BMI Calculator
Work out your Body Mass Index from your height and weight, in metric or imperial, and see your NHS weight category — underweight, healthy, overweight or obese.
BMI calculator
NHS adult categories. Metric or imperial.
A guide only — not a diagnosis. For personalised advice see the NHS BMI tool.
How BMI is calculated
Body Mass Index (BMI) is a simple measure that relates your weight to your height. The formula is your weight in kilograms divided by your height in metres squared (kg ÷ m²). It gives a single number that places you in a weight category, which is why the NHS and GPs use it as a quick screening tool.
A BMI of 18.5 to 24.9 is in the healthy range for most adults. For example, someone 1.75 m tall weighing 70 kg has a BMI of 22.9 (70 ÷ 1.75²).
NHS BMI categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 39.9 | Obese |
| 40 and above | Severely obese |
Source: NHS — BMI calculator. Different thresholds apply to children and some ethnic groups.
The limits of BMI
BMI does not tell muscle from fat, so very muscular people can read as "overweight" while not carrying excess fat. The NHS uses lower thresholds (overweight from BMI 23, obese from 27.5) for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family origin, because health risks appear at a lower BMI. Waist size is a useful extra check alongside BMI.
Frequently asked questions
How is BMI calculated?
BMI is your weight in kilograms divided by your height in metres squared (kg ÷ m²). For example, 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) = 22.9. The calculator also accepts stones, pounds, feet and inches.
What is a healthy BMI?
For most adults the NHS considers a BMI of 18.5 to 24.9 healthy. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is in the obese range.
Is BMI accurate for everyone?
BMI is a useful screening tool but not perfect. It does not distinguish muscle from fat, so very muscular people may read high. The NHS also uses lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds.
Does this BMI calculator work in stones and pounds?
Yes. Switch to imperial units to enter your height in feet and inches and your weight in stones and pounds — the calculator converts to metric and gives the same BMI.
Should I rely on BMI alone?
No. BMI is a guide, not a diagnosis. Waist size and overall health matter too. Speak to your GP or use the NHS Better Health tools for personalised advice about a healthy weight.