When to use this formula
Use this formula when your inputs match the variables and units shown below. It is most useful for checking a calculator result, recreating the calculation in a spreadsheet or understanding which input has the biggest effect.
Quick use
Use the related calculators to understand ratios and categories, then interpret results with age, sex, training status and medical context.
Formula
BMI = weight(kg) / height(m)^2; lean body mass = body weight x (1 - body fat percentage); body surface area by Mosteller = sqrt(height(cm) x weight(kg) / 3600).
Variables
Inputs include height, weight, waist, neck, hip or body-fat estimate depending on the calculator. Units must match the chosen method.
Method notes
- BMI is a broad screening ratio, not a diagnosis.
- Circumference methods depend heavily on measurement technique.
- BSA formulas are approximations and may vary by clinical context.
Example
A person weighing 70 kg at 1.75 m has BMI = 70 / 1.75^2 = 22.9.
Assumptions and limitations
These formulas can misclassify muscular people, older adults, children, pregnant people and people with medical conditions. They do not directly measure body fat unless based on validated measurement methods.
When the formula is not enough
- If the result depends on live prices, rates or official thresholds, check the latest value from the named source before relying on it.
- If the topic is medical, tax, legal, lending or safety related, use the result as a learning aid and check primary guidance before acting.
- If units or time periods differ, convert them before comparing results.
- If rounding affects the decision, keep extra precision until the final step.
Common mistakes
- Treating a category as a diagnosis.
- Comparing body-fat estimates from different methods as if interchangeable.
- Ignoring measurement error.
References
- WHO: Body mass index - WHO BMI data and classification context.
FAQ
Why look at the formula instead of only the answer?
The formula shows which inputs actually drive the result. That makes it easier to spot a wrong unit, compare two scenarios or explain the answer to someone else.
Can different calculators use different formulas for the same topic?
Yes. Some topics have multiple accepted methods or simplified variants. When that matters, the calculator should say which method it uses and what is excluded.
Are formula pages updated?
Stable math formulas need occasional review. Formulas that depend on changing rules, prices or thresholds need a dated source before the page can make stronger claims.