What BMI tells you
Body mass index (BMI) compares your weight with your height. It was designed for screening large groups of adults, and it is still the measure the World Health Organization (WHO) uses to define underweight, overweight and obesity. A single number cannot describe your health, but it is a quick, free first check that has been studied in millions of people: on average, risks of type 2 diabetes, high blood pressure and heart disease rise as adult BMI moves above the normal range.
This calculator works for adults aged 18 and over. Children and teenagers need BMI-for-age percentiles, because their healthy BMI changes as they grow.
The formula
BMI = weight (kg) ÷ height (m)²
BMI = 703 × weight (lb) ÷ height (in)²
The imperial version gives the same answer: 703 is simply the conversion factor between pounds per square inch and kilograms per square metre.
WHO categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight (pre-obesity) |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
For people of Asian background, a WHO expert consultation found that health risks start to rise at lower BMI values, and suggested extra action points at 23 and 27.5. Several Asian countries use lower national cut-offs for this reason, so the calculator mentions it when your result falls between 23 and 25.
Worked examples
Metric
Weight 70 kg, height 175 cm. Height in metres is 1.75, and 1.75² = 3.0625. BMI = 70 ÷ 3.0625 = 22.9, which is in the normal range.
Imperial
Weight 160 lb, height 5 ft 7 in (67 inches). BMI = 703 × 160 ÷ 67² = 703 × 160 ÷ 4,489 = 25.1, just inside the overweight range.
Healthy weight range for a height
Turn the formula around: weight = BMI × height². For 175 cm, the normal range is 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg. The calculator shows this range for your own height, which is often more useful than the BMI number itself.
Why waist size matters too
Fat stored around the abdomen is linked to higher risk than fat stored elsewhere, even at the same BMI. WHO reference values for waist circumference in adults of European background are:
| Increased risk | Substantially increased risk | |
|---|---|---|
| Men | above 94 cm (37 in) | above 102 cm (40 in) |
| Women | above 80 cm (31.5 in) | above 88 cm (34.6 in) |
Many Asian guidelines use 90 cm for men and 80 cm for women. If you enter a waist measurement, the calculator compares it with the WHO values above.
Limits of BMI
- Muscle weighs more than fat. Athletes and people who do heavy strength training can have a "high" BMI while having little body fat.
- Older adults lose muscle with age, so a normal BMI can hide a high body-fat percentage.
- Pregnancy changes weight for reasons BMI was never meant to measure. Use your pre-pregnancy BMI when talking to your midwife or doctor.
- Height extremes: BMI slightly overstates fatness in very tall people and understates it in very short people.
Treat BMI as a screening number, not a diagnosis. If your result is outside the normal range, or you are worried about your weight, a doctor can look at the whole picture: waist size, blood pressure, blood sugar, cholesterol and family history.
Common mistakes
- Entering height in metres in the centimetre box (1.75 instead of 175). The calculator checks for this.
- Weighing at different times of day. For tracking, weigh at the same time, ideally in the morning before eating.
- Comparing a child's BMI with adult categories. Use a BMI-for-age chart for anyone under 18.
Frequently asked questions
What is a healthy BMI?
For adults, the WHO defines 18.5 to 24.9 as the normal range. For people of Asian background, health risks start to rise from about 23.
Is BMI different for men and women?
No. The formula and the adult categories are the same for both sexes. Waist-size limits do differ: 94 cm for men and 80 cm for women in the WHO reference values.
Can I use this calculator for my child?
No. Children and teenagers need BMI-for-age percentiles from a growth chart, because healthy BMI changes with age and differs between boys and girls.
Why does my BMI say overweight when I am muscular?
BMI cannot tell muscle from fat. If you train hard and your waist measurement is in the healthy range, a BMI slightly above 25 is usually not a concern, but a doctor can confirm this.
How much should I weigh for my height?
Multiply your height in metres squared by 18.5 and by 24.9. For 1.70 m that is 53.5 kg to 72.0 kg. The calculator does this for you.
Sources
- World Health Organization. A healthy lifestyle – WHO recommendations (BMI classification for adults).
- WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet, 2004; 363: 157–163.
- World Health Organization. Waist circumference and waist–hip ratio: report of a WHO expert consultation, 2008.
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.