Methodology
How we calculate BMI
Our methodology for the BMI calculator: the formula, step-by-step calculation, authoritative sources, and limitations. Reviewed quarterly.
Formula
BMI = weight (kg) / height (m)²
Step-by-step
- 1
Convert weight to kilograms (1 lb = 0.4536 kg).
- 2
Convert height to meters (1 in = 0.0254 m; 1 ft = 0.3048 m).
- 3
Square the height in meters (multiply it by itself).
- 4
Divide the weight in kilograms by the squared height.
- 5
The result is your BMI in kg/m². Compare to WHO categories.
Authoritative sources
Every claim on this page is backed by an authoritative source.
Assumptions
What we take to be true when applying this formula.
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Adults 20 years or older. For children and teens, BMI is age- and sex-specific (percentile).
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Standard WHO BMI categories apply. Asian populations may need different cutoffs (per WHO 2004).
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Height and weight are accurate measurements (self-reported values reduce accuracy).
Limitations
What this method does NOT capture.
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BMI does not distinguish between muscle mass and fat mass. Athletes often have high BMI but low body fat.
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BMI does not account for fat distribution. Visceral (belly) fat is more dangerous than subcutaneous fat.
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BMI may misclassify older adults (who tend to lose muscle) and pregnant women.
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Ethnic differences in body composition mean standard cutoffs may not apply equally to all groups.
Editorial note
Reviewed against WHO 2004 expert consultation and CDC 2023 guidelines.
Last reviewed: 2026-06-15 • Reviewed by: CalcxApp editorial team