BMI Calculator

What Is BMI?

Body mass index compares your weight with your height, and that is the whole of it. The World Health Organization writes it as weight (kg) divided by height squared (m²), and classifies an adult BMI of 25 or more as overweight and 30 or more as obesity. What the ratio does well is flag a weight that sits far from typical for a given height. What it cannot do is say what that weight is made of.

For the split between fat and everything else, which BMI has no way of seeing, take a tape measure to the Body Fat Calculator.

What the BMI Number Is Good For

The reading places you on the four bands drawn under the number: Underweight, Normal weight, Overweight and Obesity, with the switch points at 18.5, 25 and 30.

Three things that single figure can honestly do:

• Compare you against the WHO adult cutoffs — the job the number was designed for.
• Track a direction over weeks or months. The formula and the rounding never change, so two readings are comparable as long as you measure height the same way each time.
• Decide whether a second measurement is worth taking. A waist circumference or a body-fat estimate answers questions BMI cannot.

What it carries no information about: fat, muscle, bone, or where on the body the weight sits. It also applies to adults only — nothing on this page adjusts for age.

Where the Number Came From

The ratio is much older than the acronym. The Belgian statistician Adolphe Quetelet described weight divided by height squared in the 1830s while trying to characterize the average man of a population — a statistical exercise, with no clinical intent behind it and no cutoffs attached.

The name arrived in 1972, when Ancel Keys and colleagues published "Indices of relative weight and obesity" in the Journal of Chronic Diseases. They tested several competing relative-weight measures against body-fat estimates across large samples, found Quetelet's index the best of them for population work, and called it the body mass index. That lineage explains both its strength and its weakness: it was chosen because it behaves well across thousands of people, not because it describes any one of them well.

The cutoffs at 18.5, 25 and 30 came later still, from risk data rather than from the arithmetic. Nothing in the equation makes 25 special — it is a line drawn on a continuum where the associated risk of several conditions starts climbing.

What the Calculator Asks For and What It Does

Four controls, two of which are unit pickers:

• Weight, in kilograms or pounds.
• Height, in centimeters, inches, or feet-and-inches.

The two unit pickers work independently, so kilograms with feet-and-inches is a valid combination: 70 kg at 5 ft 9 in returns 22.8, and so does 154 lb at 175 cm.

Pounds are converted at 0.45359237 kg per pound and inches at 0.0254 m per inch — the exact definitions rather than the rounded 703 shortcut — and the metric formula is applied to the converted values. The output is one number to one decimal place, with the colored band strip beneath it.

Nothing else is asked for, and nothing else is used. There is no sex control, no age control, no activity level and no goal weight, because none of those terms appear anywhere in the calculation.

Leave the weight box empty and the tool returns the amber "Check your inputs" notice rather than a number: the calculation produces a bare zero, which the page treats as a prompt to fill the field in. Leave the height box empty, or type a zero into it, and the calculation stops with a similar notice, because dividing by a height of zero has no answer.

Why This Calculator Does Not Ask for Sex

There is nothing for a male/female switch to change. BMI is mass divided by height squared; the equation has no sex term, so 70 kg at 175 cm is 22.9 for everyone who enters it. The WHO adult cutoffs at 18.5, 25 and 30 are not sex-specific either — the same four bands are published for men and women. So the form asks for two unit pickers and two numbers, and nothing else: a control that cannot change the answer has no business sitting in it.

Sex does matter one step later, in interpretation and in the methods that measure what BMI cannot. At the same BMI, women on average carry a higher proportion of body fat than men, so two identical readings do not describe two identical bodies. The tools that estimate body composition and energy needs are built with sex terms for that reason: the US Navy circumference method uses a different equation and an extra hip measurement for women, and the Mifflin-St Jeor equation ends in +5 for men and −161 for women. Those calculators ask because the answer moves. This one does not ask because it would not.

Why Health Services Screen With BMI

A high BMI is associated with higher rates of heart disease, type 2 diabetes and joint problems across large populations, and a very low one can accompany undernutrition — which is why health services screen with it despite everything it leaves out. It is also close to free: two measurements anyone can take, no equipment beyond a scale and a tape, no laboratory, no waiting. A screening test that costs nothing and catches most of what it is looking for beats an accurate test nobody administers.

What follows a reading is a question, not a conclusion. The number tells you whether the next measurement is worth taking, and the sections below tell you which measurement that should be.

