BMI Calculator

What BMI actually is

Body Mass Index is a ratio. It takes your weight and divides it by your height squared, and returns a single number. That is the whole of it.

The measure dates to the 1830s, when the Belgian mathematician Adolphe Quetelet was looking for a way to describe the distribution of body size across a population. He was not building a health test. He was building a statistical description of groups of people, and the index carried his name for over a century before it was renamed.

That origin still shapes what the number can and cannot do. An index built to describe populations tells you where an individual sits relative to a distribution. It does not tell you what that individual is made of, how they are built, or how healthy they are.

BMI stayed in use because it is cheap. It needs two measurements almost everyone already has, no equipment, and no clinician. That is a real advantage at the scale of a population and a real limitation at the scale of one person.

How the calculation works

The metric form is the original:

The imperial form people usually see is:

That 703 is a conversion constant, and it is rounded. The exact figure is 703.06957, derived from the exact definitions of the pound (0.45359237 kg) and the inch (0.0254 m).

This calculator does not use 703. It converts your entered height and weight to metric using the exact factors, then runs the metric formula. At one decimal place the two routes almost always agree. Converting exactly removes the small drift that the rounded constant introduces, so the number you see does not depend on which units you typed.

Take the default on this page: 5 ft 10 in and 165 lb. That is 70 inches and 165 pounds, which converts to 1.778 metres and 74.84 kilograms. Divide 74.84 by 1.778 squared and you get 23.7.

The same calculation works in reverse for the healthy range shown with your result. At 5 ft 10 in, a BMI of 18.5 corresponds to 128.9 lb and a BMI of 24.9 corresponds to 173.5 lb. That range is a property of the height, not a judgment about the person.

The categories

For adults 20 and older, BMI is read against six bands:

These are the adult weight-status categories published by the CDC, and they align with the World Health Organization's international classification. They are the same for everyone: they do not shift with age or sex once you are 20 or older.

The class split above 30 exists because the range beyond that point is wide. A BMI of 30.1 and a BMI of 52 are both "obesity" under a single label, and the three classes distinguish them.

These bands come from population data, and boundaries in population data are conventions, not cliffs. Nothing physical changes between 24.9 and 25.0. Someone sitting a tenth of a point either side of a line is, for practical purposes, in the same place. The band tells you roughly where in the distribution you sit. It does not describe a change of state.

Because the categories are fixed, the healthy-weight range in pounds varies only with height:

What BMI does not measure

BMI knows two things about you: how tall you are and how much you weigh. Everything it cannot distinguish follows from that.

It cannot distinguish fat, muscle and bone mass. Two people of the same height and the same weight get exactly the same BMI whatever that weight is made of. The index has no way to tell them apart, because it never received that information.

This matters for people who train. BMI reads muscle as weight like any other weight, so a muscular person can receive an elevated classification purely because the index cannot see what the weight is made of.

It cannot tell where the weight sits. Fat around the abdomen and fat distributed elsewhere are treated identically, because BMI never sees distribution at all.

It cannot see anything else about you -- not your bone structure, not your frame, not your fitness, not your blood work.

If body composition is what you actually want to know, BMI is the wrong instrument and there is a better one on this site. The Body Fat Percentage Calculator uses tape measurements to estimate what proportion of your weight is fat, which is the question BMI cannot answer. It will often give you a different picture from the one here, and when the two disagree, the disagreement is the useful information.

Worked examples

Six people, all adults, all run through the calculator on this page.

Two things are worth pulling out of that table.

The two rows at 5 ft 10 in are 20 lb apart and land in different categories. 165 lb gives 23.7 and 185 lb gives 26.5. Twenty pounds moved the index by 2.8 points, which was enough to cross a boundary. At shorter heights the same twenty pounds moves the index further, because the divisor is smaller.

