How to calculate BMI
Body mass index (BMI) is your weight divided by your height squared. In metric units: BMI = weight (kg) ÷ height (m)². In US units: BMI = 703 × weight (lb) ÷ height (in)². For example, 145 lb at 5'6" (66 inches) gives 703 × 145 ÷ 66² ≈ 23.4.
The formula and the categories are the same for adult men and women, at any age from 18.
BMI categories for adults (WHO)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class 1 |
| 35 to 39.9 | Obesity class 2 |
| 40 and above | Obesity class 3 (severe) |
BMI for men vs women
BMI categories are identical for both sexes. However, at the same BMI, women on average carry a higher share of body fat than men. That is why this calculator uses your sex to estimate body fat and ideal weight.
Does age change the result?
- Under 18, BMI is interpreted with age- and sex-specific growth charts (BMI-for-age percentiles), not adult categories.
- As we age, muscle mass tends to decrease and body fat to increase at the same BMI. The Deurenberg formula used here accounts for that.
- After 65, a slightly higher BMI is often considered healthier than a low one, as it protects against undernutrition.
Ideal weight: the Devine formula
The Devine formula, widely used in medicine, gives a reference weight based on height and sex:
- Men: 50 kg (110 lb) + 2.3 kg (5.1 lb) for every inch over 5 feet
- Women: 45.5 kg (100 lb) + 2.3 kg (5.1 lb) for every inch over 5 feet
Treat it as a reference point, not a target: the whole healthy BMI range is considered healthy.
Limits of BMI
BMI cannot tell muscle from fat: a very muscular athlete can have a high BMI without excess fat. It also says nothing about where fat is stored. Waist size is a useful complement: a waist above about 35 inches (88 cm) for women or 40 inches (102 cm) for men is linked to higher cardiovascular risk.
FAQ
What is a healthy BMI for a woman?
For adult women, a BMI between 18.5 and 24.9 is considered a healthy weight by the WHO, the same range as for men.
How much should I weigh for my height?
At 5'6", a healthy BMI corresponds to roughly 115 to 154 lb. Enter your height in the calculator to get your own range.
Is BMI accurate for athletes?
Not really: muscle is denser than fat, which can push BMI up without any excess body fat.
How is BMI calculated for children?
The calculation is the same, but the result must be plotted on growth charts by age and sex. Your pediatrician can help interpret it.