How Do You Calculate BMI?

The calculator applies the standard definition — mass in kilograms over height in meters squared. The World Health Organization and the NHS publish the same formula and the same adult cutoffs.

In metric it is used directly:

Metric

BMI = weight (kg) ÷ [height (m)]²

The pounds-and-inches version usually quoted on charts is:

Imperial

BMI = 703 × weight (lb) ÷ [height (in)]²

This calculator does not use the rounded 703. It converts first, at 0.45359237 kg per pound and 0.0254 m per inch, which is the same constant carried to full precision: 0.45359237 ÷ 0.0254² = 703.0696. The two routes disagree by 0.0099 percent — at 150 lb and 65 in the shortcut gives 24.9586 and the exact conversion gives 24.9611, a gap of 0.0025 of a point at a BMI of 25, which never moves the first decimal place.

Either way the answer is in kilograms per square meter, and it is that figure the bands at 18.5, 25 and 30 apply to.

Step by Step

By hand, in metric, it is three operations. The calculator does the same three after converting whatever units you chose.

• Weigh yourself and write the figure down in kilograms. If you have pounds, multiply by 0.45359237 — 150 lb is 68.039 kg.
• Measure your height and convert it to meters. Centimeters divide by 100; inches multiply by 0.0254, so 70 in is 1.778 m.
• Square the height in meters. 1.778 × 1.778 = 3.1613.
• Divide the weight in kilograms by that squared height. 68.039 ÷ 3.1613 = 21.5.
• Round to one decimal place, then read the figure against 18.5, 25 and 30 — not against the band color, which follows the rounded number and can sit a hundredth either side of a cutoff.

Step three is the one people skip. Dividing by the height instead of the height squared is the most common hand-calculation error, and on the worked figures above it turns a BMI of 21.5 into 38.3 — a number that looks alarming and means nothing.

Entering Your Height Correctly

More wrong answers come from the height box than from anywhere else on this page, in two different ways.

The first is the feet-and-inches format. The digits after the decimal point are read as whole inches, not as tenths of a foot, and inches above 11 are capped:

The gap between 5.1 and 5.10 is nine inches and it is the single largest error this page can produce: at 200 lb, typing 5.10 returns 28.7 and typing 5.1 returns 37.8. If the format bothers you, switch the unit picker — 5 ft 10 in is 70 inches or 177.8 centimeters, and all three routes agree. A 165 lb person entering 5.11, 71, or 180.34 gets 23.0 every time.

The second way is measurement itself. Height goes into the denominator squared, so a small error there moves the answer more than the same error in weight would:

• At 180 lb, entering 5 ft 9 in returns 26.6, 5 ft 10 in returns 25.8 and 5 ft 11 in returns 25.1 — 1.5 points of spread across two inches of guesswork. One more inch, at 6 ft 0 in, returns 24.4 and moves the label from Overweight to Normal weight.
• At 70 kg, 173 cm returns 23.4, 175 cm returns 22.9 and 177 cm returns 22.3, about 0.26 of a point per centimeter.
• Measure without shoes, heels together against a wall, first thing in the morning if you want the flattering version — spinal compression costs most adults a little height over the course of a day.

Rounding your height up by an inch is not a harmless simplification. At average adult weights it is worth about three-quarters of a BMI point, in the direction you were hoping for.

The BMI Formula Explained

Dividing by height squared is what makes one scale usable for a short adult and a tall one; raw weight is not comparable between them. Three consequences follow from the shape of the equation, and all three show up in the readout:

• Weight moves the result in a straight line. At 5 ft 10 in, one pound is worth 0.143 BMI points, so a whole point costs 6.97 lb; at 175 cm, one kilogram is worth 0.327.
• Height moves it faster, because it is squared and sits in the denominator. At 70 kg, 175 cm gives 22.9, 174 cm gives 23.1 and 176 cm gives 22.6 — about 0.26 of a point per centimeter, so a generous height estimate flatters the reading.
• The output is rounded to one decimal place, which makes readings near a cutoff into ties. At 5 ft 10 in, 174 lb is 24.9661 and displays as 25.0.

The squaring is also why the same number of pounds means different things to different people. One pound is 0.183 BMI points at 5 ft 2 in and 0.128 at 6 ft 2 in — so a tall person can gain 42 percent more weight than a short one before the reading moves by the same amount. It is the reason the healthy band is 35.5 lb wide at 5 ft 2 in and 50.6 lb wide at 6 ft 2 in.

