Body mass index (BMI) compares your weight with your height to give a rough idea of whether you’re a healthy weight. Enter your height and weight in metric or imperial units to see your BMI, your NHS weight category and the healthy weight range for your height.
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How to use the BMI calculator
- Choose metric (cm and kg) or imperial (feet, inches, stone and pounds).
- Enter your height and weight.
- Tick the background box if it applies to you. People from some ethnic backgrounds have a higher health risk at a lower BMI.
- Read your BMI, category and healthy weight range.
This calculator is for adults aged 18 and over. Children’s BMI is compared with others of the same age and sex, so use the NHS children’s BMI calculator instead.
How BMI is calculated
BMI = weight in kilograms ÷ (height in metres)².
For example, someone who is 1.75m tall and weighs 72kg has a BMI of 72 ÷ (1.75 × 1.75) = 23.5.
BMI categories for adults
| Category | Most adults | South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background |
|---|---|---|
| Underweight | below 18.5 | below 18.5 |
| Healthy weight | 18.5 to 24.9 | 18.5 to 22.9 |
| Overweight | 25 to 29.9 | 23 to 27.4 |
| Obese | 30 to 39.9 | 27.5 or above |
| Severely obese | 40 or above | 40 or above |
The lower thresholds come from NICE guidance, because type 2 diabetes and heart disease risk rises at a lower BMI in these groups.
What BMI doesn’t tell you
- It doesn’t measure body fat, so muscular people can show as overweight.
- It can understate body fat in older adults, who tend to have less muscle.
- It doesn’t show where fat is stored. Waist size matters too: NICE guidance suggests keeping your waist to less than half your height.
- It isn’t used during pregnancy.
If your result worries you, or you’re thinking about losing or gaining weight, speak to your GP or a registered dietitian. Aim for gradual change; very low-calorie diets should only be followed with medical support.
Frequently asked questions
What is a healthy BMI?
For most adults, 18.5 to 24.9. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, 18.5 to 22.9.
Is BMI accurate?
It’s a quick screening measure, not a diagnosis. It works well across large groups but can be misleading for very muscular people, older adults and some ethnic groups.
Can I use this BMI calculator for children?
No. Children’s BMI is compared with growth charts for their age and sex. Use the NHS BMI calculator for children.
What's a healthy waist size?
NICE guidance suggests keeping your waist to less than half your height, measured around your middle halfway between your lowest rib and your hip bone.
Is my data stored?
No. Your height and weight stay in your browser and aren’t sent or stored.
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