BMI Calculator
Body Mass Index (BMI) is a quick screening number that relates your weight to your height. It does not measure body fat directly and does not account for muscle mass, frame size, age or sex, but it is widely used as a simple first indicator. Enter your height and weight in either metric or imperial units to get your BMI and see which of the four standard categories it falls into.
How to use the bmi calculator
- Choose metric (centimetres and kilograms) or imperial (inches and pounds).
- Enter your height and weight.
- Read your BMI value and its category below the result.
Formula
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)².
Categories: under 18.5 underweight · 18.5–24.9 normal · 25–29.9 overweight · 30 and above obese (World Health Organization adult cut-offs).
Worked examples
Metric example
A person 170 cm tall weighing 68 kg: 170 cm = 1.70 m, 1.70² = 2.89. 68 ÷ 2.89 ≈ 23.5, which is in the normal range.
Imperial example
A person 67 inches tall weighing 150 lb: 703 × 150 ÷ 67² = 703 × 150 ÷ 4,489 ≈ 23.5 — the same BMI as the metric example above, since both describe a similar build.
The full BMI category table, and what BMI does not measure
The four broad bands shown above break down further into WHO sub-categories, useful if your result is near a boundary:
| BMI range | Category |
|---|---|
| Below 16.0 | Severely underweight |
| 16.0 – 18.4 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
BMI's biggest blind spot is that it cannot see body composition — it treats a kilogram of muscle the same as a kilogram of fat. Two people with the same height and weight can have very different health profiles depending on how much of that weight is muscle versus fat, and where fat is distributed. Waist circumference or waist-to-hip ratio are often used alongside BMI because they capture abdominal fat, which is more strongly linked to metabolic risk than total weight alone. If your BMI is borderline or you have a muscular or very lean build, treat BMI as one data point among several, not a verdict.
Common mistakes to avoid
- Treating BMI as a direct measure of body fat — it cannot tell the difference between muscle and fat, so muscular or very athletic people are often classed as "overweight" despite low body fat.
- Using the general adult categories for children or teenagers, who need age- and sex-specific BMI-for-age charts instead.
- Mixing units — entering centimetres while imperial is selected (or vice versa) gives a meaningless result.
Frequently asked questions
What is a healthy BMI?
For most adults, 18.5 to 24.9 is classed as the normal/healthy weight range by the WHO, though this is a population-level guideline, not a personal target.
Is BMI accurate for athletes or very muscular people?
Not reliably. Muscle is denser than fat, so muscular people can have a high BMI while carrying little body fat. Waist circumference or body-fat measurement are better indicators in these cases.
Does BMI apply to children?
No, not this general-adult version — children and teens are assessed on BMI-for-age percentile charts because healthy body composition changes as they grow.
Is BMI a medical diagnosis?
No. It is a screening tool only. Talk to a doctor or dietitian for advice specific to your health.
Reviewed September 29, 2026 by the CalcSolver editorial team. Found a mistake? Tell us; see our editorial policy.