How to Calculate BMI

Step-by-step BMI calculation in metric and imperial units, with worked examples and common mistakes.

Use Calculator

Metric (cm, kg): step-by-step

  1. Convert height to meters: m = cm ÷ 100
  2. Square the height: m² = m × m
  3. Compute BMI: BMI = kg ÷ m²

Example (Metric)

Height = 175 cm → 1.75 m
Weight = 70 kg
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9

Imperial (ft/in, lb): step-by-step

Convert height to total inches, then use the common US formula:

  • Total inches: in = (ft × 12) + in
  • BMI: BMI = 703 × lb ÷ in²

Example (Imperial)

Height = 5'9" → (5×12)+9 = 69 in
Weight = 154 lb
BMI = 703 × 154 ÷ (69×69) = 22.7

Common mistakes (that mess up your BMI)

  • Using centimeters directly in the metric formula (you must convert to meters first)
  • Mixing units (kg with inches, or pounds with meters)
  • Rounding too early
  • Applying adult BMI categories to kids/teens
Calculate BMI now See BMI formulas Use BMI chart

Disclaimer: BMI is a screening tool and doesn’t directly measure body fat. For medical questions, talk to a qualified clinician.

Quick links