Child BMI Calculator: BMI-for-Age Percentile (CDC)
Calculate a child or teen’s BMI and CDC BMI-for-age percentile (ages 2–19) from age or birth date, sex, height and weight, with the category, z-score, percent of the 95th percentile, extended percentiles for severe obesity and a healthy weight range for their height.
How it works
BMI = weight(kg) ÷ height(m)² · z = ((BMI ÷ M)^L − 1) ÷ (L × S) with the CDC L, M, S values for the child’s age in months and sex · percentile = Φ(z) · above the 95th percentile (CDC 2022 extended method): percentile = 90 + 10 × Φ((BMI − P95) ÷ σ) · categories: < 5th underweight, 5th–< 85th healthy, 85th–< 95th overweight, ≥ 95th obesity, ≥ 120% of P95 or BMI ≥ 35 severe obesity
A child’s BMI is worked out exactly like an adult’s, weight in kilograms divided by height in metres squared, but it is read differently. Healthy body fatness changes a lot as children grow and differs between boys and girls, so the BMI is compared with other children of the same age and sex on the CDC BMI-for-age growth charts. The calculator does that lookup for you: it takes the CDC L, M and S values for the child’s age in months (from years and months, or from the birth date and the date measured) and turns the BMI into a z-score and a percentile. Below 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. CDC also defines severe obesity as a BMI at or above 120% of the 95th percentile (or 35 or more), and class 3 obesity from 140% (or 40). Above the 95th percentile the original curves are unreliable, so the calculator switches to CDC’s 2022 extended method, as the official CDC calculator does. It also shows the healthy BMI and weight range for the child’s current age and height, a category table and the percentile curves.
Worked example
An 8-year-old boy (96 months) who is 128 cm tall and weighs 25 kg: BMI = 25 ÷ 1.28² = 25 ÷ 1.6384 = 15.3 kg/m². The CDC values for a boy of 96 months are about L = −3.19, M = 15.77 and S = 0.1017, so z = ((15.26 ÷ 15.77)^−3.19 − 1) ÷ (−3.19 × 0.1017) ≈ −0.34, which is about the 37th percentile: a healthy weight. At this age the healthy range is a BMI of about 13.8 to 17.9, or 22.6–29.4 kg at his height, and his BMI is 76% of the 95th percentile.
Sources: CDC — BMI for children and teens · CDC — Growth chart data files (BMI-for-age LMS parameters) · CDC — Extended BMI-for-age growth chart data files (2022) · CDC — Child and teen BMI categories
Frequently asked questions
Why can’t I use adult BMI categories for my child?
Because children’s body composition changes quickly with age and differs by sex. A BMI of 17 might be healthy at one age and overweight at another, so children are assessed by where their BMI falls on age- and sex-specific percentile charts, not by the adult 18.5–25 bands.
Does this calculator give the percentile?
Yes. It uses the CDC’s own reference values (the L, M and S parameters behind the growth charts) for the child’s age in months and sex, so you do not need to plot the BMI by hand. Above the 95th percentile it uses CDC’s 2022 extended method, the same one the official CDC calculator uses.
What is percent of the 95th percentile?
It is the child’s BMI divided by the BMI at the 95th percentile for their age and sex. At 100% or more the child is in the obesity range; at 120% or more it is severe obesity (class 2) and at 140% or more class 3. Doctors use it to track very high BMIs, where percentiles all bunch up near 99.
Should I enter the birth date or the age?
Either works. The birth date and the date measured give the exact age in months, which matters most for young children and around puberty, when the cut-offs move quickly. Years and months is close enough for most checks.
Is one BMI reading enough to worry about?
Not on its own. Growth happens in spurts, and a single point matters far less than the trend across several visits. A pediatrician looks at the whole growth curve alongside diet, activity and development before drawing conclusions.
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Educational estimate, not medical advice — for clinical decisions consult a healthcare professional.