Use this BMI calculator to work out your body mass index and – if you enter your waist – your waist-to-height ratio as well. Together, the two values give a better picture than BMI alone.
How BMI is calculated
Body mass index relates body weight to height: weight in kilograms divided by height in metres squared. For example, 72 kg at 1.75 m gives a BMI of 23.5.
WHO categories for adults
- below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 or more: obesity
Why measure your waist too?
BMI does not distinguish muscle from fat and says nothing about where fat is stored. Abdominal fat in particular is closely linked to heart and metabolic disease. The waist-to-height ratio (waist divided by height) reflects this better. A simple rule of thumb: keep your waist to less than half your height, i.e. a ratio below 0.5. Measure standing, after breathing out normally, at about the level of the navel.
Limits of BMI
In very muscular people BMI often comes out too high; in older people with little muscle mass it tends to be too low. Children and teenagers are assessed with age- and sex-specific percentiles, and BMI is not meaningful in pregnancy. An unusual value is a reason to talk to your GP – not a diagnosis.
Frequently asked questions
What is an ideal BMI?
For adults, a BMI between 18.5 and 24.9 is classed as healthy weight. There is no single ideal value within this range; waist size, activity, diet and blood tests matter more.
Does BMI apply to children?
No. In children and teenagers, BMI is assessed with percentile charts for age and sex. This calculator is for adults only.
Is waist-to-height ratio better than BMI?
Studies show that it often predicts the risk of heart and metabolic disease slightly better than BMI alone, because it takes abdominal fat into account. Both values together are the most informative.
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Note: This calculator provides general information and does not replace medical advice, diagnosis or treatment. Your entries are not stored.