Metric: BMI = kg ÷ (m × m). 70 kg at 1.75 m → 70 / 3.0625 = 22.9. Imperial: BMI = (lb × 703) ÷ (in × in). The tool converts to metric internally for accuracy. The WHO categories are the same regardless of unit system — BMI itself is unitless.

Quick self-check or to fill in a form that asks for it (insurance, fitness apps, gym intake). Comparing a value across populations or studies. Tracking direction of change over time (rising, stable, falling) — the trend is more useful than any single reading.

BMI does not measure body composition. Muscle weighs more than fat, so a fit, muscular person can score "overweight" while having low body-fat. Conversely, a low-muscle person can score "normal" while being unhealthy ("skinny fat"). It's an adult metric. For children and adolescents (under 18) use the age- and sex-specific BMI percentile charts instead. Pregnancy isn't supported. BMI doesn't apply during pregnancy; talk to your healthcare provider. Ethnicity matters. Several health bodies (NHS, WHO Asia-Pacific guidance) use lower thresholds (overweight ≥23, obese ≥27.5) for South Asian, Chinese, and other groups, because cardiovascular risk rises at lower BMIs. Tall vs short people. The squared-height formula systematically over-classifies tall people as "underweight" and short people as "overweight" — alternative formulas (Trefethen's BMI uses height^2.5) try to correct for this. Not medical advice. If you're concerned about your weight, talk to a clinician. They have the rest of the picture (waist circumference, blood pressure, blood-work, lifestyle) that a single number doesn't.

BMI Calculator

Body Mass Index from height and weight. Metric or imperial. Shows the WHO category — not medical advice.

Units

Height (cm)

Weight (kg)

Result

What is this for?

Body Mass Index (BMI) is a single number — weight in kilograms divided by the square of height in metres — used by the World Health Organization and many health systems as a quick screen for body-weight categories. It's not diagnostic; it's a flag. The classic adult thresholds are: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and over obese (split into Class I/II/III at 35 and 40). This tool computes the value and the category from height and weight in metric or imperial.

How it's calculated

Metric: BMI = kg ÷ (m × m). 70 kg at 1.75 m → 70 / 3.0625 = 22.9.
Imperial: BMI = (lb × 703) ÷ (in × in). The tool converts to metric internally for accuracy.
The WHO categories are the same regardless of unit system — BMI itself is unitless.

When to use it

Quick self-check or to fill in a form that asks for it (insurance, fitness apps, gym intake).
Comparing a value across populations or studies.
Tracking direction of change over time (rising, stable, falling) — the trend is more useful than any single reading.

Common gotchas

BMI does not measure body composition. Muscle weighs more than fat, so a fit, muscular person can score "overweight" while having low body-fat. Conversely, a low-muscle person can score "normal" while being unhealthy ("skinny fat").
It's an adult metric. For children and adolescents (under 18) use the age- and sex-specific BMI percentile charts instead.
Pregnancy isn't supported. BMI doesn't apply during pregnancy; talk to your healthcare provider.
Ethnicity matters. Several health bodies (NHS, WHO Asia-Pacific guidance) use lower thresholds (overweight ≥23, obese ≥27.5) for South Asian, Chinese, and other groups, because cardiovascular risk rises at lower BMIs.
Tall vs short people. The squared-height formula systematically over-classifies tall people as "underweight" and short people as "overweight" — alternative formulas (Trefethen's BMI uses height^2.5) try to correct for this.
Not medical advice. If you're concerned about your weight, talk to a clinician. They have the rest of the picture (waist circumference, blood pressure, blood-work, lifestyle) that a single number doesn't.