BMI Calculator

What BMI actually measures

Weight divided by height squared. That is the entire formula, and it is worth knowing because it explains both why BMI is useful and why it is limited.

It was devised in the 1830s by a Belgian statistician studying populations, not patients. It does what it was built for — describing groups — and it has been doing a job it was never designed for ever since.

It cannot tell muscle from fat

The formula has no way to know what the weight is made of. A muscular athlete and a sedentary person of the same height and weight get the same number, and one of them is being described badly.

It also takes no account of where weight is carried, which matters medically, and it is known to fit some ethnic groups better than others — several health bodies use lower thresholds for South Asian populations for exactly this reason.

The three classes of obesity

Most calculators stop at one band above 30 and call it obese. The World Health Organization does not: it splits that range into class I (30.0 to 34.9), class II (35.0 to 39.9) and class III (40.0 and above), and Brazil’s Ministry of Health uses the same three grades. The split exists because the risk attached to each differs enough to change what a clinician does about it, so collapsing them throws away the part of the answer that carries information.

Where the weight sits matters more than how much

This is the limitation BMI cannot fix from the inside, and the reason this tool asks for a waist measurement. Weight carried around the middle tracks with type 2 diabetes and cardiovascular disease more closely than total weight does, and two people with identical BMIs can sit far apart on that measure.

Waist-to-height ratio is the simplest way in, and in 2025 NICE began recommending it be measured alongside BMI for anyone with a BMI below 35. The guidance puts 0.4 to 0.49 as healthy, 0.5 to 0.59 as increased and 0.6 or more as high — and the memorable version is that your waist should be less than half your height. Its real advantage is that unlike BMI the same thresholds hold across sexes and ethnicities, and it stays useful in people with a lot of muscle.

Waist alone has thresholds of its own, and two sets of them are in circulation. The WHO and Brazil’s Ministry of Health mark increased risk at 94 cm for men and 80 cm for women; the WHO and the American NHLBI mark substantially increased risk at 102 cm and 88 cm. Both are shown here rather than one being picked, because both are genuinely in use and a reader is better served knowing where they sit against each.

Together they say more than either alone. The NHLBI clinical guidelines publish a grid crossing BMI band with waist measurement, and the tool reads your position off it: a BMI in class I with a waist under the threshold is “high” risk relative to a healthy weight, and the same BMI with a waist over it is “very high”. One measurement moves you a whole step.

The numbers beside BMI, and what each is for

BMI Prime is simply your BMI divided by 25, the top of the healthy band. It turns the figure into a ratio: 1.0 is exactly at the upper edge, 1.2 is twenty per cent above it. Nothing new is measured, but a proportion is easier to hold in your head than a number whose scale you have to remember.

The ponderal index divides weight by height cubed rather than squared, and it exists because a body is a three-dimensional object. BMI’s square drifts with stature: for the same build it reads high on tall people and low on short ones. At the extremes of height the ponderal index is the more honest figure, which is exactly where BMI is most often quoted at someone unfairly.

Estimated body fat comes from a 1991 formula by Deurenberg and colleagues that predicts fat percentage from BMI, age and sex. It is shown as a range, not a figure, and that is deliberate: the paper reports a standard error of 4.1 percentage points, so a single number would be claiming a precision three inputs cannot support. It is also a population formula, and it is known to overestimate fat-free mass in older adults.

Relative Fat Mass is the better estimate of the two, and it appears whenever a waist is given. Published by Woolcott and Bergman in 2018, it uses only height, waist and sex — and in the NHANES data it was developed and validated on, it predicted whole-body fat measured by DXA more accurately than BMI did, with fewer false negatives. It has since been validated externally. Both figures are shown rather than one replacing the other: they disagree by a few points, and a reader who has seen one of them on another site deserves to know which formula produced which.

Either percentage is also given in kilograms, because a percentage is an abstraction and a kilogram is not. Knowing that roughly 30 kg of a 92 kg body is fat says something that “33%” does not.

Lower thresholds for Asian populations

A WHO expert consultation published in the Lancet in 2004 found that the proportion of Asian people at high risk of type 2 diabetes and cardiovascular disease is substantial at BMIs below the general cut-off of 25. It identified lower public-health action points — 23.0 and 27.5 — rather than replacing the thresholds outright. When your figure would fall in a different band under those points, the tool says so, because that is the case where the distinction changes the reading.

Why there is no “ideal weight” here

The formulas people look for — Devine, Robinson, Miller, Hamwi — were written to calculate drug doses from height, and were never validated as targets for a healthy body. They persist online as personal goals anyway, which is a misuse we would rather not add to. The healthy weight range this tool does show is derived from the BMI band itself, which is at least the thing that band is for.

Read it as one number among several

BMI is a screening measure. It is a cheap, fast way to decide whether a longer conversation is worth having, and it is not a diagnosis of anything on its own.

This tool shows the band and also the weight range that band corresponds to at your height, because a range is more useful than a verdict. Nothing here is medical advice, and a number that concerns you is a reason to speak to a doctor rather than to a website.

Where these numbers come from

Every threshold on this page is from a named source rather than from rounding somebody else’s calculator: the band structure and the obesity classes from the World Health Organization and Brazil’s Ministry of Health; the waist thresholds from the WHO and the NHLBI’s clinical guidelines, whose Table IV-2 is the combined grid; the waist-to-height bands from NICE guideline NG246; the waist-to-hip thresholds from the WHO; the lower Asian action points from the 2004 WHO expert consultation in the Lancet; and the two body fat estimates from Woolcott and Bergman, 2018, and from Deurenberg and colleagues, 1991, the latter reported with the standard error that paper gives.

Nothing is sent anywhere

Every figure on this page is worked out in your own browser. No request is made, nothing is stored between visits, and the page keeps working with the connection switched off.