BMI calculator
For adults aged 18 and over. Categories: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 and over obesity. Estimates only.
BMI is your weight in kilograms divided by your height in metres squared. For adults, the WHO calls a BMI of 18.5–24.9 a healthy weight, 25–29.9 overweight and 30 or more obesity. A 170 cm, 70 kg adult has a BMI of about 24.2.
How BMI is calculated
The formula is simple: BMI = weight (kg) ÷ height (m)². For someone 1.70 m tall who weighs 70 kg, that is 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 ≈ 24.2. The calculator above does the same maths and also turns the healthy range back into kilograms for your height.
What the BMI categories mean
| BMI | WHO category (adults) | Example, 170 cm |
|---|---|---|
| Under 18.5 | Underweight | under 53.5 kg |
| 18.5–24.9 | Healthy weight | 53.5–72 kg |
| 25–29.9 | Overweight | 72–86.7 kg |
| 30 and over | Obesity | 86.7 kg and over |
The same cut-offs apply to men and women. They are for adults aged 18 and over; children and teenagers use age- and sex-specific growth charts instead.
What BMI does not tell you
BMI cannot tell muscle from fat. A muscular athlete can score as "overweight" while carrying little fat, and an older adult can have a "healthy" BMI with low muscle mass. It also says nothing about where fat sits. Fat around the waist is linked with higher health risk, which is why the NHS suggests also checking that your waist is less than half your height. Our guide on BMI vs waist size explains how to use both.
Some groups, including many people of South Asian, Chinese and Black African or Caribbean background, have higher health risk at lower BMIs, so a lower threshold may be used for them. Pregnancy also changes what a healthy weight means. In these cases, and if your result is far outside the healthy range, talk to a doctor.
What to do with your number
- In the healthy range: keep an eye on your waist and habits rather than chasing a different number.
- Above the range: a loss of even 5–10% of body weight can improve health. Use the calorie calculator to find a sensible target and read how to set a realistic goal weight.
- Below the range: do not try to lose weight; if you have lost weight without trying, see a doctor.
Frequently asked questions
Is BMI accurate?
BMI is a useful screening tool for groups of people, but for one person it can mislead because it does not separate muscle from fat or show where fat is stored. Combine it with your waist measurement.
Is BMI different for men and women?
No. Adults use the same BMI cut-offs regardless of sex. Women tend to carry more body fat than men at the same BMI, which is one of BMI's known limits.
What is a healthy BMI for my age?
For adults the healthy range is 18.5–24.9 at every age. Children and teenagers need age- and sex-specific growth charts, not adult cut-offs.
How much weight should I lose if my BMI is high?
Losing 5–10% of your body weight already brings health benefits. A pace of about 0.5–1 kg a week is usually considered safe; your doctor can help set a target.
Sources
This article is general information, not medical advice. If you have a health condition, are pregnant or under 18, talk to a doctor or registered dietitian before changing your diet.