BMI Calculation Examples: Metric, Imperial and Feet-and-Inches

Four worked entries, each one run through the calculator on this page.

The height is converted to meters and squared, then divided into the weight:

BMI = 68 ÷ (1.70 × 1.70)
BMI = 68 ÷ 2.89 = 23.5

Metric — 68 kg, height set to 170 cm

23.5 — inside the healthy band, which at 170 cm runs from 54 kg (18.7) to 72 kg (24.9).

By hand, using the rounded 703 constant:

BMI = 703 × 150 ÷ (65 × 65)
BMI = 105,450 ÷ 4,225 = 24.96

Imperial — 150 lb, height set to 65 inches

The calculator displays 25.0. Its exact constants give 24.9611, and one decimal place rounds that onto the cutoff — a boundary reading rather than a verdict.

The 10 after the point is read as inches, giving 70 inches:

5.10 → 70 in → 1.778 m
200 lb → 90.718 kg
BMI = 90.718 ÷ (1.778 × 1.778) = 28.7

Feet-and-inches — 200 lb, height typed as 5.10

28.7 — the overweight band. Type the same height as 5.1 and the answer becomes 37.8, because 5.1 is read as 5 ft 1 in.

The same two operations, landing on the far side of the third cutoff:

BMI = 85 ÷ (1.68 × 1.68)
BMI = 85 ÷ 2.8224 = 30.1

Metric, over the line — 85 kg, height set to 168 cm

30.1 — the first tenth of the obesity band. At the same height, 84 kg returns 29.8 and stays in overweight; the whole class-I band from 30.0 to 34.9 spans 85 kg to 98 kg.

BMI Chart: Weight Ranges by Height

A BMI chart answers the question the raw number does not: what weight would put me somewhere else. Every figure in the tables below came out of this calculator.

First, the classification itself. The four bands drawn under your result are the WHO adult bands; the obesity classes are the standard subdivisions used in the NIH clinical guidelines.

The strip under the readout draws four of these bands — Underweight, Normal weight, Overweight, Obesity — across a scale from 15 to 40. It does not split obesity into classes.

Healthy Weight by Height, in Pounds

Whole pounds, with the boundaries taken from the true value rather than the rounded display. The healthy column is every pound that computes to at least 18.5 and less than 25.

A handful of those boundary pounds display right on a cutoff without being over it. At 5 ft 10 in, 174 lb computes to 24.9661 and shows as 25.0 while still belonging to the healthy column; 209 lb computes to 29.9881 and shows as 30.0 while still belonging to overweight. The number to trust is the one in the table, not the tenth on the screen.

Healthy Weight by Height, in Kilograms

The same bands in metric, in whole kilograms.

These ranges are wide on purpose — a 20-kilogram spread at 180 cm is not a target. For a narrower figure to aim at, and the classical formulas behind it, run the Ideal Weight Calculator.

BMI at a Glance: Pounds Against Height

Find your weight down the left, your height across the top, and read the BMI at the intersection. Values are exactly what the calculator returns.

Read a row across and the effect of the squared denominator is obvious: 200 lb is a BMI of 39.1 at five foot flat and 25.0 at six foot three. Read a column down and the spacing is even, because weight enters the equation in a straight line.

BMI at a Glance: Kilograms Against Height

How Much Weight One BMI Point Is Worth

The chart above hides a fact worth pulling out: the bands are not the same size for everyone. A taller frame gets a wider healthy range and a slower-moving number.

In metric the same pattern holds: the healthy band is 16.6 kg wide at 160 cm and 21.1 kg wide at 180 cm. This is also why a short person's reading is jumpier week to week — the same two pounds of water weight move a 5 ft 2 in BMI by 0.37 and a 6 ft 2 in BMI by 0.26.

How to Read Your Result

The calculator returns one figure and one colored strip. The figure is exact to a tenth; the strip is a rounding of a rounding, and the two can disagree at the edges.

What Each Band Is Telling You

The four labels are screening categories, and each one implies a different next step rather than a different verdict.

None of these labels is a statement about your health, because none of the inputs is a measurement of your health. They describe where your weight sits relative to your height, and that is the extent of the claim.

