BMI is a useful population screening tool but an imperfect individual measure. It doesn't distinguish muscle from fat, so very muscular people can read as 'overweight,' and it doesn't capture fat distribution. Combine it with waist measurement and body fat percentage for a fuller picture.

BMI Calculator

Q: How is BMI calculated?

BMI is weight in kilograms divided by height in metres squared: BMI = kg ÷ m². In imperial units it is 703 × pounds ÷ inches². Someone 5'9" (175 cm) weighing 160 lb (73 kg) has a BMI of about 23.6.

Q: What is a healthy BMI?

For adults the WHO classifies 18.5–24.9 as a healthy weight, under 18.5 as underweight, 25–29.9 as overweight, and 30 or above as obese. These ranges apply to adults over 20; children use age- and sex-specific percentiles.

Q: Does BMI apply to children?

No, the fixed adult BMI categories shown here apply only to adults over 20. Children and teens are assessed using age- and sex-specific BMI-for-age percentile charts rather than fixed thresholds.

Q: Why do athletes often get an overweight BMI result?

Because muscle tissue is denser than fat tissue, a lean and muscular person can weigh enough for their height to fall in the overweight BMI category while carrying very low body fat. Body fat percentage is a more accurate individual measure in these cases.

Calculate your body mass index in metric or imperial units, see your WHO weight category, and find the weight range considered healthy for your height.

Written by the Calculator Index team · Categories: World Health Organization · Last reviewed August 2026

Your BMI

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How BMI is calculated

Metric: BMI = weight(kg) ÷ height(m)²
Imperial: BMI = 703 × weight(lb) ÷ height(in)²

Worked example

175 cm, 73 kg: 73 ÷ (1.75)² = 73 ÷ 3.0625 = BMI 23.8 — within the healthy range.

BMI categories (adults, WHO)

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obese

The BMI scale, visualized

The four adult BMI categories as defined by the World Health Organization.

What BMI does and doesn't tell you

BMI is a fast, free screening number that correlates with health risk across populations — but for an individual it has real blind spots. It can't tell muscle from fat, so athletes often read "overweight" while carrying very little fat, and it says nothing about where fat sits (visceral belly fat is the riskier kind). Treat BMI as one input, not a verdict. For a fuller picture, add the body fat calculator and consider waist circumference. To act on the number, the calorie calculator and TDEE calculator translate a weight goal into daily targets. Read our explainer: What does my BMI actually mean?

Common mistakes when interpreting BMI

Treating BMI as a body fat measurement. BMI only relates height to weight — it cannot distinguish muscle from fat, which is why a muscular athlete and a sedentary person of the same BMI can have very different body compositions.
Applying adult ranges to children or teens. Pediatric BMI uses age- and sex-specific percentile charts, not the fixed adult thresholds shown here — never apply this calculator's categories to anyone under 20.
Reacting to a single reading instead of a trend. Day-to-day weight fluctuates with hydration and food intake; BMI trends over weeks and months are far more meaningful than any single measurement.
Ignoring where fat is distributed. Two people with an identical BMI can carry very different health risk if one holds more weight around the abdomen — waist circumference adds information BMI can't capture on its own.

Frequently asked questions

How is BMI calculated?

Weight(kg) ÷ height(m)², or 703 × lb ÷ in². 175 cm / 73 kg ≈ 23.8.

What is a healthy BMI?

18.5–24.9 for adults. Under 18.5 underweight, 25–29.9 overweight, 30+ obese.

Is BMI accurate?

Good for populations, imperfect for individuals — it ignores muscle mass and fat distribution.

Does BMI apply to children?

No — this calculator's fixed categories are for adults over 20. Children and teens are assessed with age- and sex-specific BMI percentile charts, which a pediatrician uses differently than the adult thresholds shown here.

Why do athletes often get an "overweight" BMI result?

Because muscle is denser than fat, a lean, muscular person can weigh enough for their height to land in the "overweight" BMI range while carrying very low body fat. Body fat percentage is a more accurate gauge for highly muscular individuals.

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Medical disclaimer: BMI is a general screening estimate for adults over 20, not a diagnosis or medical advice. It does not apply to children, pregnant people, or athletes without additional context. Consult a healthcare provider about your individual health. Categories: World Health Organization. Last reviewed: July 2026.