BMI Explained: How to Calculate and Interpret It
Body Mass Index (BMI) is a quick, widely used screening number that relates your weight to your height. It is easy to calculate and useful as a rough guide — but it has real limits. Here is how it works and how to read it sensibly.
What is BMI?
BMI is a single number that estimates whether your weight is in a healthy range for your height. It is used by clinicians and health bodies for population screening because it is cheap and fast — not as a precise diagnosis.
The BMI formula
BMI = weight (kg) ÷ height (m)². For example, 74 kg and 1.78 m gives 74 ÷ (1.78 × 1.78) = 23.4.
If you use pounds and inches, multiply the result by 703: BMI = (lb ÷ in²) × 703. The BMI Calculator does either for you instantly.
BMI categories
- Under 18.5 — underweight
- 18.5 to 24.9 — normal range
- 25 to 29.9 — overweight
- 30 and above — obese
The limits of BMI
BMI does not distinguish muscle from fat, so very muscular people can read as "overweight" while having low body fat. It also ignores fat distribution, age and ethnicity. Treat it as one data point, not the whole picture.
Beyond BMI
For a fuller view, pair BMI with a reference healthy weight range and your energy needs. The Ideal Weight Calculator gives a target range for your height, and the BMR & Calorie Calculator estimates how much energy your body uses each day.
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Last updated: July 6, 2026