BMI Calculator
Quick answer
BMI is weight in kilograms divided by height in metres squared. The World Health Organization bands are: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese.
Body mass index
Choose your units, then enter height and weight.
BMI
22.9
kg/m² — compare with the bands below
Under 18.5 underweight · 18.5–24.9 normal · 25–29.9 overweight · 30+ obese. Lower thresholds apply for people of Asian descent; children use percentile charts.
Body mass index was designed in the nineteenth century to describe populations, not individuals. It is still used because it needs only two measurements and predicts population-level health risk reasonably well, which is a different claim from predicting any single person's health.
Use it as one number among several. Waist circumference, blood pressure and activity level tell you things BMI cannot, and they disagree with it often enough to matter.
The category boundaries, and why they are not universal
The WHO bands — under 18.5, 18.5 to 24.9, 25 to 29.9, and 30 or more — were derived largely from European-ancestry populations. Several health authorities use lower thresholds for people of South Asian, Chinese and other Asian descent, where cardiometabolic risk rises at a lower BMI.
Children and teenagers are not assessed on these bands at all. Paediatric BMI is read against age-and-sex percentile charts, because body composition changes rapidly through growth.
Where BMI is simply wrong
BMI cannot distinguish muscle from fat. A trained athlete can register as overweight or obese while carrying very little body fat, and an inactive person can sit in the normal band while carrying a great deal of it — sometimes called normal-weight obesity.
It also ignores where fat sits. Visceral fat around the organs carries far more metabolic risk than the same mass on hips and thighs, which is why waist-to-height ratio is often a better single screening number: keep your waist under half your height.
Measuring properly
Weigh yourself at the same time of day, ideally in the morning before eating, because body mass swings by one to two kilograms across a day from food and fluid alone.
Measure height without shoes, standing straight against a wall. A two-centimetre error in height changes BMI by roughly 0.6 units, which is enough to move a borderline reading across a category line.
Frequently asked questions
- What is a healthy BMI?
- The WHO defines 18.5 to 24.9 as the normal range for adults. Lower thresholds are used by several health authorities for people of Asian descent, and children are assessed on percentile charts instead.
- Is BMI accurate for athletes?
- No. Muscle is denser than fat, so a muscular person can score as overweight while carrying low body fat. Body fat percentage or waist measurement is more informative for trained individuals.
- What is the imperial BMI formula?
- Multiply weight in pounds by 703, then divide by height in inches squared. This calculator applies that conversion automatically when you choose imperial units.
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Updated 2026-09-20 · all calculators. Results are estimates for guidance, not professional advice.