How to Calculate BMI – Step by Step
Whether you prefer to calculate manually or use our free online tool, this guide walks you through every method for accurately calculating your Body Mass Index.
The BMI Formula
BMI uses your weight and height. There are two versions of the formula depending on which unit system you use.
Metric Formula
Height must be in metres (not centimetres). Convert by dividing cm by 100.
Imperial Formula
Height must be in total inches. Convert feet + inches (e.g. 5'7" = 67 inches).
Example Calculations
Example 1 — Metric
Weight: 68 kg | Height: 165 cm → 1.65 m
1.65 × 1.65 = 2.7225
68 ÷ 2.7225 = 24.98 → Normal Weight
Example 2 — Imperial
Weight: 180 lbs | Height: 5'10" = 70 inches
70 × 70 = 4900
(180 × 703) ÷ 4900 = 25.82 → Overweight
Using the BMI Hero Calculator
Skip the manual math entirely. Our free online tool calculates your BMI instantly as you type.
Select Metric (kg/cm) or Imperial (lbs/ft+in) using the toggle at the top of the calculator.
Type your weight or use the slider. The calculator supports any realistic range.
For metric, enter centimetres. For imperial, enter feet and inches separately.
Your BMI, category, healthy weight range, and BMI prime appear in real time — no button needed.
BMI for Different Groups
Adults (18+)
Standard WHO BMI categories apply. Calculate using the formula above and compare to the chart on our BMI chart page.
Teenagers (13–17)
BMI is calculated identically but interpreted using age-and-sex-specific percentile charts. See our BMI for Teenagers guide for details.