How to Calculate and Interpret Your BMI Category
In metric units, BMI is weight in kilograms divided by height in meters squared. In imperial units, it's 703 times weight in pounds divided by height in inches squared. The result is compared against four standard categories: underweight, healthy weight, overweight, and obese.
The BMI formula itself is simple arithmetic, but working through it by hand — especially in imperial units, with the less intuitive 703 multiplier — is where most manual calculation mistakes happen.
The calculation, step by step
Metric: divide weight (kg) by height (m), then divide again by height (m) a second time — equivalent to dividing by height squared. For someone weighing 70 kg at 1.75 m tall: 70 ÷ 1.75 ÷ 1.75 = 22.9.
Imperial: divide weight (lb) by height (in) squared, then multiply by 703. For someone weighing 154 lb at 68 inches tall: (154 ÷ 68 ÷ 68) × 703 ≈ 23.4. The 703 constant exists specifically to make the imperial formula produce the same scale of number as the metric version.
What each category means
Below 18.5 is classified as underweight. 18.5 to 24.9 is considered a healthy weight range. 25 to 29.9 is overweight. 30 and above is classified as obese. These thresholds are set by the World Health Organization and used consistently across most medical contexts, applying the same way regardless of age (for adults) or sex.
A note on interpreting the number
The category a BMI falls into is a general screening indicator, not an individual diagnosis — the separate guide on BMI's limitations covers exactly where this matters most, particularly for athletes and older adults.