• Below 18.5 — Underweight. Worth investigating rather than celebrating. Persistent low weight can accompany undernutrition, malabsorption or an eating disorder, and unintentional loss into this band is the version that most warrants a conversation with a clinician.
• 18.5 to 24.9 — Normal weight. The band the cutoffs were drawn to enclose. It is wide: at 5 ft 10 in it stretches from 129 lb to 174 lb, which is 45 pounds of perfectly ordinary variation.
• 25.0 to 29.9 — Overweight. On its own this is a prompt for a second measurement, not a diagnosis. A waist circumference tells you far more about whether the extra mass is the kind that matters.
• 30.0 and above — Obesity, split by the NIH guidelines into class I (30.0–34.9), class II (35.0–39.9) and class III (40.0 and up). The strip does not name the class; the number does.

If the reading has you planning a change, the arithmetic of how fast to make it lives in the Calorie Deficit Calculator.

Boundary Readings and the Rounding Rule

The band color is picked from the displayed figure, not from the full-precision one, and the comparison is inclusive. A reading of exactly 25.0 is colored as Normal weight; exactly 30.0 is colored as Overweight; exactly 18.5 is colored as Underweight.

That produces a small set of readings where the color and the classification table disagree, and they are all genuine:

• 5 ft 10 in, 174 lb — true 24.9661, displayed 25.0, colored Normal weight. It really is a healthy-band weight.
• 5 ft 10 in, 209 lb — true 29.9881, displayed 30.0, colored Overweight. It really is below the obesity line.
• 5 ft 6 in, 155 lb — true 25.0174, displayed 25.0, colored Normal weight. This one genuinely is overweight by 0.0174 of a point, and the strip is being generous.
• 5 ft 4 in, 108 lb — true 18.5380, displayed 18.5, colored Underweight. It is inside the healthy band by half a tenth.
• 160 cm at 64 kg, and 180 cm at 81 kg — both land on exactly 25.0000, display 25.0 and are colored Normal weight. The classification table puts an exact 25.0 in Overweight, which is why the kilogram table ends the healthy column at 63 kg and 80 kg.

The practical rule: anything that displays within a tenth of 18.5, 25.0 or 30.0 is a boundary reading, and a boundary reading is not a category. One pound either side settles it — at 5 ft 10 in, 173 lb displays 24.8 and 175 lb displays 25.1.

This is also why a reading should never be compared against a cutoff to more precision than the measurements deserve. Your height is probably accurate to half an inch and your weight to half a pound, which is more uncertainty than the entire boundary zone contains.

What a Change in the Number Is Worth

Crossing a label takes more weight than most people expect, and the benefit of losing weight does not wait for the crossing. Both of those are visible in the arithmetic.

In the first row, a 10 percent loss is 20 pounds of real work and the label still says Overweight — the healthy band does not begin until 174 lb, a 13.0 percent loss. In the second, 10 percent moves a class-I obesity reading down into overweight. In the third, 10 percent takes 3.4 points off and still leaves the reading above 30.

The lesson is not that the effort was wasted. It is that the label is a coarse instrument for tracking progress: it changes three times in a lifetime while your weight changes every week. Track the number, which moves by 0.143 per pound at 5 ft 10 in, and let the label take care of itself.

It also cuts the other way. If your reading is 25.1 and you want it under 25, that is two pounds at 5 ft 10 in — 175 lb displays 25.1, 174 lb displays 25.0 and 173 lb displays 24.8 — which tells you how little the difference between those two labels means.

When the Marker Sits at the End of the Strip

The gauge is drawn from 15 to 40, which covers the great majority of adult readings but not all of them. A value outside that window parks the marker at whichever end it ran off while the number itself stays exact: 140 kg at 175 cm still prints 45.7, and the strip simply shows the marker pinned at the far right of the Obesity band.

At the other end, a blank weight box does not produce a low reading — it produces the amber "Check your inputs" notice, because the calculation returns a bare zero and the page reads that as an unfilled field rather than an answer. If you see that panel, one of the two number boxes is empty or zero.

Limits: When BMI Does Not Apply

The honest case against BMI is not that it is wrong. It is that it answers a narrower question than the one people ask it. Here is where the answer stops being useful, and what to measure instead.

Muscle Reads Exactly Like Fat

The NHS states the limitation plainly on its own adult BMI tool: the calculation cannot tell the difference between muscle and fat. Mass is mass to this equation. At 5 ft 10 in and 200 lb it returns 28.7 whether that 200 pounds arrived through fifteen years of training or fifteen years of sitting down.

