How to Calculate BMI at Home (Step by Step)

If you have a tape measure and a bathroom scale, you can work out your Body Mass Index in under a minute — no app required. Here is how the calculation works, what your number means, and where BMI quietly falls short.

The BMI formula

BMI is your weight divided by the square of your height. In metric units the formula is:

BMI = weight (kg) ÷ height (m)²

If you use pounds and inches, multiply the result by 703:

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

Worked example (metric)

Say you weigh 72 kg and you are 1.70 m tall. Square the height first: 1.70 × 1.70 = 2.89. Then divide your weight by that: 72 ÷ 2.89 = 24.9. That puts you at the top of the "normal weight" range.

Worked example (imperial)

Same person at 158 lb and 67 inches tall. Square the height: 67 × 67 = 4,489. Then 158 ÷ 4,489 = 0.0352. Multiply by 703: 24.7. Same answer, as expected, allowing for rounding.

What your BMI means

The World Health Organization uses these adult ranges: under 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30 or above is obese.

The limits of BMI

BMI is a screening tool, not a diagnosis. It cannot tell muscle from fat, so muscular athletes can look "overweight" by BMI alone. It also does not apply to children, teens, or pregnant people, who need different references. Treat your number as a starting point — combine it with waist measurement, body fat percentage, and how you actually feel.

Want to skip the maths? Use our BMI Calculator — it handles both metric and imperial in one tap.