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Calculate body mass index
Calculates BMI from height and weight in either unit system. BMI ignores body composition, so it is a poor measure for athletes and a screening indicator rather than a diagnosis.
22.9 — Normal weight
BMI ignores body composition, so it is a poor measure for athletes, and it is not a diagnosis.
How to use it
- 1Choose metric or imperial units.
- 2Enter your height and your weight.
- 3Read the BMI value and the band it falls into.
Frequently asked questions
- How is BMI calculated?
- Weight in kilograms divided by height in metres squared. In imperial units the same figure comes from pounds divided by inches squared, multiplied by 703 — the constant exists purely to make the two systems agree.
- What do the bands mean?
- Under 18.5 is classed as underweight, 18.5 to 24.9 as the normal range, 25 to 29.9 as overweight, and 30 and above as obese. These are the WHO cut-offs for adults.
- Why is BMI a poor measure for athletes?
- Because it only knows your height and your mass — it cannot tell muscle from fat. A rower and a sedentary person of the same height and weight get the same number, which is the clearest illustration of what the measure does not see.
- Does it apply to children or during pregnancy?
- No. Children are assessed against age-and-sex percentile charts rather than fixed thresholds, and BMI is not meaningful during pregnancy. This calculator is for non-pregnant adults, and it is a screening indicator rather than a diagnosis — a doctor interprets it alongside everything else.