How it works
BMI compares weight to height. In metric units it's weight in kilograms divided by height in meters squared. In US units, multiply by 703 to convert.
BMI = kg ÷ m² · BMI = 703 × lb ÷ in²
| BMI | Category (adults) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
These are the standard adult ranges. Children and teens use age- and sex-specific percentiles instead, so this calculator is for adults 20 and older.
Worked example
A person who is 5 ft 9 in (69 inches) and weighs 160 lb:
BMI = 703 × 160 ÷ 69² = 112,480 ÷ 4,761 = 23.6, in the healthy weight range.
In metric, 175 cm and 70 kg: BMI = 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9.
Frequently asked questions
Is BMI accurate?
It's a quick, rough screen. Very muscular people can show as overweight, and it can miss excess body fat in people with little muscle. Waist size and other measures add context.
Can kids use this BMI calculator?
No. For ages 2 to 19, BMI is compared against growth charts for age and sex, so these adult categories don't apply.
Why multiply by 703 for pounds and inches?
703 converts lb/in² into kg/m², so the US formula gives the same number as the metric one.
Does BMI differ for men and women?
The adult formula and categories are the same for everyone, although body fat at a given BMI tends to differ.