BMI calculator for adults
Work out your body mass index (BMI) in kilograms and centimetres, or stones, pounds, feet and inches. Add your waist size for a better picture of health risk.
For adults 18 and over. BMI is a quick screening number, not a diagnosis, and it can’t tell muscle from fat. It isn’t used for children or during pregnancy. Nothing you enter leaves this device.
What your BMI means
BMI = weight in kilograms ÷ (height in metres)². For example, 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) = 22.9.
- Under 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 to 34.9: obesity class I
- 35 to 39.9: obesity class II
- 40 or more: obesity class III
Lower cut-offs for some backgrounds
- People of Asian background tend to carry more fat around the organs, and the risk of type 2 diabetes and heart disease rises at a lower BMI. The WHO suggests action points of 23 (increased risk) and 27.5 (high risk) for many Asian populations.
- The UK’s NICE uses 27.5 as the obesity threshold for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.
- The Australian Government notes that a healthy BMI may be higher for people of Polynesian background, and for older people and athletes with a lot of muscle.
Waist size matters too
- Fat around the middle carries more health risk than fat elsewhere. Measure halfway between your lowest rib and the top of your hip bone, breathing out normally.
- Women: 80 cm or more = increased risk; 88 cm or more = greatly increased risk.
- Men: 94 cm or more = increased risk; 102 cm or more = greatly increased risk.
- A simple rule: keep your waist to less than half your height.
- Waist cut-offs don’t apply in pregnancy or to children.
Limits of BMI
- It doesn’t separate muscle from fat, so very muscular people can score “overweight” while being healthy.
- It doesn’t show where fat sits, which is why waist size helps.
- It isn’t used for children and teenagers (growth charts are used instead) or in pregnancy.
- Your health is more than a number: blood pressure, blood sugar, cholesterol, fitness, sleep and how you feel all count.
When to see a doctor
- Your BMI is under 18.5, or 25 or more (23 or more for Asian background), or your waist is above the cut-offs.
- You’ve lost weight without trying, or gained weight quickly without a clear reason.
- Weight is affecting your joints, sleep (snoring, daytime sleepiness), mood or fertility.
- You’re thinking about weight-loss medicines or surgery.
Common questions
What is a healthy BMI for adults?
For most adults, 18.5 to 24.9. For people of Asian background, health risks start to rise from a BMI of 23. A healthy BMI may be a little higher for older adults and people with a lot of muscle.
How do I calculate BMI?
Divide your weight in kilograms by your height in metres squared. For example, 70 kg and 1.75 m: 70 ÷ (1.75 × 1.75) = 22.9. This calculator also converts stones, pounds, feet and inches.
Is BMI accurate?
It’s a useful quick screen for groups of adults but an imperfect measure for individuals, because it can’t tell muscle from fat or show where fat is stored. Waist size adds useful information.
What waist size is healthy?
In Australia, risk increases from 80 cm for women and 94 cm for men, and is greatly increased from 88 cm for women and 102 cm for men. Keeping your waist under half your height is another good guide.
Can I use this BMI calculator for my child?
No. For children and teenagers, BMI is compared with growth charts for their age and sex. Your GP or child health nurse can plot it for you.
Sources
- Australian Government Department of Health: BMI and waist measurement. adult BMI categories; waist cut-offs; different ranges for some groups; not for children
- WHO Expert Consultation. Appropriate body-mass index for Asian populations. Lancet 2004. action points of 23 and 27.5
- NICE: lower BMI thresholds for some ethnic groups. 27.5 for South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds
- WHO growth reference for 5 to 19 years. BMI for age in children and teens