BMI Calculator
BMI is weight in kilograms divided by height in metres squared. It is a population screening statistic that has been widely repurposed as an individual health measure, which it was never designed to be. Worth computing, worth understanding, and not worth over-interpreting.
How to use it
- Enter your height in centimetres.
- Enter your weight in kilograms.
- Read the BMI and the standard WHO category.
The formula and the categories
BMI equals weight in kilograms divided by the square of height in metres. Someone 175 cm and 70 kg has a BMI of 70 divided by 1.75 squared, which is 22.9.
The WHO thresholds are: under 18.5 underweight, 18.5 to 24.9 normal, 25.0 to 29.9 overweight, and 30.0 and above obese, with obesity subdivided at 35 and 40.
Two things about these thresholds are worth knowing. They are round numbers chosen for convenience rather than points where risk changes sharply, so 24.9 and 25.1 are not meaningfully different despite falling in different categories. And they were derived from European-descent populations. The WHO recommends lower thresholds for many Asian populations — commonly 23 for overweight and 27.5 for obese — because metabolic risk appears at lower BMI values in those groups.
What BMI cannot distinguish
The formula uses only height and weight, so it cannot tell what the weight consists of or where it is located. Both omissions matter.
Muscle is denser than fat, so a muscular person can register as overweight or obese with low body fat. A 180 cm rugby forward at 100 kg has a BMI of 30.9, formally obese, while potentially carrying 12 percent body fat. The reverse also occurs: sarcopenic obesity, where low muscle mass and high fat coexist at a normal BMI, is common in older adults and is associated with worse outcomes than the number suggests.
Fat distribution matters more than total quantity for metabolic risk. Visceral fat around the organs is strongly associated with insulin resistance and cardiovascular disease; subcutaneous fat on hips and thighs much less so. Two people at identical BMI with different distributions have materially different risk profiles, and BMI is blind to this.
The exponent is also a known approximation. Squaring height assumes body mass scales with the square of stature, which is not quite right, so BMI systematically overstates for tall people and understates for short people. Proposed alternatives using an exponent nearer 2.5 fit the data better but have not displaced the standard.
Better measures to use alongside it
BMI is cheap and requires no equipment, which is its genuine advantage. Adding one or two other measures improves the picture substantially.
- Waist circumference, measured at the navel. Above 102 cm for men or 88 cm for women indicates elevated risk regardless of BMI, and it captures the visceral fat that BMI misses.
- Waist-to-height ratio, which should ideally be under 0.5. It is a better predictor of cardiometabolic risk than BMI in most studies and needs only a tape measure.
- Waist-to-hip ratio, above about 0.90 for men or 0.85 for women indicating central adiposity.
- Body fat percentage, via bioelectrical impedance, DEXA, or skinfold calipers. DEXA is the most accurate available in practice; impedance scales are convenient and vary considerably with hydration.
- Actual metabolic markers — fasting glucose, HbA1c, lipid panel, blood pressure — which measure the risk directly rather than estimating it from body shape.
Where BMI is still the right tool
The criticisms above are all valid and none of them make BMI useless. It remains appropriate for the job it was built for.
At population scale it works well. Tracking the mean BMI of a country over decades, comparing regions, or evaluating a public health intervention are all cases where individual variation averages out and the measure is informative. It is also standardised worldwide and computable from data that is easy to collect, which is why epidemiology continues to use it.
For an individual it is reasonable as a starting point that indicates whether a closer look is warranted. A BMI of 35 in someone who does not train warrants attention; a BMI of 22 alongside good waist measurements and normal blood work is reassuring. It is also useful for tracking change in yourself over time, since your own height and body composition are relatively stable and a rising BMI in one person is more informative than a comparison between people.
What it should not be is a diagnosis, an eligibility criterion applied without clinical judgement, or a target in itself. Clinical guidance has moved toward treating it as one input among several, which is the correct reading.
At a glance
| Formula | kg divided by metres squared |
|---|---|
| WHO normal range | 18.5 to 24.9 |
| Asian population thresholds | Commonly 23 and 27.5 |
| Transmitted | Nothing, health data stays in the page |
Frequently asked questions
Is BMI accurate for athletes?
No. Muscle is denser than fat, so a muscular person can register as obese at low body fat. A 180 cm, 100 kg athlete has a BMI of 30.9 regardless of composition.
Do the categories apply to everyone?
Not uniformly. They were derived from European-descent populations, and the WHO recommends lower thresholds — commonly 23 and 27.5 — for many Asian populations.
What should I measure instead?
Waist circumference and waist-to-height ratio, which capture visceral fat that BMI misses. A ratio under 0.5 is the usual target.
Is BMI useless then?
No. It works well at population scale and is a reasonable individual starting point. It is not a diagnosis or a target on its own.
Read more
Health and fitness math — Every formula here is an estimate with a known error band. Knowing the band is what makes them useful.