How BMI is calculated
Body Mass Index is your weight in kilograms divided by the square of your height in metres. If you enter feet, inches, stones and pounds, the calculator converts them first (1 lb = 0.45359237 kg, 1 inch = 2.54 cm). Example: 70 kg and 1.75 m gives 70 ÷ 1.75² = 22.9.
NHS BMI ranges for adults
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 to 39.9: obese
- 40 or above: severely obese
The NHS says people from Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds have a higher risk of some health problems at a lower BMI. For them, a BMI of 23 or more may mean a higher risk, and 27.5 or more a high risk.
Limits of BMI
BMI can't tell fat from muscle, so athletes may show as "overweight" while being very healthy. It also isn't used on its own for children and teenagers (the NHS uses age- and sex-specific centiles), or during pregnancy. Waist size is a useful extra check: for most adults, a waist of more than half your height can mean more health risk.
Worked example (imperial)
5 ft 10 in (1.778 m) and 12 st (76.2 kg): 76.2 ÷ 1.778² = 24.1, a healthy weight. The healthy range for that height is about 58.5–78.7 kg (9 st 3 lb to 12 st 6 lb).
This tool is for information only, not medical advice. Nothing you enter leaves your device.
Frequently asked questions
What is a healthy BMI in the UK?
18.5 to 24.9 for most adults, according to the NHS.
How do I work out BMI in stones and pounds?
Choose ft/in and st/lb and enter your height and weight. The calculator converts to metric and works it out.
Is BMI accurate for muscular people?
Not always. BMI doesn’t separate muscle from fat, so very muscular people can get a high BMI.
How to use the BMI Calculator
- Enter your numbers.
- Choose the operation if needed.
- Read the answer and the working.
- Use the table below to check your understanding.
Why use this BMI Calculator
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Nothing you enter is uploaded or stored.
Instant results
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Works on phones, tablets and desktops.
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Explained
Formulas, tables and sources on the page.
BMI Calculator: chart and reference data
Source: NHS – Obesity
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 39.9 | Obese |
| 40 or above | Severely obese |
BMI Calculator compared with the alternatives
| Option | Shows working | Cost | Availability |
|---|---|---|---|
| This calculator | Yes | Free | Any device |
| Scientific calculator | No | £10–£20 | Needs the device |
| Spreadsheet | With formulas | Licence or account | Needs setup |
Who uses it in the UK
- GCSE and A-level revision
- University and coursework grades
- Electronics and science labs
- Sports stats and health checks
More questions
Does it show the working?
Where it helps, yes – the result section explains the steps so you can check your homework or report.
Can I use it for GCSE or A-level revision?
Yes. It is a good way to check answers, but make sure you can do the method by hand, as calculators are not allowed in every exam paper.
How accurate is it?
It uses standard double-precision arithmetic, accurate to about 15 significant figures. Results are rounded only for display.
Is my data stored?
No. Everything is calculated in your browser.
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Related free tools
Sources
- NHS – BMI healthy weight calculator
- NHS – Obesity: diagnosis
- NICE – Overweight and obesity management (NG246)
Note: This free tool runs entirely in your browser. No data is sent or stored. Results are for information only.