Kids BMI Calculator

For ages 2 to 20. The result is a screening figure, not a diagnosis. It cannot replace the growth record a pediatrician keeps over years.

Why a child's BMI is read as a percentile

A child's BMI is calculated exactly like an adult's: weight divided by height squared. What changes is how the number is read.

Typical BMI values change as children grow, and they change differently for girls and boys. A BMI of 17 is at the 86th percentile for a 4-year-old boy and at the 4th percentile for a 16-year-old boy.

The CDC growth charts solve this by comparing a child with children of the same sex and age. The answer is a percentile: the 60th percentile means the BMI is higher than that of about 60 in 100 children of that age and sex in CDC's reference group.

BMI percentile categories for children and teens

CDC uses these categories for ages 2 through 19. From age 20 the fixed adult ranges in the adult BMI calculator apply.

The category names are CDC's. They describe where a measurement falls on a chart. They do not describe a child, and they say nothing about effort, habits or health on their own.

How the BMI percentile is calculated

CDC publishes three numbers for every month of age and each sex: L, M and S. M is the median BMI, S describes the spread and L corrects for the fact that BMI is not distributed symmetrically. They turn a BMI into a z-score, and the z-score into a percentile:

z = ((BMI ÷ M)^L − 1) ÷ (L × S)
percentile = 100 × Φ(z)

Φ is the standard normal distribution. For BMIs above the 95th percentile CDC published a second method in 2022, the extended BMI-for-age charts. The calculator uses it for those BMIs:

percentile = 90 + 10 × Φ((BMI − 95th percentile BMI) ÷ sigma)

• Data. The L, M, S and sigma values are read from CDC's two published data files, the 2000 growth chart file and the 2022 extended file. They were fetched on October 12, 2026 and stored unchanged; how we calculate lists the checks.
• Age. CDC lists each row at the middle of a month. An age of 10 years 6 months is looked up at 126.5 months.
• Rounding. The percentile is rounded down to a whole number, so the 84.9th shows as the 84th and stays in the healthy weight range, as CDC's limits require.

How to calculate a child's BMI percentile: a worked example

A boy aged 10 years 6 months who is 4 feet 7 inches tall and weighs 75 pounds:

1. BMI: 703 × 75 ÷ (55 × 55) = 17.43.
2. CDC values for boys at 126.5 months: L = −2.6739, M = 16.9151, S = 0.12366.
3. z = ((17.43 ÷ 16.9151)^−2.6739 − 1) ÷ (−2.6739 × 0.12366) = 0.233.
4. Φ(0.233) = 0.5921, so the BMI is at the 59th percentile.
5. That is between the 5th percentile (BMI 14.4) and the 85th (BMI 19.8) for his age: healthy weight.

BMI-for-age table: the 5th, 50th, 85th and 95th percentiles

BMI at the 5th, 50th, 85th and 95th percentile on each birthday, calculated from the CDC data. A BMI between the 5th and the 85th column is in the healthy weight range for that age.

What a BMI percentile cannot tell you

• It is one point in time. A pediatrician looks at how a child's measurements move over years. A steady line along any percentile reads differently from a sudden change.
• It does not measure body fat. BMI cannot tell fat from muscle and bone. That is as true for children as for adults.
• Home measurements are rough. For a 4-year-old boy who weighs 36 pounds, a height of 40 inches gives the 56th percentile and a height of 41 inches the 29th. Measure without shoes, and weigh in light clothing.
• It is a U.S. reference. The charts describe American children. WHO uses a different reference and different cut-offs for ages 5 to 19.

If the result is outside the healthy weight range, the useful next step is a conversation with your child's doctor, who can check the measurements and look at the whole picture. Changes to what a child eats are best planned with that doctor.