The 5 ft 6 in and 5 ft 9 in rows land in the same category from different directions. 195 lb at 5 ft 6 in gives 31.5 and 215 lb at 5 ft 9 in gives 31.7. The taller person weighs 20 lb more and gets a slightly higher index, because height enters the formula squared and weight enters it directly. Height carries more leverage in the formula than weight does.

Who the index fits poorly

BMI was built from population data, and it works least well for people whose bodies differ from that population in ways the two inputs cannot capture.

People who carry significant muscle. Covered above in section 4.

Older adults. Body composition changes with age even when weight does not. The same BMI at 70 and at 30 can describe quite different bodies.

People who are pregnant. Weight gain during pregnancy is expected and BMI has no way to account for it. The index is not designed for this and should not be read during it.

People with limb loss or significant differences in body structure. The height-and-weight relationship the index assumes does not hold.

Very tall and very short adults. Height enters the formula squared while weight enters it directly, so the index does not scale proportionally across the height range. The same bands are applied at every height.

One further note on population. The World Health Organization has identified BMI figures of 23.0 and 27.5 as additional public-health action points, because the relationship between BMI and health risk varies across populations and is not identical everywhere. These are points at which public-health attention may be warranted at a population level. They are not an alternative set of categories, and this calculator does not apply them. The classification here is the international one shown in section 3.

Two people, one number

Here is the limitation as a single case rather than a general statement.

Two adults are both 5 ft 11 in and both weigh 200 lb. Run either of them through this calculator and the answer is the same: BMI 27.9, Overweight. It has to be. The inputs are identical, so the output is identical.

Now suppose, hypothetically, that the two are built very differently - that one carries considerably more muscle and less fat than the other. Nothing in the calculation would change. The result would still be 27.9 for both.

BMI does not separate them, because BMI never received the information that would. It was given a height and a weight, and it returned the only thing those two numbers can produce.

This is not an error in the calculation. The arithmetic is correct in both cases. It is a limit on what the question can answer, and the honest response is to change the question -- which is what the Body Fat Percentage Calculator does.

Common mistakes when reading a BMI number

Treating a band as a diagnosis. BMI is a screening measure. It flags where someone sits in a distribution, and screening measures are starting points for a conversation, not conclusions.

Reading precision that is not there. The index is reported to one decimal place, which looks precise. It is built from two measurements that are usually rounded, and it is a population statistic to begin with. The difference between 24.8 and 25.1 is not a meaningful difference about a person.

Forgetting that the categories are conventions. The boundaries were chosen. They are useful and they are widely agreed, but nothing in the body changes when the number crosses one.

Using it on someone under 20. For ages 2 through 19 the index is read against age- and sex-specific percentile charts, not these bands. This calculator does not do that, and applying adult categories to a child gives a meaningless answer.

Assuming it measures health. It measures weight relative to height. Fitness, blood pressure, blood glucose, strength, sleep and everything else that bears on health are outside what two numbers can express.

Expecting it to describe an athlete. See section 4. It will not.

What the number is for, and what it is not for

What it is for. BMI is a fast, free, universally available screening measure. It gives a rough position in a population distribution using two measurements anyone can take. At the scale of a population it is genuinely useful, which is why public-health bodies use it. At the scale of one person it is a first look.

It is also useful as a tracking number over time for the same individual, because a person's height is roughly constant, so changes in the index reflect changes in weight. It says nothing about what that weight is made of.

What it is not for. It is not a diagnosis. It is not a measure of body composition. It is not a measure of health. It does not account for muscle, fat distribution, frame, age, training history or anything else.

This page does not tell you what to do with the number. It reports the index, the category it falls into, and the healthy-weight range for the height you entered. Those are facts about a calculation. What they mean for you, and whether anything should follow from them, is a question for you and for a clinician who can see the rest of the picture.

If you want the body-composition question answered instead, use the Body Fat Percentage Calculator. If you are working on energy intake, the Calorie Calculator and the Macros Calculator sit alongside this one.

Frequently asked questions

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