BMI calculator
Enter your height and weight in whichever units you actually think in — including stones and pounds. Adults only.
Why the thresholds change with ethnicity
Most BMI calculators apply one set of cut-offs to everybody. That is not what UK clinical guidance says. NICE recommends lower thresholds for adults of Asian, Middle Eastern, Black African or African-Caribbean family background, because the risk of type 2 diabetes and heart disease rises at a lower BMI in these groups.
| Category | General | Lower-threshold groups |
|---|---|---|
| Underweight | below 18.5 | below 18.5 |
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25 – 29.9 | 23 – 27.4 |
| Obese | 30 and above | 27.5 and above |
The difference is not small. A BMI of 24 is squarely in the healthy range on the general scale and above it on the lower one. If a calculator ignores this, it gives a false reassurance to the people the guidance was written for.
How BMI is worked out
The formula is weight in kilograms divided by height in metres squared:
BMI = kg ÷ (m × m)
So someone 1.70 m tall weighing 70 kg has a BMI of 70 ÷ (1.70 × 1.70) = 24.2.
Imperial inputs are converted first — one pound is 0.45359237 kg, one inch is 2.54 cm, one stone is 14 pounds — so the arithmetic is identical whichever units you type in.
The obesity classes
Above 30, clinical guidance divides obesity into three classes. They matter because treatment thresholds differ, not because the labels are useful in themselves:
- Class 1: 30 to 34.9
- Class 2: 35 to 39.9
- Class 3: 40 and above
Common questions
Is BMI useful at all?
At population level, yes — it tracks health risk well across large groups, which is why it is still used. For an individual it is a rough first look. Waist measurement is often more informative about health risk than BMI alone, because where fat sits matters.
Why does my BMI differ between websites?
Usually rounding, or unit conversion done at different points. A difference of 0.1 or 0.2 is normal. A difference of more than about 0.5 means one of them has a bug — check the arithmetic against the formula above.
Does BMI work for children?
Not this calculator. Children are assessed against age-and-sex percentile charts rather than fixed cut-offs, because healthy body composition changes as they grow. Ask a GP or use an NHS child BMI tool.
Is my height and weight sent anywhere?
No. The arithmetic runs in your browser and this page makes no network requests at all, so there is nothing to log. That matters more here than on most pages, because height, weight and ethnicity are health data. Turn your wi-fi off once the page has loaded and the calculator still works — which is the test you can actually run.