What is BMI (Body Mass Index)?
Body Mass Index (BMI) is a simple number that compares your weight with your height. Doctors, dietitians and the World Health Organization (WHO) use it as a quick screening tool to see whether an adult’s weight is likely to be healthy, too low or too high. Our free BMI calculator works for both women and men aged 18 and over, and lets you use kilograms and centimetres or pounds, feet and inches.
BMI does not measure body fat directly, but in large population studies a higher BMI is closely linked with a higher risk of type 2 diabetes, high blood pressure, heart disease, fatty liver and some cancers. That makes it a useful first check — as long as you understand its limits (explained below).
How the BMI calculator works
The BMI formula divides your weight in kilograms by your height in metres squared:
BMI = weight (kg) ÷ height (m)²
For example, a woman who weighs 68 kg and is 1.65 m tall has a BMI of 68 ÷ (1.65 × 1.65) = 25.0. If you use US units, the calculator converts pounds and feet/inches for you (the US formula is 703 × weight in lb ÷ height in inches²). The tool also shows your healthy weight range — the weights that give a BMI of 18.5 to 24.9 at your height — and your BMI Prime, which is your BMI divided by the upper healthy limit.
BMI chart for adults: low, normal and high values
These are the standard WHO BMI categories for adult women and men. The same cut-offs are used for both sexes and all adult ages.
| BMI (kg/m²) | Category | Level |
|---|---|---|
| Below 18.5 | Underweight | Low |
| 18.5 – 24.9 | Healthy (normal) weight | Normal |
| 25.0 – 29.9 | Overweight | High |
| 30.0 – 34.9 | Obesity class I | Very high |
| 35.0 – 39.9 | Obesity class II | Very high |
| 40 and above | Obesity class III | Very high |
Asian BMI cut-offs (South Asian, Arab and East Asian adults)
Research shows that people of South Asian and East Asian background develop diabetes and heart disease at a lower BMI than people of European background, often because more fat is stored around the waist and organs. The WHO therefore suggests lower action points, which many doctors in Pakistan, India and the Gulf also use. Choose “South Asian / East Asian” in the calculator to apply them:
| Asian BMI | Category | Level |
|---|---|---|
| Below 18.5 | Underweight | Low |
| 18.5 – 22.9 | Healthy weight | Normal |
| 23.0 – 27.4 | Overweight / increased risk | High |
| 27.5 and above | Obesity / high risk | Very high |
Benefits of knowing your BMI
- Quick health check: it takes seconds and needs only a scale and a tape measure.
- Early warning: a rising BMI is often the first sign that blood sugar, blood pressure or cholesterol may start to climb.
- A clear target: the healthy weight range gives you a realistic goal instead of a number from social media.
- Track progress: checking your BMI every month shows whether your diet and activity changes are working.
- Better conversations with your doctor: BMI is the starting point for many medical guidelines, including weight-loss and diabetes screening.
Health warnings and limitations of BMI
- BMI cannot tell fat from muscle. Athletes and very muscular people may show as “overweight” while having little body fat. Use our body fat calculator for a better picture.
- It ignores where fat is stored. Belly fat is more harmful than fat on the hips. Check your waist-to-height ratio as well.
- Not for children or pregnancy. Children and teens need BMI-for-age percentiles — use the child BMI calculator. During pregnancy, use the pregnancy weight gain calculator.
- Older adults naturally lose muscle, so a BMI slightly above 25 may be acceptable after 65. Ask your doctor.
- Underweight is a risk too. A BMI under 18.5 can mean poor nutrition, hormonal problems or an eating disorder. Seek help if you have lost weight without trying.
How to reach a healthy BMI
If your BMI is above the healthy range, aim to lose 0.25–0.5 kg a week. Work out your daily needs with the calorie calculator, fill half your plate with vegetables, choose wholegrains such as brown rice, oats or whole-wheat roti, and include protein at every meal to stay full. Swap sugary drinks for water and walk after meals. Our guides on sustainable weight loss and insulin resistance explain more.
If your BMI is below 18.5, add nourishing, energy-dense foods — nuts, dates, full-fat yoghurt, eggs, lentils and olive oil — and eat every 3–4 hours. Read our guide to healthy weight gain for underweight adults.
Frequently asked questions
What is a healthy BMI for adults?
For most adults a healthy BMI is 18.5 to 24.9. For people of South Asian, East Asian or Arab background, many doctors use a lower healthy range of 18.5 to 22.9 because health risks start to rise at a BMI of 23.
Is BMI different for women and men?
No. Adult BMI uses the same formula and the same categories for women and men. However, at the same BMI women usually carry more body fat than men, so pairing BMI with a waist measurement gives a fuller picture.
How do I calculate BMI by hand?
Divide your weight in kilograms by your height in metres, then divide the answer by your height in metres again. For example, 80 kg ÷ 1.75 m ÷ 1.75 m = 26.1. In US units, multiply your weight in pounds by 703 and divide by your height in inches squared.
Why does my BMI say overweight when I am muscular?
BMI only uses height and weight, so it cannot tell muscle from fat. Muscular people can have a high BMI and still be healthy. A body fat percentage or waist-to-height ratio check is more accurate for you.
Can I use this BMI calculator for my child?
No. Children and teenagers are still growing, so their BMI must be compared with growth charts for their age and sex. Use our child BMI calculator, which gives a BMI-for-age percentile for ages 2 to 19.
How often should I check my BMI?
Once a month is enough for most people. Weigh yourself at the same time of day, ideally in the morning before eating, because weight naturally changes by 1–2 kg during the day.
Sources
- World Health Organization. A healthy lifestyle – WHO recommendations (BMI classification).
- WHO Expert Consultation. Appropriate body-mass index for Asian populations. The Lancet 2004;363:157–163.
- U.S. Centers for Disease Control and Prevention (CDC). Adult BMI categories.
- NICE guideline NG246 (2025). Overweight and obesity management.