♥Free calculator

BMI calculator

Work out your body mass index (BMI) in metric or imperial units, see which category it falls in, and find the healthy weight range for your height.

For adults aged 18 and over. For children and teenagers, BMI must be compared with age-specific growth charts.

Your BMI
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UnderweightHealthy weightOverweightObese
Healthy weight for your height
BMI Prime

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How to use the BMI calculator

Choose metric (kilograms and centimetres) or imperial (stone and pounds, feet and inches), then enter your weight and height. Your BMI, category and healthy weight range appear right away.

The BMI formula

BMI = weight (kg) ÷ height (m)²

In imperial units: BMI = 703 × weight (lb) ÷ height (in)². The calculator converts imperial units for you.

BMI categories for adults

BMICategory
Below 18.5Underweight
18.5 – 24.9Healthy weight
25 – 29.9Overweight
30 and aboveObese

These are the World Health Organization categories for adults. Some health services use lower thresholds for people of South Asian, Chinese, Black African or African-Caribbean background, for whom health risks can start at a lower BMI (for example, 23 instead of 25).

Worked example

Someone who weighs 72 kg and is 178 cm tall:

BMI = 72 ÷ 1.78² = 72 ÷ 3.1684 ≈ 22.7, which is in the healthy range.

Their healthy weight range is 18.5 × 3.1684 to 24.9 × 3.1684, which is about 58.6 kg to 78.9 kg.

What BMI does not tell you

  • Muscle vs fat: athletes can have a high BMI and very little body fat.
  • Where fat is stored: fat around the waist carries more health risk. A waist measurement of less than half your height is a useful extra check.
  • Age, sex and pregnancy: BMI is not suitable during pregnancy, and for children and teenagers it must be compared with age-specific growth charts.

BMI is a screening tool, not a diagnosis. If you are worried about your weight, talk to your doctor.

Frequently asked questions

What is a healthy BMI?
For most adults, 18.5 to 24.9. Your doctor may use a different range depending on your ethnicity, age and health.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value below 1.0 means you are below that limit. For example, 1.12 means you are 12% above it.
Is BMI accurate for muscular people?
Not very. BMI cannot tell muscle from fat, so very muscular people may be classed as overweight. A body-fat or waist measurement gives a better picture.
Can children use this calculator?
No. For under-18s, BMI has to be compared with age- and sex-specific percentile charts, which this adult calculator does not do.

Last reviewed: November 13, 2026

Results are estimates for general information only and are not financial, tax, medical or legal advice. Check important figures with a qualified professional.