BMI Calculator
Calculate Body Mass Index in metric or imperial units, with the standard WHO weight categories.
Why use this tool?
Calculate your Body Mass Index from height and weight and see which WHO category it falls in. BMI is a rough screening measure: it doesn't account for muscle, age or body shape.
How to use it
- Choose metric or imperial units.
- Enter your height and weight.
- Read your BMI and category.
How BMI is calculated
Body Mass Index is your weight divided by the square of your height. In metric units that is kilograms ÷ meters²; in imperial units it is 703 × pounds ÷ inches². The 703 factor converts the imperial result to the same scale, so both give the same number.
Example: 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. In imperial: 160 lb and 5 ft 9 in (69 inches) gives 703 × 160 ÷ 4,761 = 23.6.
The WHO categories
For adults, the World Health Organization uses these ranges: below 18.5 is underweight, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 or above is obesity. The obesity range is often split further into classes (30-34.9, 35-39.9 and 40+).
Some health bodies use lower thresholds for people of Asian descent, because the same BMI is associated with higher health risk in those populations. If that applies to you, a doctor may interpret your number differently.
What BMI can and can't tell you
BMI is a screening number, not a diagnosis. It can't tell muscle from fat, so athletes with a lot of muscle often show as overweight while being lean. It also ignores where fat is carried; waist circumference is a useful second measure, since fat around the abdomen carries more risk.
The adult categories don't apply to children and teenagers, whose BMI is compared against growth charts for their age and sex, and they are less reliable during pregnancy and for older adults who have lost muscle. Use the result as a starting point for a conversation with a health professional, not as a verdict.
Frequently asked questions
What is a healthy BMI?
For most adults, 18.5 to 24.9 according to the WHO. Below or above that range is a prompt to look further, not proof of a health problem.
Why does my BMI say overweight when I'm muscular?
Muscle is denser than fat, and BMI only uses weight and height. Muscular people often score above 25 while having low body fat. A body-fat or waist measurement gives a fuller picture.
Can I use this for my child?
No. Children's BMI has to be compared with age- and sex-specific growth charts (percentiles). The adult categories shown here would be misleading.