How Is BMI Calculated? The Formula Explained

BMI is calculated using a simple formula that divides your weight by the square of your height. The exact formula depends on whether you’re using metric or imperial units.

Metric Formula

BMI = weight (kg) ÷ height (m)²

For example, a person weighing 70 kg who is 1.75 m tall has a BMI of: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.

Imperial Formula

BMI = 703 × weight (lb) ÷ height (in)²

The constant 703 converts the result into the same scale as the metric formula so that category thresholds (18.5, 25, 30) apply consistently regardless of which units you use.

Why Height Is Squared

Weight tends to scale roughly with the square of height across a population — taller people are proportionally wider and deeper, not just longer — so squaring height helps normalise BMI across different heights more fairly than dividing by height alone would.

Try It Yourself

Rather than calculating by hand, you can get an instant result with our BMI Calculator, which supports both metric and imperial units and shows your category immediately.

Source: CDC — About Adult BMI.