BMI calculator
Body mass index from height and weight, with the WHO adult categories and an honest account of what the number does and does not tell you.
WHO adult BMI categories
| BMI | Category | What it means in practice |
|---|---|---|
| Below 16.0 | Severe thinness | A clinical concern; needs professional assessment |
| 16.0 – 18.4 | Underweight | Below the reference range for the population |
| 18.5 – 24.9 | Healthy weight range | The band most adults fall into |
| 25.0 – 29.9 | Overweight range | Elevated risk at population level |
| 30.0 – 34.9 | Obese class I | Population-level risk marker |
| 35.0 – 39.9 | Obese class II | Higher population-level risk |
| 40.0 and above | Obese class III | Highest population-level risk band |
The formula
BMI = weight (kg) ÷ height (m)². The imperial version multiplies by 703 instead: BMI = 703 × weight (lb) ÷ height (in)². Both give the same answer.
Because the height term is squared, BMI is very sensitive to measurement error in height. A 2 cm error at 175 cm changes the result by about 0.5 — enough to cross a category boundary. Measure height properly, with shoes off and heels against a wall.
Four things BMI gets wrong
- It cannot see body composition. Muscle and fat weigh the same per kilogram, so a lean athlete and a sedentary person can share a BMI while having completely different health profiles.
- It ignores where fat sits. Abdominal fat carries different risk from fat stored on the hips and thighs, and BMI cannot distinguish them.
- It uses one table for everyone. The WHO applies lower overweight thresholds to some Asian populations because the same BMI corresponds to different fat levels, and children need age-specific percentile curves rather than fixed cut-offs.
- It treats a range as a target. The healthy band spans more than six BMI points. Two people at opposite ends of it can have quite different compositions.
Better measures to pair with it
Waist circumference adds distribution information that BMI lacks. Waist-to-height ratio — keeping your waist under half your height — is a simple, well-supported screen. A body-fat measurement or a DEXA scan measures composition directly. None of these is a substitute for a clinician's assessment, and all of them are more informative than BMI alone for anyone who is muscular.
Frequently asked questions
How do I calculate BMI?
Divide your weight in kilograms by your height in metres squared. At 1.75 m and 70 kg: 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.9. That is the metric formula used worldwide.
What is a healthy BMI range?
The WHO adult categories put 18.5 to 24.9 in the healthy-weight range, below 18.5 as underweight, 25 to 29.9 as overweight, and 30 and above as obese. These are population screening thresholds, not individual diagnoses.
Is BMI accurate for athletes?
No, and it is a well-known limitation. Muscle is denser than fat, so muscular people frequently score in the overweight or obese bands while carrying very little fat. BMI was designed as a population measure, not a body-composition test.
Does BMI work the same for everyone?
No. The WHO also publishes lower overweight thresholds for some Asian populations, and BMI is not used in the same form for children, where age- and sex-specific percentile curves are used instead. Older adults and pregnant people are also poorly served by a single threshold table.
What is the difference between BMI and body fat percentage?
BMI is a ratio of weight to height and cannot distinguish muscle from fat or show where fat is stored. Body fat percentage measures composition directly. Waist circumference is a third, often more informative, measure because it captures abdominal fat distribution.
Should I use BMI to decide anything about my health?
Use it as one data point at most. Any health decision should rest on a conversation with a clinician who can look at the whole picture — composition, distribution, bloodwork, history and lifestyle.