BMI and body size
BMI uses weight divided by height squared. It is useful for population screening, but it does not directly measure fat, muscle, bone density or fat distribution.
Body fat estimates
Body fat calculators often use circumference, skinfold or formula-based estimates. Measurement technique, hydration, sex, age and body shape can affect results.
Ideal weight estimates
Ideal weight formulas are rough reference equations, not personal medical targets. They can differ substantially and should be framed as comparison tools.
Common mistakes
- Treating BMI category as a diagnosis.
- Comparing body fat estimates from different methods as if they were interchangeable.
- Using a single number without considering age, sex, training status and medical context.
- Ignoring measurement error from tape placement or posture.
Which body composition estimate fits the question
BMI answers a body-size question, not a body-composition question. Waist-to-height ratio adds central-fat context by comparing waist circumference with height. A 90 cm waist at 180 cm gives a waist-to-height ratio of 0.50. Body surface area formulas such as Mosteller answer a different question again, often used for clinical dose or physiology context rather than fitness interpretation.
For user-facing calculators, the safest structure is to show each estimate with its own limitation. BMI can be calculated from two values, waist-to-height needs a reliable waist measure, and body-fat formulas often depend on sex-specific circumference assumptions. None of these should be framed as a diagnosis.
Useful calculators
FAQ
Can BMI tell me my body fat?
No. BMI uses height and weight only.
Why do body fat calculators disagree?
They use different formulas and measurement inputs, so differences are expected.
Should health decisions rely on these calculators?
No. Use them as estimates and discuss health concerns with a qualified professional.
References
- WHO: Body mass index, accessed 2026-05-16.
Last reviewed: 2026-05-16.