What is BMI?
Body Mass Index (BMI) is a simple number that compares your weight with your height. Doctors and health workers use it as a quick first check to see whether an adult's weight is in a healthy range. It does not measure body fat directly, but for most people it gives a reasonable idea of whether they may be underweight or carrying extra weight.
How is BMI calculated?
The formula is: BMI = weight (kg) ÷ height (m)². For example, if you weigh 68 kg and are 1.70 m tall, your BMI is 68 ÷ (1.70 × 1.70) = 68 ÷ 2.89 = 23.5.
If you use pounds and inches, the formula is BMI = 703 × weight (lb) ÷ height (in)². This calculator converts everything for you, so you can use whichever units you know.
BMI categories
| Category | Standard (WHO) | Asian cut-offs |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25 – 29.9 | 23 – 27.4 |
| Obese | 30 and above | 27.5 and above |
Why are there Asian cut-offs? Research has found that many people of South Asian, East Asian and South-East Asian origin face a higher risk of type 2 diabetes and heart disease at a lower BMI than people of European origin. For this reason, a World Health Organization expert group suggested lower "action points" of 23 and 27.5 for Asian populations. Indian clinical guidelines go further and treat a BMI of 23–24.9 as overweight and 25 or more as obese. If you are of Asian origin, tick the checkbox to see your result on the Asian scale.
Limits of BMI
- Muscle vs fat: athletes and bodybuilders can have a "high" BMI because muscle is heavy, even with little body fat.
- Where fat is stored matters: fat around the stomach is linked to more health risk. Measuring your waist is a useful extra check.
- Children and teenagers need age- and sex-specific BMI charts, so this calculator is for adults only.
- Pregnancy and older age change body composition, so BMI is less useful then.
BMI is a good starting point for a conversation with your doctor, not a final answer about your health.
Frequently asked questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. On the Asian scale, the healthy range is 18.5 to 22.9.
How do I convert feet to centimetres?
Multiply feet by 30.48 and inches by 2.54, then add them. For example, 5 feet 7 inches is 152.4 + 17.8 = 170.2 cm. The calculator does this for you when you choose lb and feet.
Is BMI accurate for women?
The same BMI formula and categories are used for adult men and women. Women naturally have a little more body fat at the same BMI, which is taken into account in the healthy range.
Can I use this for my child?
No. Children and teenagers under 18 need BMI-for-age growth charts. Ask your paediatrician to check your child's growth.