What BMI is
Body mass index (BMI) compares your weight with your height. Doctors and public health bodies use it as a quick first check of whether someone's weight might raise their risk of health problems like type 2 diabetes, heart disease and high blood pressure. It's a screening number, not a diagnosis.
The BMI formula
BMI = weight (kg) ÷ height (m)²
BMI = 703 × weight (lb) ÷ height (in)²
Metric example: 68 kg and 170 cm (1.70 m).
1.70 × 1.70 = 2.89. Then 68 ÷ 2.89 = 23.5.
Imperial example: 150 lb and 5 ft 7 in (67 in).
67 × 67 = 4,489. Then 703 × 150 ÷ 4,489 = 23.5.
BMI categories for adults
These are the World Health Organization categories for adults aged 20 and over, for both men and women:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
Some health services use lower cut-offs for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, because health risks can rise at a lower BMI. The UK's NHS, for example, treats 23 and above as a raised risk for these groups. Your doctor can tell you which thresholds apply to you.
What BMI doesn't tell you
- Muscle vs. fat. Muscle is denser than fat, so athletes and very muscular people can have a high BMI while carrying little fat.
- Where fat is stored. Fat around the waist is linked to higher health risk than fat on the hips. A waist measurement adds useful information: as a rough guide, try to keep your waist under half your height.
- Age and body changes. Older adults naturally lose muscle, so the same BMI can mean more body fat.
- Pregnancy. BMI isn't meaningful during pregnancy.
BMI for children and teens
Don't use the adult categories above for anyone under 20. Children's BMI is compared with growth charts for their exact age and sex and given as a percentile. Your pediatrician or a national health service's child BMI tool is the right source.
If your result is outside the healthy range
A single number is only a starting point. A doctor can look at your BMI together with blood pressure, blood tests, family history and lifestyle. If you want to change your weight, our calorie calculator estimates how much energy your body uses each day, which is a useful baseline to discuss with a professional.
This calculator is for general information and isn't medical advice. Talk to a qualified health professional about your weight and health.
Frequently asked questions
Is BMI different for men and women?
The adult formula and categories are the same for both. Women usually carry more body fat than men at the same BMI, which is one reason BMI is only a rough guide.
How is the healthy weight range worked out?
It uses the BMI formula in reverse: the weights that give a BMI of 18.5 and 24.9 at your height.
Do I need to convert units first?
No. Switch between kg/cm and lb/ft at the top of the calculator.
Last reviewed November 2026.