How the BMI calculator works
Body mass index (BMI) compares your weight with your height to give a rough idea of whether you're a healthy weight. It's the measure the NHS and GPs use as a first check for adults. The formula is:
BMI = weight in kg ÷ (height in metres × height in metres)
If you enter stones and pounds or feet and inches, the calculator converts them first (1 stone = 14lb, 1lb = 0.4536kg, 1 inch = 2.54cm). The result is rounded to one decimal place and compared with the adult ranges below.
NHS BMI ranges 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 to 37.4 |
| Severely obese | 40 or more | 37.5 or more |
The lower thresholds come from NHS and NICE guidance, because people from these backgrounds have a higher risk of type 2 diabetes, heart disease and stroke at a lower BMI. NICE suggests reducing the higher obesity thresholds by 2.5 for these groups, which is where the 37.5 figure comes from.
Worked example
Someone who is 175cm tall and weighs 70kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. That's in the healthy weight range for everyone. The healthy range at 175cm is 56.7kg to 76.3kg (about 8st 13lb to 12st 0lb), or up to 70.1kg using the lower thresholds.
At the same height, a weight of 90kg gives a BMI of 29.4, which counts as overweight for most adults but obese for someone of South Asian background.
In imperial: 6ft (1.829m) and 15 stone (95.3kg) gives a BMI of 28.5.
What BMI doesn't tell you
BMI is a quick screening tool, not a diagnosis. It has some well-known limits:
- It can't tell muscle from fat. Rugby players and bodybuilders often have an "overweight" BMI with very little body fat.
- It doesn't show where fat is stored. Fat around the waist carries more health risk. The NHS suggests keeping your waist measurement to less than half your height, so a 175cm person should aim for under 87.5cm (about 34 inches).
- It's not for children or teenagers. For anyone aged 2 to 17, BMI is compared with growth charts for their age and sex. Use the NHS child BMI calculator instead.
- It's not for pregnancy. Weight gain in pregnancy is expected; your midwife will use your BMI from your booking appointment.
- Older adults naturally lose muscle, so a BMI in the healthy range can hide low muscle mass.
What to do next
If your BMI is in the overweight or obese range, losing even 5% of your body weight can lower your risk of type 2 diabetes and high blood pressure. The NHS suggests aiming to lose 0.5kg to 1kg (1 to 2lb) a week. Our calorie calculator estimates how much you need to eat to maintain or lose weight gently, and the alcohol units calculator shows how many calories might be coming from drinks.
If your BMI is under 18.5, or you're worried about your weight for any reason, speak to your GP. Being underweight can be a sign of an underlying health problem and can affect your bones and immune system.
Frequently asked questions
What is a healthy BMI for adults?
For most adults in the UK, a BMI of 18.5 to 24.9 is a healthy weight. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, the NHS treats 23 or more as overweight.
How do I work out my BMI in stones and pounds?
Convert your weight to kg (stones × 6.35 plus pounds × 0.4536) and your height to metres (inches × 0.0254), then divide the weight by the height squared. Or just choose Imperial in the calculator above.
Is BMI accurate if I'm muscular?
Not really. BMI only looks at height and weight, so people with a lot of muscle can be classed as overweight or obese. A waist measurement under half your height is a useful extra check.
What BMI is classed as obese?
A BMI of 30 or more is obese for most adults, and 40 or more is severely obese. The threshold is 27.5 for people from ethnic groups with a higher risk at lower BMIs.
Can I use this BMI calculator for my child?
No. Children's BMI has to be compared with growth charts for their age and sex, so use the NHS BMI calculator for children and teenagers, or ask your GP or health visitor.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.