BMI formula
Worked example: a person 1.70 m tall who weighs 68 kg. Height squared is 1.70 × 1.70 = 2.89. BMI = 68 ÷ 2.89 = 23.5. On the WHO scale that is a normal weight. On the Asian scale it is overweight, because the Asian cut-off for overweight is 23.
In imperial units, 5 ft 7 in is 67 inches and 150 lb gives 703 × 150 ÷ 67² = 703 × 150 ÷ 4,489 = 23.5. The factor 703 converts pounds and inches to kilograms and metres.
WHO BMI categories for adults
The World Health Organization (WHO) uses these ranges for adults aged 18 and over, for both men and women:
| Category | BMI (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5–24.9 |
| Pre-obese (overweight) | 25.0–29.9 |
| Obesity class I | 30.0–34.9 |
| Obesity class II | 35.0–39.9 |
| Obesity class III | 40.0 and above |
The calculator rounds your BMI to one decimal place and then applies the table, so the category always matches the number you see.
BMI cut-offs for Asian and Indian adults
Many people of Asian origin have more body fat, and a higher risk of type 2 diabetes and heart disease, at a given BMI than people of European origin. A WHO expert consultation in 2004 suggested extra action points for Asian populations. India's consensus guidelines for obesity use lower cut-offs:
| Category | WHO (international) | Asian / Indian |
|---|---|---|
| Normal weight | 18.5–24.9 | 18.5–22.9 |
| Overweight | 25.0–29.9 | 23.0–24.9 |
| Obesity | 30.0 and above | 25.0 and above |
Tick Use Asian cut-offs to apply these ranges. The healthy weight range then uses BMI 18.5–22.9.
Healthy weight for your height
Rearranging the formula gives the weight for any BMI: weight = BMI × height². At 1.70 m, a BMI of 18.5 is 53.5 kg and a BMI of 24.9 is 72.0 kg, so the WHO healthy range is about 53–72 kg. With Asian cut-offs the upper limit is 22.9 × 2.89 = 66.2 kg.
When BMI is not the right measure
- Children and teenagers. Under 18, body fat changes with age and sex, so doctors use BMI-for-age percentiles from growth charts, not the adult table.
- Pregnancy. Weight gain in pregnancy is expected. Use your pre-pregnancy BMI and your midwife's or doctor's advice.
- Very muscular people. Muscle is dense. Athletes and people who lift weights can have a high BMI with little body fat.
- Older adults. Muscle mass falls with age, so a normal BMI can hide low muscle and high fat.
BMI does not show where fat is stored. Fat around the waist carries more health risk. Measuring your waist alongside BMI gives a fuller picture.
Tips for measuring
- Weigh yourself in the morning, after using the toilet and before eating, in light clothes.
- Measure height without shoes, standing straight against a wall.
- Use the same scale each time. Track the trend over weeks, not day-to-day swings.
To plan how much to eat for a target weight, use the calorie calculator. To switch between kilograms and pounds or centimetres and feet, use the unit converter.
This calculator gives general information only. It is not medical advice. Talk to a doctor or a registered dietitian about your weight and health.
Frequently asked questions
What is a healthy BMI?
How do I calculate BMI by hand?
Is BMI different for men and women?
Can children use this BMI calculator?
Why does India use lower BMI cut-offs?
Is BMI accurate for athletes?
Last updated . How we check our tools. Sources: WHO fact sheet: obesity and overweight; WHO expert consultation, The Lancet 2004 (BMI for Asian populations).