What is BMI?
Body mass index (BMI) is a simple screening number that compares your weight with your height. It is widely used by doctors and health organisations as a first check for whether weight may be affecting health, but it does not measure body fat directly.
In imperial units: BMI = 703 × weight (lb) ÷ height (in)². Example: 70 kg and 175 cm → 70 ÷ 1.75² = 22.9.
BMI categories for adults
| Category | WHO (standard) | 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 |
| Obesity | 30 and above | 27.5 and above |
Research shows that many South Asian and East Asian people have higher health risks at a lower BMI, so lower cut-offs are often used, including in India. Tick “Use Asian BMI cut-offs” to apply them.
Limitations of BMI
- It does not tell muscle from fat. Athletes and very muscular people can have a high BMI without excess fat.
- It ignores where fat is stored. Fat around the waist carries more health risk; waist measurement adds useful information.
- Age, sex and ethnicity matter. Older adults may have more fat at the same BMI, and risk differs between populations.
- It is not for children and teenagers. They need age- and sex-specific BMI percentiles.
BMI is a starting point, not a diagnosis. If you are worried about your weight or health, talk to a doctor or another qualified health professional.
Frequently asked questions
How is BMI calculated?
Divide your weight in kilograms by your height in metres squared. In imperial units, multiply your weight in pounds by 703 and divide by your height in inches squared.
What is a healthy BMI?
For most adults the World Health Organization classes a BMI from 18.5 to 24.9 as a healthy weight. With Asian cut-offs the healthy range is 18.5 to 22.9.
Is BMI accurate?
BMI is a useful, quick screening measure for groups of people, but it cannot distinguish muscle from fat or show where fat is stored, so it can be misleading for individuals.
Can children use this BMI calculator?
No. Children and teenagers need BMI-for-age percentiles, which compare them with others of the same age and sex. This calculator is for adults aged 18 and over.
This calculator is for general information only and is not medical advice.