BMI Formula Explained

This reference explains the BMI formula used by our free BMI Calculator. Use it to understand variables, units, and common mistakes before you calculate.

The formula

BMI = weight (kg) ÷ height (m)²

Variables and units

• weight — Body mass (kg or lb (convert to kg))
• height — Standing height (m (convert from cm or ft/in))

Worked example

Inputs: Weight 70 kg, height 175 cm (1.75 m)

1. Convert height to meters: 175 cm = 1.75 m
2. Square height: 1.75² = 3.0625 m²
3. Divide weight by height²: 70 ÷ 3.0625 = 22.86

Result: BMI ≈ 22.9 (normal weight range)

When to use this formula

Use this formula when you need a quick bmi estimate with values you already know. Our BMI Calculator applies the same relationship automatically so you can verify handwritten work.

Common mistakes

• Using height in centimeters instead of meters in the metric formula
• Comparing BMI to body-fat percentage thresholds directly
• Ignoring waist circumference when BMI is borderline

Frequently asked questions