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BMI calculator

Height and weight in, body mass index out — with what the number can and cannot tell you, said plainly.

BMI22.9
CategoryHealthy range
Healthy-range weight for your height—
Waist-to-height ratio— add your waist

Formula: 70 kg ÷ (1.75 m)² = 22.9

How BMI works

For people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, NICE uses lower thresholds — overweight from 23 and obesity from 27.5 — because health risks such as type 2 diabetes start at a lower BMI. Choose your family background above and the calculator uses the right bands. Adding your waist gives the waist-to-height ratio, which NICE recommends alongside BMI: keeping your waist to less than half your height is a simple target that works for everyone (NICE NG246).

Body mass index is weight in kilograms divided by height in metres squared. The WHO screening bands: under 18.5 underweight · 18.5–24.9 healthy range · 25–29.9 overweight · 30+ obese. Imperial entries are converted exactly (1 lb = 0.45359237 kg, 1 in = 2.54 cm) before the same formula runs.

BMI is a screening number, not a diagnosis. It cannot tell muscle from fat — a rugby player can read "overweight" while lean — and it can understate risk in older adults. UK guidance uses lower thresholds (increased risk from 23) for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds. If your number worries you, that is a conversation for a GP or practice nurse, not a webpage.
Nothing you type is sent anywhere.

The calculator runs entirely in your browser and works offline. No cookies, no third-party scripts, no record of your body on any server.

Common questions

Why does one kilogram barely move the number?

Because height is squared in the denominator, BMI changes by roughly 0.3 per kilogram for someone of average height. Small day-to-day weight fluctuation — water, food, time of day — moves it less than the noise in bathroom scales.

Is waist size a better measure?

Waist-to-height ratio catches central fat that BMI misses; UK guidance suggests keeping your waist to less than half your height. The two numbers answer different questions and are most useful together.

What about children?

This page is for adults. Children's BMI is read against age-and-sex growth centiles, which is a different calculation — the NHS provides a dedicated child BMI tool.

The BMI formula and WHO bands are long-standing and stable. Calculations verified by automated known-answer tests (e.g. 70 kg · 1.75 m → 22.9). Page last reviewed: 14 August 2026. This is a calculation aid, not medical advice.

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