This matters for a genuinely small group — heavily trained athletes, rugby forwards, powerlifters, some manual workers. It is also the most over-claimed exemption on the internet. A reading of 28.7 is far more often what it looks like than a hidden athlete, and the test is easy: if you cannot point to the training that built the mass, the muscle explanation is probably not yours.

It runs in reverse too. Someone in the healthy band with very little muscle and a high proportion of fat gets a reassuring number that describes nothing reassuring — a pattern common enough in older adults to have its own literature.

Children, Teenagers and Anyone Under 18

The bands on this page are the adult bands, and the calculator has no age field to switch on. The NHS tool that uses the same cutoffs tells you not to use it if you are under 18, and runs a separate calculator for ages 2 to 17.

For children and teenagers the same BMI number is computed and then compared against age- and sex-specific growth charts, and reported as a centile rather than a category — because a growing body's expected ratio of weight to height changes every year. A 13-year-old at 1.60 m and 55 kg returns 21.5 here, which the strip files comfortably under the adult healthy band. That figure says nothing at all about whether 21.5 is typical for a 13-year-old, which is the only question worth asking about it.

Pregnancy

BMI is not valid during pregnancy, and the NHS adult tool says so directly: do not use it if you are pregnant. The weight gained is fluid, placenta, and a second person, none of which the ratio was built to interpret, and a rising number through the second and third trimesters is exactly what is supposed to happen.

What does get used is your pre-pregnancy BMI, which sets the recommended range of total gestational weight gain. For that calculation, use the Pregnancy Weight Gain Calculator.

Older Adults

The equation has no age term, so a reading of 23.0 is reported identically for a 25-year-old and an 80-year-old. The bodies are not identical. Muscle mass declines with age while fat is redistributed toward the abdomen, so the same BMI at 80 sits on top of a different body composition than it did at 25 — usually more fat and less lean tissue for the same number.

Height also shrinks. Losing an inch of standing height between 40 and 75 is unremarkable, and at 150 lb an inch is worth 0.8 of a BMI point — 5 ft 6 in returns 24.2 and 5 ft 5 in returns 25.0 for exactly the same person on exactly the same scale. If you are tracking your BMI across decades, re-measure your height rather than reusing the figure from your driver's license.

Why 25 Is Not the Same Line Everywhere

The cutoffs came from populations that were overwhelmingly European, and the risk they describe does not arrive at the same BMI in every population. The NHS states it directly: people from an Asian, Black African, African-Caribbean or Middle Eastern ethnic background have a higher chance of developing health problems at a lower BMI.

A WHO expert consultation reviewing Asian populations reached the same conclusion in The Lancet in 2004. It found that the proportion of Asian people at high risk of type 2 diabetes and cardiovascular disease is substantial at BMIs below the existing overweight cutoff of 25, with observed risk rising from around 22 to 25 and high risk from 26 to 31. It recommended keeping the international classification intact while adding public-health action points at 23.0, 27.5, 32.5 and 37.5 kg/m².

In pounds and kilograms, those two lower action points sit a long way from the standard ones:

This calculator draws the standard international bands, because those are the ones the WHO classification specifies and the ones most readers are being measured against. If your health service works to a different set, read the number against theirs rather than against the strip.

What to Measure When BMI Is Not Enough

BMI's blind spot is distribution: it does not care whether the weight sits around your middle or anywhere else, and abdominal fat is the part most closely tied to cardiovascular and metabolic risk. Two cheap measurements fill that gap.

None of these replaces BMI so much as finishes the sentence it starts. A BMI of 27 with a 33-inch waist and a BMI of 27 with a 44-inch waist are the same reading and two different situations.

• Waist circumference. The NHLBI puts the risk thresholds at more than 35 inches for women and more than 40 inches for men. Measure at the top of the hip bone, after breathing out, tape snug but not compressing.
• Waist-to-height ratio. Divide waist by height in the same units. It needs no separate thresholds by sex and captures the same distribution signal BMI misses.
• A body-fat estimate. Circumference methods, skinfolds and DEXA all answer the question BMI cannot: how much of that mass is fat.

For what the reference method actually measures and how the circumference shortcuts compare with it, see the DEXA Body Fat Calculator.

And whatever you measure, measure it the same way each time. A trend built from consistent measurements beats a single accurate one.