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BMI calculator — your body mass index.
Your body mass index (BMI) is your weight divided by your height squared — a quick population screen for whether your weight sits in a healthy range. This calculator uses the standard formula (kg ÷ m²), maps you onto the WHO category table, and works in metric or imperial. Results update live, no signup.
BMI screens weight for height — it can't see muscle. If you train, also check body fat.
Save my numbers Track your weight trend in Nishaana.BMI categories.
The World Health Organization cut-offs for adults. Your result highlights below — but read the limits before you act on it.
| Category | BMI range | What it means |
|---|---|---|
| Underweight | below 18.5 | May signal too little mass, energy, or muscle |
| Normal weight | 18.5 – 24.9 | The WHO healthy-weight range for most adults |
| Overweight | 25.0 – 29.9 | Raised risk — but very common for muscular lifters |
| Obese (class I) | 30.0 – 34.9 | Higher metabolic and cardiovascular risk |
| Obese (class II) | 35.0 – 39.9 | Substantially higher health risk |
| Obese (class III) | 40.0 and above | Severe — clinical support usually warranted |
BMI is a screening tool, not a diagnosis. It ignores muscle mass entirely — a lean, well-trained lifter can read "overweight" while carrying single-digit body fat. It also can't judge fat distribution, age, or sex. For anyone who trains, pair BMI with body fat or FFMI.
The BMI formula, explained.
No black box. The exact equation in both unit systems, plus a worked example.
BMI = kg ÷ m² Weight in kilograms divided by height in metres, squared. Adopted by the WHO as the standard adult weight screen.
BMI = 703 × lb ÷ in² The same index in pounds and inches. The 703 constant converts the units so both formulas give the identical BMI.
80 ÷ (1.78)² = 25.2 An 80 kg person who is 1.78 m tall has a BMI of 25.2 — just into the "overweight" band, though a muscular build could explain it.
Adolphe Quetelet devised the index in the 1830s; it was named "body mass index" by Ancel Keys in 1972 and adopted by the World Health Organization for population screening. See the WHO obesity classification.
Related body calculators.
BMI is the crude screen. These give the composition picture it can't — then track it in the browser.
Body-fat calculator
The US Navy method BMI can't give you — actual fat percentage.
Open toolFFMI calculator
A muscle-aware index that fixes BMI's blind spot for lifters.
Open toolIdeal weight calculator
Devine, Robinson, Miller, and Hamwi target weights side by side.
Open toolNutrition tracker
Log food against your numbers, right in the browser.
Open toolBody mass index questions.
What is a healthy BMI?
The World Health Organization defines a healthy BMI as 18.5 to 24.9 for most adults. Below 18.5 is classed as underweight, 25.0 to 29.9 as overweight, and 30.0 and above as obese. These cut-offs are population screening thresholds, not a diagnosis — a muscular or older person can sit outside the range and still be perfectly healthy.
How is BMI calculated?
BMI is your weight in kilograms divided by your height in metres squared: BMI = kg ÷ m². For example, an 80 kg person who is 1.78 m tall has a BMI of 80 ÷ (1.78 × 1.78) = 25.2. In imperial units the formula is 703 × pounds ÷ inches². This calculator runs that math for you and maps the result onto the WHO category table.
Is BMI accurate for muscular people or athletes?
No — this is BMI's biggest limitation. BMI only knows your height and weight; it cannot tell muscle from fat. A lean, well-trained lifter can carry enough muscle to land in the "overweight" or even "obese" band while having low body fat. If you strength train, treat BMI as a rough population screen and use body-fat percentage or FFMI for a real read on your composition.
What is the difference between BMI and body-fat percentage?
BMI is a ratio of total weight to height and does not distinguish tissue types. Body-fat percentage measures how much of your weight is actually fat versus lean mass — muscle, bone, water, and organs. Two people with the same BMI can have very different body-fat percentages. For anyone who trains, body-fat percentage is the more meaningful number.
Does BMI work the same for men and women?
The BMI formula and the WHO category cut-offs are identical for men and women. Women naturally carry a higher body-fat percentage than men at the same BMI, so the same number can mean different compositions across sexes. BMI also isn't designed for children, pregnancy, or the very elderly, who use different reference charts.
Is this BMI calculator free?
Yes. It runs entirely in your browser, updates instantly as you type, and needs no signup or download. Save your result to Nishaana to track it over time alongside your body-fat percentage, weight trend, and training.
Save your numbers. Track them.
Free in your browser — Nishaana keeps your BMI, body fat, and weight trend in one place and charts where you're heading, no app store needed.
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