The short answer
Body mass index (BMI) = weight in kilograms ÷ (height in metres × height in metres)
For someone who weighs 70 kg and is 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9.
A BMI between 18.5 and 24.9 is classed as a healthy weight for most adults. BMI is a quick screening tool: useful for spotting possible risk, but not a diagnosis of health on its own.
How to calculate BMI
Metric:
- Square your height in metres (multiply it by itself).
- Divide your weight in kilograms by that number.
Imperial:
BMI = 703 × weight in pounds ÷ (height in inches × height in inches)
For 154 lb and 5 ft 9 in (69 inches): 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7.
Many health services offer an official online BMI calculator if you prefer not to do the maths.
What the numbers mean
The World Health Organization's categories for adults:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class I |
| 35 to 39.9 | Obesity class II |
| 40 or above | Obesity class III |
These apply to adults aged 18 and over, and are not used during pregnancy.
Lower cut-offs for some ethnic groups
People of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds tend to develop type 2 diabetes and heart disease at a lower BMI than white European populations.
A WHO expert consultation suggested additional public health action points of 23 and 27.5 for Asian populations. Some national guidelines, such as the UK's NICE guidance, use a BMI of 23 to indicate increased risk and 27.5 to indicate high risk for these groups. Several Asian countries use their own lower cut-offs too.
What BMI does not tell you
BMI is simple and cheap, which is why it is widely used. But it has real limits:
- It does not measure body fat directly. Muscle weighs more than fat for its size, so athletes can register as "overweight".
- It does not show where fat is stored. Fat around the waist and organs (visceral fat) carries more health risk than fat on the hips and thighs.
- It can miss risk in people with a "normal" BMI who carry excess weight around the middle.
- It changes meaning with age. Older adults lose muscle, so the same BMI can mean more body fat.
That is why it is best used alongside other measures.
Waist measurement: a useful second check
A simple guide used by several health authorities: keep your waist to less than half your height (a waist-to-height ratio below 0.5).
- Height 170 cm: aim for a waist under 85 cm.
- Height 5 ft 6 in (66 in): aim for under 33 inches.
How to measure: find the point halfway between your lowest rib and the top of your hip bone, wrap a tape measure around it, breathe out normally, and read the number. Do not pull the tape tight.
What to do with your result
- Healthy range: keep up regular activity and a balanced diet.
- Above the healthy range: even a modest loss, of around 5 to 10 percent of body weight, can lower blood pressure, blood sugar and cholesterol. Small, sustainable changes work better than crash diets. Our guides to daily calorie needs and the benefits of walking are good starting points.
- Below the healthy range: talk to a doctor, particularly if you have lost weight without trying.
- Any result, with other risk factors (high blood pressure, family history of diabetes, a large waist): ask for a health check. Blood pressure and blood sugar tests give a much fuller picture. See how to lower blood pressure naturally.
Children and teenagers
Children's BMI is calculated the same way but interpreted differently: it is compared with growth charts for age and sex to give a percentile. Use a calculator designed for children, or ask a doctor or school nurse.
Key takeaways
- BMI = kg ÷ m². A healthy adult range is 18.5 to 24.9.
- Risk starts at lower BMIs for many Asian and other ethnic groups, often from 23.
- BMI cannot distinguish muscle from fat or show where fat sits.
- Check your waist too: aim for less than half your height.
This article is general information, not medical advice. Speak to a doctor or other health professional about your own weight and health.