Why does children’s BMI use percentiles instead of fixed categories?

Children grow, so a single BMI number means different things at different ages. The CDC publishes BMI-for-age curves by sex; a child’s percentile tells you how their BMI compares to other U.S. children of the same age and sex.

What age range does this tool cover?

Ages 2 through 20 years, which is the range the CDC 2000 BMI-for-age LMS reference data covers. For infants under 2, clinicians use the WHO Child Growth Standards instead.

Which percentile is considered “healthy weight”?

The CDC defines healthy weight as a BMI from the 5th to below the 85th percentile for a child’s age and sex.

Should I act on this result?

No — treat it as a screening number, not a diagnosis. Talk to the child’s pediatrician about their full growth history and any concerns.

No. The calculation runs entirely in your browser; nothing is sent to a server or saved between visits.

Children BMI Calculator

Written by Golam Rabbani, Founder & Lead Engineer — Last updated 2026-05-01

This interactive tool requires JavaScript. Read the formula and worked example below; you can compute the result by hand.

How to use this children bmi calculator

Enter the child’s age in completed years and (optionally) extra months.
Pick the child’s sex at birth — CDC publishes separate boy and girl reference curves.
Enter the weight, switching between kg and lbs with the unit toggle (existing values convert automatically).
Enter the height, switching between cm and in with the unit toggle (existing values convert automatically).
Press Calculate to see the BMI, the CDC BMI-for-age percentile, and the weight-status category.
Use Copy to save the result line or Reset to clear every field.

About this children bmi calculator

The children BMI calculator computes Body Mass Index for a child or teen aged 2–20 years and looks up the result against the U.S. CDC BMI-for-age growth charts. Unlike adult BMI, the same number means very different things at different ages — a BMI of 18 is normal for a 16-year-old but high for a 4-year-old — so the tool uses the child’s age in months plus their sex to pick the correct LMS curve and convert BMI to a percentile.

The math is BMI = weight (kg) ÷ height (m)². The CDC 2000 pediatric LMS reference data (bundled with this tool) provides the L, M, and S parameters for every age and sex; we then compute a z-score and convert it to a percentile using the standard-normal distribution. The result is then mapped to the four CDC weight-status bands: underweight (<5th), healthy weight (5th–<85th), overweight (85th–<95th), and obesity (≥95th).

Worked example: an 8-year-old boy weighing 30 kg at 130 cm has BMI = 30 ÷ 1.30² ≈ 17.8, which falls near the 78th percentile of CDC BMI-for-age for boys — inside the healthy-weight band.

This calculator is for general information using the CDC 2000 pediatric LMS reference data and is not a substitute for an evaluation by a qualified clinician. This calculator is for general educational purposes only and is not medical advice — consult a licensed healthcare professional for personal guidance.

FAQ

Why does children’s BMI use percentiles instead of fixed categories?: Children grow, so a single BMI number means different things at different ages. The CDC publishes BMI-for-age curves by sex; a child’s percentile tells you how their BMI compares to other U.S. children of the same age and sex.
What age range does this tool cover?: Ages 2 through 20 years, which is the range the CDC 2000 BMI-for-age LMS reference data covers. For infants under 2, clinicians use the WHO Child Growth Standards instead.
Which percentile is considered “healthy weight”?: The CDC defines healthy weight as a BMI from the 5th to below the 85th percentile for a child’s age and sex.
Should I act on this result?: No — treat it as a screening number, not a diagnosis. Talk to the child’s pediatrician about their full growth history and any concerns.
Is my data stored?: No. The calculation runs entirely in your browser; nothing is sent to a server or saved between visits.