How the BMI formula works
Body Mass Index is a ratio of weight to the square of height. The calculator uses the standard forms:
- Metric:
BMI = weight (kg) ÷ height (m)² - Imperial:
BMI = 703 × weight (lb) ÷ height (in)²
In metric mode you enter height in centimetres, which the calculator divides by 100 first. The 703 in the imperial formula is just the unit conversion between pounds-per-inch-squared and kilograms-per-metre-squared. The result is shown to one decimal place and matched against the standard adult cut-offs.
Worked examples
- 82 kg at 180 cm: 1.80² = 3.24, and 82 ÷ 3.24 = 25.3, which falls in the overweight range.
- 150 lb at 67 in: 703 × 150 = 105,450, and 67² = 4,489, giving 23.5, in the normal range.
- 180 lb at 5 ft 9 in (69 in): 703 × 180 = 126,540, and 69² = 4,761, giving 26.6.
The categories
| BMI | Category used here |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25 to 29.9 | Overweight |
| 30 and above | Obese |
These are the World Health Organization adult thresholds, widely adopted by health agencies. They are applied identically at every age and sex here. For children and teenagers, BMI is interpreted against age- and sex-specific percentile charts, so this calculator's categories do not apply to them.
What BMI can and cannot tell you
- It does not measure body fat. A muscular athlete can land in the overweight range while having low body fat, and an older adult with little muscle can have a normal BMI but a high fat percentage.
- It ignores fat distribution. Waist circumference and waist-to-height ratio capture abdominal fat, which BMI does not.
- Thresholds vary by population. Some health bodies use lower cut-offs for people of South or East Asian descent, because health risks can rise at lower BMI values.
- It is a screening number, not a diagnosis. Use it as one data point and talk to a doctor about anything health-related. This page is information, not medical advice.
Common input mistakes
The most frequent error is entering imperial height in feet. The imperial inputs here take total inches, so 5 ft 9 in is 69, not 5.9. Another is mixing units, such as kilograms with inches. Check the unit labels, or convert beforehand with the unit converter.
To go from BMI to daily energy needs, see the TDEE calculator and the calorie calculator. The guide BMI and daily calorie needs goes further.