BMI Formula
The BMI formula in metric and imperial units, with worked examples for both. Plain-English explanation of why height is squared.
Reviewed Jan 15, 2025
Quick answer
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)². The 703 is just the unit-conversion constant baked in.
BMI = kg ÷ m² (or 703 × lb ÷ in²)
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Find your Body Mass Index and see where it sits on the standard healthy-range scale.
Result
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How it works
BMI is a screening tool that estimates whether your weight is in a healthy range relative to your height. It doesn't measure body fat directly.
Formula
BMI = weight (kg) ÷ height (m)²
Common mistakes
- Mixing units — entering height in cm but weight in pounds.
- Treating BMI as a diagnosis. It's a screening signal, not a verdict.
- Using adult ranges for children or teens.
- Ignoring muscle mass — athletes often score 'overweight' but have low body fat.
Source & methodology
Healthy ranges follow WHO adult cut-offs: underweight <18.5, healthy 18.5–24.9, overweight 25–29.9, obese ≥30.
Related calculators
Last updated
Reviewed Apr 15, 2026.
Calcxo gives you the answer plus the reasoning behind it.
Why is height squared?
Squaring height roughly normalizes weight across different body sizes. It's an empirical choice that fits adult populations reasonably well — not a physical law.
Worked example, both units
A concrete walkthrough you can follow line by line.
A person who is 175 cm (≈ 68.9 in) and 72 kg (≈ 158.7 lb).
Metric
72 ÷ (1.75)² = 72 ÷ 3.0625 ≈ 23.5
Imperial
703 × 158.7 ÷ (68.9)² ≈ 23.5
Result
Same BMI either way: ≈ 23.5.