How the bmi calculator works
Body mass index compares your weight with your height:
BMI = 703 × weight (lb) ÷ height (in)²
The healthy weight range is the weight that gives a BMI of 18.5 to 24.9 at your height: weight = BMI × height (m)². BMI Prime is BMI ÷ 25. The result is classified with the WHO adult cut-offs in the table above.
BMI is a screening tool. It does not tell muscle from fat, and it does not account for age, sex or ethnicity, so use it alongside waist size and a conversation with your doctor.
Worked example
Someone who is 5 ft 9 in (69 inches) and weighs 165 lb:
That is in the healthy range (18.5 to 24.9). BMI Prime is 24.4 ÷ 25 ≈ 0.97. At 69 inches the healthy range runs from about 125 lb to 169 lb. In metric, 175 cm and 75 kg gives 75 ÷ 1.75² ≈ 24.5.
Frequently asked questions
What is a healthy BMI?
For adults, the World Health Organization and CDC classify a BMI of 18.5 to 24.9 as healthy weight, under 18.5 as underweight, 25 to 29.9 as overweight and 30 or higher as obesity. Obesity is further split into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 and above).
How do you calculate BMI?
BMI is weight in kilograms divided by height in meters squared. In US units it is 703 × weight in pounds ÷ height in inches squared. Both formulas give the same number.
Is BMI accurate?
BMI is a quick screening number, not a diagnosis. It does not measure body fat directly, so very muscular people can score as overweight and older adults with low muscle mass can score as normal. Waist circumference and body fat measurements add useful context.
Does BMI work for children and teens?
No. For ages 2 to 19 the CDC uses BMI-for-age percentiles that compare a child with others of the same age and sex. This calculator uses the adult cut-offs, which do not apply to children.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A BMI Prime of 1.00 is exactly at the limit, 0.74 to 0.99 is in the healthy range, and 1.20 means you are 20% above the upper limit.
Are BMI ranges different for Asian adults?
Some health bodies, including the WHO expert consultation for Asian populations, suggest lower action points because health risks rise at lower BMIs. A common guide is 23 or higher for increased risk and 27.5 or higher for high risk. Ask your doctor which thresholds apply to you.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.