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Health & fitness

BMI Calculator

Body mass index in metric or imperial units with WHO categories, and an honest note on what BMI cannot tell you.

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Quick answer

BMI = weight (kg) ÷ height (m)². 70 kg at 175 cm is 22.9, “normal” spans 18.5 to 24.9. BMI is a population screening ratio, not a diagnosis: it cannot separate muscle from fat, which is why athletes routinely read “overweight”.

Asian cut-offs are lower, India and much of Asia use them.

BMI
22.9
Category (WHO standard)
Normal (18.5 – 24.9)
Healthy weight for this height
56.7 – 76.6 kg

Formulas last verified 2026-10-10. Results are estimates, not financial advice. How we calculate.

One ratio, honestly presented

BMI = kg / m²

Weight divided by height squared. A 70 kg person at 1.75 m scores 70 ÷ 3.0625 = 22.9. The imperial form multiplies by 703 to land on the same scale: BMI = (lb / in²) × 703. The calculator takes either unit system; the number means the same thing.

The WHO bands

BMICategory70 kg example, weight at 1.75 m
< 18.5Underweightunder 56.7 kg
18.5 – 24.9Normal56.7 – 76.3 kg
25 – 29.9Overweight76.6 – 91.6 kg
≥ 30Obese91.9 kg and above

The third column is the useful reframing: at a given height, the "normal" band is nearly a 20 kg range. BMI is not prescribing a weight; it is bounding a wide corridor.

What BMI cannot see

The formula contains two numbers, so everything else is invisible to it:

  • Composition. Muscle outweighs fat per litre, so athletes routinely read "overweight", a 90 kg, 1.80 m rugby player is BMI 27.8 regardless of being lean. The mirror error is "normal-weight obesity": an in-band BMI over low muscle and high fat.
  • Distribution. Visceral fat around the organs carries more metabolic risk than fat elsewhere, and BMI cannot tell the two apart. Waist circumference is the cheap companion measurement that partially can.
  • Ethnicity. The 18.5/25/30 cutoffs were derived largely from European populations. WHO-endorsed guidance for Asian populations uses action points of 23 and 27.5, because metabolic risk rises at lower BMIs; Indian guidelines similarly treat 23+ as overweight. The band your result falls in can legitimately depend on which thresholds apply to you.
  • Age. Adult cutoffs do not apply to children and teens, who are assessed by age-and-sex percentile charts, and they blur for older adults, where slightly higher BMIs have not shown the same risks.

Screening, not diagnosis

BMI earns its ubiquity at population scale, across millions of people it tracks health outcomes well enough to be useful, and it costs a scale and a tape measure. Applied to one person it is a prompt, not a verdict: a flag worth checking against waist size, blood pressure, blood sugar, lipids and history, which is a doctor's job, not a formula's. This page computes the standard ratio and reports the standard bands for information; it is not medical advice, and a surprising result in either direction is best taken to a professional rather than to a search engine.

Nearby numbers

Weight management, where it is warranted, runs on energy arithmetic rather than ratios, the calorie calculator estimates maintenance needs from the Mifflin-St Jeor equation and shows the macro split. Tracking change over time is percentage work (percentage calculator), and BMI-for-age assessments for younger people start from an exact age, the age calculator gives it to the day.

Frequently asked questions

How is BMI calculated?

BMI = weight in kilograms ÷ height in metres squared. A 70 kg person at 1.75 m: 70 ÷ (1.75)² = 22.9. In imperial units the formula is (pounds ÷ inches²) × 703, which gives the same number. It is deliberately crude, two measurements and one division, which is exactly why it became the universal screening ratio.

What are the BMI categories?

The WHO bands for adults: under 18.5 is classed underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30+ obese (with obesity subdivided further at 35 and 40). The cutoffs are population-level conventions, not biological switches, nothing changes in your body between 24.9 and 25.0. They exist so studies and clinics can speak a common language.

Why does BMI misjudge athletes?

BMI weighs mass, not composition. Muscle is denser than fat, so a muscular 90 kg rugby player at 1.80 m reads 27.8, “overweight”, while carrying little fat. The reverse error also exists: a sedentary person can hold a “normal” BMI with high body fat and low muscle, sometimes called normal-weight obesity. For individuals, composition measures beat the ratio.

Do BMI thresholds differ by ethnicity?

Yes. Research shows people of Asian descent tend to carry more visceral fat and face higher metabolic risk at lower BMIs, so WHO-endorsed guidance for Asian populations often uses action points of 23 (increased risk) and 27.5 (high risk) instead of 25 and 30. India’s guidelines similarly class 23+ as overweight. If those thresholds apply to you, read your result against them.

Is BMI useful at all, then?

At population scale, genuinely: it correlates with health risk across large groups and costs nothing to measure, which is why researchers and public-health bodies keep it. For an individual it is a screening prompt, not a verdict, a flag that says “worth a closer look”, ideally alongside waist circumference, blood pressure, blood sugar and lipids, which carry far more personal information.

What should I do about a high or low BMI?

Treat it as a reason for a conversation, not a diagnosis. A BMI outside the normal band alongside other signals, waist size, family history, lab results, is worth raising with a doctor, who can interpret the whole picture. This page is informational; it computes a ratio and reports the standard bands, and none of that substitutes for medical advice tailored to you.

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