BMI Calculator
Free BMI calculator — type in weight and height to get your body mass index and the WHO weight category. Runs in your browser, no sign-up. Metric units (kg, cm).
Type your weight in kilograms and height in centimetres. The calculator returns your BMI and the WHO category it falls in.
The formula
BMI = weight (kg) ÷ height (m)²
A 70 kg adult who is 1.75 m tall: 70 ÷ (1.75 × 1.75) = 22.9 — comfortably
in the normal-weight band.
WHO adult categories
| BMI | Category |
|---|---|
| under 18.5 | Underweight |
| 18.5–24.9 | Normal weight |
| 25.0–29.9 | Overweight |
| 30 and above | Obese |
The boundaries are statistical, not biological. Don’t read too much into a result that’s one or two tenths over a line.
What BMI doesn’t tell you
BMI is a screening tool, not a diagnosis. It is a function of two numbers (weight and height) and cannot distinguish lean mass from fat mass. So:
- A 100 kg, 1.85 m powerlifter at 12% body fat will register as “overweight” with a BMI of 29.2. They’re not.
- A 55 kg, 1.65 m sedentary adult at 30% body fat will register as “normal” with a BMI of 20.2. They may still be at metabolic risk.
For an actual health picture, body fat percentage and waist circumference matter more than BMI. BMI is useful at the population level (a straightforward, repeatable proxy for weight status) — less useful for any individual on the edge of a category.
A note on units
This calculator uses metric (kg, cm). For imperial: BMI = weight(lb) ÷ height(in)² × 703. Or convert: 1 lb ≈ 0.454 kg, 1 in = 2.54 cm.
Worked examples
-
70 kg at 175 cm — normal weight
BMI: 22.9 (normal weight).
-
100 kg at 175 cm — obese
BMI: 32.7 (obese).
Frequently asked questions
What is BMI and how is it calculated?
Body Mass Index is weight in kilograms divided by height in metres squared: BMI = weight ÷ height². A 70 kg adult who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. It's a single number that gives a rough sense of whether body weight is appropriate for height.
What do the categories mean?
WHO standard adult categories: under 18.5 = underweight, 18.5–24.9 = normal weight, 25.0–29.9 = overweight, 30 and above = obese. The category boundaries are population averages; individuals can sit on either side of a boundary without that translating into a health problem.
Is BMI a reliable measure of health?
It's a screening tool, not a diagnosis. It can't distinguish lean mass from fat mass, so very muscular people (athletes, weight-trained individuals) often score "overweight" without being unhealthy, and people with low muscle mass can score "normal" while still having unhealthy body composition. Waist circumference and waist-to-height ratio capture body fat distribution better.
Does BMI work for children, elderly, or pregnant people?
Adults only. For children and teens, BMI is interpreted against age- and sex-specific percentile charts, not the adult categories. Older adults often have lower lean mass at the same BMI, so a slightly higher BMI may actually be protective. Pregnant people use pre-pregnancy BMI as a reference for weight gain ranges.
What about ethnic differences?
Some health bodies use different thresholds for South and East Asian populations (overweight starting at 23, obese at 27.5) because cardiovascular risk rises at lower BMI in those groups. This calculator uses the WHO global standard; check local guidelines if those apply to you.