BMI Calculator
Calculate your Body Mass Index from height and weight, see your WHO category, healthy weight range, BMI Prime and ponderal index, with CDC percentiles for children and teens.
How it works
BMI = weight(kg) ÷ height(m)² · BMI Prime = BMI ÷ 25 · Ponderal index = weight(kg) ÷ height(m)³ · Child z = ((BMI ÷ M)^L − 1) ÷ (L × S)
Weight in kilograms divided by height in meters squared. US units are converted first (1 lb = 0.4536 kg, 1 in = 2.54 cm). For adults (20 and over) the WHO ranges are: under 18.5 underweight (with severe, moderate and mild thinness below 16, 17 and 18.5), 18.5–24.9 normal, 25–29.9 overweight, and 30 or more obesity, split into class I (30–34.9), class II (35–39.9) and class III (40+). The optional lower cut-offs (overweight from 23, obesity from 27.5) follow the WHO expert consultation for Asian populations and UK NICE guidance, because risk rises at a lower BMI in those groups. BMI Prime divides your BMI by 25, the top of the normal range, so 1.00 sits exactly on the line and 0.74–0.99 is normal. The ponderal index divides weight by height cubed, which treats very tall and very short people more evenly. For ages 2–19, the calculator converts BMI into a percentile with the CDC 2000 growth-chart LMS values for the child’s age and sex: under the 5th percentile is underweight, 5th to under 85th healthy, 85th to under 95th overweight, 95th or above obesity, and 120% of the 95th percentile (or a BMI of 35) severe obesity. A waist measurement adds the waist-to-height ratio, where under 0.5 is the healthy target.
Worked example
At 5′8″ (1.727 m) and 165 lb (74.84 kg): BMI = 74.84 ÷ 1.727² = 74.84 ÷ 2.983 = 25.1, just over the normal line, so the category is overweight. BMI Prime is 25.1 ÷ 25 = 1.00, the ponderal index is 74.84 ÷ 1.727³ = 14.5 kg/m³, and the normal-BMI weight range for that height is about 122–164 lb, so losing roughly 1 lb reaches it. For a 10-year-old boy who is 4′6″ and 70 lb, the BMI of 16.9 sits around the 55th percentile for his age on the CDC chart — a healthy weight, even though the same number would read as underweight for an adult.
Sources: WHO — BMI classification · CDC — Child and teen BMI categories · CDC — 2000 growth charts, BMI-for-age LMS data · NICE — Keep your waist to less than half your height
Frequently asked questions
Is BMI accurate for athletes?
Not very. Muscle is denser than fat, so muscular people often read "overweight" while carrying little fat. The waist-to-height ratio and body-fat percentage tell more for athletic builds.
How is BMI worked out for children and teens?
From age 2 to 19 the same BMI number is compared with children of the same age and sex on the CDC growth charts and turned into a percentile. Under the 5th percentile is underweight, the 5th to under the 85th is a healthy weight, the 85th to under the 95th is overweight, and the 95th or above is obesity. Enter the child’s age and sex to see the percentile and the healthy range for their height.
What is BMI Prime?
Your BMI divided by 25, the upper limit of the normal range. A BMI Prime of 0.90 means you are 10% below that limit and 1.20 means 20% above it, which makes the number easy to read as a percentage over or under.
What is the ponderal index?
Weight divided by height cubed instead of squared. Because it scales with height more evenly, it stays more consistent for very tall or very short people, and it is also used for newborns. Adult values of roughly 11–15 kg/m³ are typical.
Why are there lower cut-offs for some backgrounds?
Research shows that people of South Asian, Chinese and other Asian, Middle Eastern, Black African and African-Caribbean descent face a higher risk of type 2 diabetes and heart disease at a lower BMI. WHO and the UK’s NICE therefore suggest treating 23 as the start of overweight and 27.5 as the start of obesity for these groups.
What BMI should I aim for?
For most adults 18.5–24.9 is the statistical normal band, but the healthiest weight for you depends on body composition, age and health markers. BMI does not apply during pregnancy. Discuss targets with your doctor.
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This is a general educational estimate, not medical advice. Bodies differ — for decisions about diet, training, medication or health conditions, talk to a doctor or registered dietitian.