BMI Calculator
Calculate your Body Mass Index instantly using height and weight.
How BMI Is Calculated, and What It Actually Measures
Body Mass Index is calculated as weight in kilograms divided by height in metres, squared: BMI = kg ÷ m². It was originally devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way to describe population-level weight patterns — not as an individual diagnostic tool, which is worth knowing given how it's used today.
Worked example: a person who is 170 cm (1.70 m) tall and weighs 65 kg has a BMI of 65 ÷ (1.70 × 1.70) = 65 ÷ 2.89 = 22.5, placing them in the "Normal Weight" band. If that same person's height were 165 cm instead, the identical 65 kg would produce a BMI of 23.9 — still "Normal," but noticeably higher, which is why BMI is so sensitive to small height differences and shouldn't be compared directly between people of different heights without the score itself.
Where the standard 18.5–24.9 "normal" range comes from: the World Health Organization set these cutoffs using data primarily from European populations. Several national health bodies — including those in India, Singapore, and other South and East Asian countries — use lower cutoffs (commonly 23 for "overweight" and 27.5 for "obese") because research has shown that health risks linked to excess weight tend to appear at lower BMI values in these populations. If you fall in the 23–25 range and your national guidelines use Asian cutoffs, you may be classified differently than the WHO scale above suggests.
What BMI doesn't measure: it can't distinguish muscle from fat, doesn't account for where fat is distributed on the body (which matters more for cardiovascular risk than total weight), and uses the same formula regardless of age or sex — even though body composition changes substantially between, say, a 25-year-old and a 70-year-old at the same height and weight. This is why a BMI reading is best treated as a starting point for a conversation with a healthcare provider, not a standalone verdict.
Groups Where BMI Is Known to Be Less Accurate
Athletes and highly muscular people
Muscle is denser than fat, so a strength athlete can register as "overweight" or "obese" by BMI while carrying very low body fat. Body fat percentage or waist circumference are better indicators here.
Older adults
Muscle mass naturally declines with age even if weight stays constant, which can make an older adult's BMI look "normal" while masking a real loss of lean muscle — sometimes called sarcopenic obesity.
Pregnant women
BMI calculations don't apply to pregnancy — weight gain guidelines during pregnancy are tracked separately by a healthcare provider using pre-pregnancy weight as the baseline.
Children and teenagers
For anyone under 20, BMI is interpreted using age- and sex-specific percentile charts rather than the fixed adult cutoffs used above, since healthy body composition changes constantly through growth.
Frequently Asked Questions
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