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 was invented in the 1830s for population statistics, not individual diagnosis. Adolphe Quetelet's original Quetelet Index aimed to characterise the "average man" across a population. Its use as an individual diagnostic tool came a century later and was always controversial. For a single person, waist circumference (men >102 cm / women >88 cm indicates elevated risk) is generally a better cardiovascular signal than BMI alone. The "overweight paradox" is real but contested. Multiple large meta-analyses find that adults in the "overweight" BMI range (25-29.9) have slightly lower all-cause mortality than the "normal" range. Possible explanations: residual confounding (smokers and the chronically ill tend to weigh less), the population-statistics origin of the threshold, and that small reserves of fat may be protective in illness or injury. Don't interpret the BMI categories as a fitness target. The thresholds are not the same everywhere. WHO Asia-Pacific guidance shifts the overweight threshold to 23 and the obese threshold to 27.5 for South Asian, Chinese, and Southeast Asian populations, because cardiovascular and diabetes risk rises at lower BMIs in those groups. The US/Europe thresholds (25/30) were derived from European-descent populations and don't transfer cleanly. The squared-height denominator is mathematically arbitrary. Quetelet picked it because it produced numbers that varied less with height in his sample. For very tall and very short people, the formula systematically misfires — Trefethen's revision (height^2.5) is more accurate but never displaced the original because the institutional cost of switching is enormous. If you're under 165 cm or over 195 cm, treat your BMI as approximate. Trends matter more than absolute values. Whatever your starting BMI, a stable trend over years signals different things than a rapid change. Rapid unintended loss in adults often signals underlying disease; rapid gain often signals lifestyle or medication change. A clinician interpreting the same number alongside the trend, alongside lab work and your individual history, will reach a much better conclusion than any web calculator.

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.

Expert notes

BMI was invented in the 1830s for population statistics, not individual diagnosis. Adolphe Quetelet's original Quetelet Index aimed to characterise the "average man" across a population. Its use as an individual diagnostic tool came a century later and was always controversial. For a single person, waist circumference (men >102 cm / women >88 cm indicates elevated risk) is generally a better cardiovascular signal than BMI alone.
The "overweight paradox" is real but contested. Multiple large meta-analyses find that adults in the "overweight" BMI range (25-29.9) have slightly lower all-cause mortality than the "normal" range. Possible explanations: residual confounding (smokers and the chronically ill tend to weigh less), the population-statistics origin of the threshold, and that small reserves of fat may be protective in illness or injury. Don't interpret the BMI categories as a fitness target.
The thresholds are not the same everywhere. WHO Asia-Pacific guidance shifts the overweight threshold to 23 and the obese threshold to 27.5 for South Asian, Chinese, and Southeast Asian populations, because cardiovascular and diabetes risk rises at lower BMIs in those groups. The US/Europe thresholds (25/30) were derived from European-descent populations and don't transfer cleanly.
The squared-height denominator is mathematically arbitrary. Quetelet picked it because it produced numbers that varied less with height in his sample. For very tall and very short people, the formula systematically misfires — Trefethen's revision (height^2.5) is more accurate but never displaced the original because the institutional cost of switching is enormous. If you're under 165 cm or over 195 cm, treat your BMI as approximate.
Trends matter more than absolute values. Whatever your starting BMI, a stable trend over years signals different things than a rapid change. Rapid unintended loss in adults often signals underlying disease; rapid gain often signals lifestyle or medication change. A clinician interpreting the same number alongside the trend, alongside lab work and your individual history, will reach a much better conclusion than any web calculator.