BMI Calculator
Calculate body mass index from weight and height, in metric or imperial units.
BMI is a screening measure, not a diagnosis — see the categories below.
Body mass index (BMI) divides weight in kilograms by height in metres squared. A person who weighs 70 kg and stands 1.75 m tall has a BMI of 70 ÷ 1.75² ≈ 22.9, which falls in the WHO “normal” range of 18.5–24.9. BMI screens for weight categories; it does not diagnose health on its own.
What BMI is
Body mass index is a single number that relates a person’s weight to their height. It was designed as a quick population-level screening measure: dividing mass by height squared roughly normalises for body size, so people of different heights can be compared on one scale. The result places someone into a broad weight category, but it says nothing directly about body composition, fat distribution, or fitness.
Because the metric formula squares height in metres, a small change in height has a large effect on the denominator. The imperial version reaches the same number using pounds and inches with a conversion constant of 703.
Imperial: BMI = 703 × weight(lb) ÷ height(in)²
Worked example: 70 kg, 1.75 m
Square the height, then divide the weight by it:
- 1 Take your weight and height. Use kilograms and metres for metric, or pounds and inches for imperial.
- 2 Square the height. 1.75 m × 1.75 m = 3.0625 m².
- 3 Divide weight by squared height. 70 ÷ 3.0625 ≈ 22.86.
- 4 Round and read the category. BMI ≈ 22.9 sits inside the 18.5–24.9 normal range.
WHO BMI categories for adults
Standard World Health Organization cut-offs for adults aged 20 and over.
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese | 30.0 and above |
How to read your result — and its limits
BMI is a population screening tool, not a diagnosis. A category tells you how your weight-to-height ratio compares with statistical reference ranges; it does not measure body fat directly or account for where fat is stored. Treat it as one starting signal, not a verdict on health.
The number can mislead for several groups. Muscular athletes often score as “overweight” or “obese” because muscle is denser than fat, even with low body fat. In older adults, muscle loss can hide excess fat behind a “normal” BMI. The standard cut-offs were derived largely from European populations, and some health bodies use lower thresholds for people of South and East Asian descent, where cardiometabolic risk rises at a lower BMI. BMI is also not validated the same way for children and pregnant people.
Not medical advice. This calculator is for general information and education only. For an assessment of your weight and health, talk to a qualified healthcare professional.