What it does
This calculator works out your Body Mass Index (BMI) from your height and weight, and tells you which NHS weight band it falls in. Enter your figures in metric (kilograms and centimetres) or imperial (stone, pounds, feet and inches) and the result updates as you type. Nothing is sent to a server.
How it works
BMI compares your weight against your height by dividing your weight in kilograms by the square of your height in metres:
Squaring the height is what lets a single number work across different builds: a taller person can weigh more for the same BMI. Imperial entries are converted first — 1 stone is 6.35029 kg, 1 pound is 0.453592 kg, 1 foot is 30.48 cm and 1 inch is 2.54 cm — then the same division is applied.
Worked example
Take someone weighing 70 kg at 175 cm tall. First put the height in metres: 175 ÷ 100 = 1.75 m. Square it: 1.75² = 3.0625. Then divide the weight by that figure: 70 ÷ 3.0625 = 22.9. A BMI of 22.9 sits in the Healthy weight band.
NHS weight categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 and above | Obese |
The bands above follow the NHS BMI categories for adults.
Who uses it & limits
BMI is used by the NHS, GPs and public-health services as a quick first screen for weight-related health risk across a population. It is cheap to measure and needs only a set of scales and a tape measure, which is why it is the standard starting point before any more detailed assessment.
It is a screen, not a diagnosis, and it has real limits:
- Muscle mass: BMI cannot tell muscle from fat, so very muscular people (athletes, regular weight-trainers) can read as "overweight" while carrying little excess fat.
- Not for under-18s: children and teenagers are assessed against age-and-sex growth centiles, not these adult bands.
- Pregnancy: BMI is not a meaningful measure during pregnancy.
- Ethnicity: the NHS notes that the risk thresholds are lower for some Black, Asian and other minority ethnic groups, who may face increased health risk at a BMI below 25.
If your result concerns you, treat it as a prompt to speak to a GP rather than a verdict. See also how we build and verify our tools.
Frequently asked
What is a healthy BMI?
For most adults the NHS considers a BMI from 18.5 up to 24.9 a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.
How do I work out my BMI in stone and pounds?
Convert your weight to kilograms (multiply stone by 6.35029 and pounds by 0.453592, then add them) and your height to metres, then divide the weight by the height squared. Switching this calculator to imperial does all of that for you.
Is BMI accurate?
It is an accurate calculation but a blunt health measure. It reliably converts your height and weight into a single number, yet that number does not account for muscle, body-fat distribution, age or ethnicity, so it works best as a screening tool rather than a complete picture of health.
Why is BMI not used for children?
A growing child's healthy weight changes with age and sex, so a fixed adult band would be misleading. Children are plotted on growth centile charts instead.