BMI Calculator

BMI categories for adults

The left column is the WHO international classification, which applies to adults aged 20 and over. The right column is the WHO Asian-Pacific classification, published after evidence that cardiometabolic risk rises at a lower BMI in South and East Asian populations. Switch the calculator between them with the cut-points control above.

BMI chart — metric (cm and kg)

Find your height down the left and your weight across the top. The cell where they meet is your BMI, coloured by WHO category.

BMI chart — imperial (ft/in and lb)

Find your height down the left and your weight across the top. The cell where they meet is your BMI, coloured by WHO category.

How BMI is calculated

BMI is weight divided by height squared. The metric form is the definition; the imperial form is the same calculation with a conversion factor of 703 folded in, which is why that number appears from nowhere in American textbooks.

Metric:    BMI = weight (kg) / height (m)²
Imperial:  BMI = 703 × weight (lb) / height (in)²

Worked example

Someone who is 175 cm and 70 kg has a height of 1.75 m, so 1.75² = 3.0625 m². Dividing 70 by 3.0625 gives 22.9 — inside the healthy band. The same person in imperial is 5 ft 9 in, which is 69 inches, and 154.3 lb: 703 × 154.3 / 69² = 22.8. The small difference is rounding in the unit conversion, not a different formula.

The derived indices

BMI Prime      = BMI / 25
Ponderal Index = weight (kg) / height (m)³
Healthy range  = 18.5 × height (m)²  to  24.9 × height (m)²

BMI Prime restates the index as a fraction of the overweight threshold, so 1.00 is the threshold itself and 0.90 is nine tenths of the way to it. The Ponderal Index uses the cube of height, which corrects the bias BMI carries at unusual heights. Inverting the two cut-points against your height is what produces the healthy weight range shown above.

Where BMI falls short

BMI is a screening tool built on a population average, and it was never designed to describe an individual. It is genuinely useful for spotting trends across large groups and for flagging that a closer look might be warranted. These are the cases where the number misleads.

It cannot tell muscle from fat

This is the central limitation. BMI reads total mass, and muscle is roughly 18% denser than fat. A trained athlete with visible abdominal definition can land squarely in the "obese" band, and someone sedentary with little muscle can sit mid-band while carrying a high fat percentage — a pattern sometimes called normal-weight obesity, which carries real metabolic risk that BMI records as normal. A body fat calculatoranswers the question BMI cannot.

It says nothing about where fat sits

Visceral fat around the organs is considerably more dangerous than subcutaneous fat on the hips and thighs, and two people with the same BMI can differ sharply in how much of each they carry. Waist circumference and waist-to-height ratio capture this and BMI does not — which is why clinicians increasingly read them alongside it rather than relying on BMI on its own.

It is biased by height

Dividing by height squared assumes bodies scale as squares. They do not, so BMI reads slightly high for tall people and slightly low for short ones. The Ponderal Index shown above is the usual correction.

The cut-points are not universal

The 25 and 30 thresholds were derived largely from European-descent populations. Risk of type 2 diabetes and cardiovascular disease rises at a lower BMI in South and East Asian populations, which is why the WHO published the lower 23 and 27.5 cut-points this page offers as an alternative. Body composition at a given BMI also differs across other groups.

It does not apply to everyone

Children and teenagers need age-and-sex percentiles from a growth chart, not fixed bands. BMI categories do not apply during pregnancy. In adults over about 65, height loss inflates BMI while muscle loss means the same figure represents more fat, and the BMI associated with the lowest mortality shifts slightly upward.

Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. BMI is a screening figure that cannot assess your body composition, fat distribution, or health on its own.Speak to a doctor or another qualified clinician about any health decision.Last reviewed 16 September 2026.

Frequently asked questions