

Whichever you choose, keep the tape figures themselves. Six months from now the percentage will be a number you cannot audit, while the centimeters will still tell you where the tape went.