Body mass index is a useful screening tool, but the way you read it for a child is different from the way you read it for an adult. An adult who scores 21 is simply "normal weight." A child who scores 21 might be underweight, healthy, overweight, or obese — depending on whether they are a two-year-old boy or a sixteen-year-old girl. This child BMI calculator computes the raw BMI from weight and height, then places it on a CDC-style BMI-for-age percentile curve for the child's sex and completed age, so the result is a percentile and a weight-status label rather than a blunt adult number.
Approximation for quick screening. This tool provides an approximate estimate only. Exact percentiles require WHO/CDC reference tables and browser-side interpolation; consult a clinician for medical decisions. The engine here models the CDC BMI-for-age curves with a closed-form LMS approximation rather than running the official CDC Epi Info lookup tables, so the reported percentile can differ by a few points from a pediatric clinic reading — especially near the extremes. Use it to track your child's trend and to flag when to ask the pediatrician; never rely on it as a medical diagnosis.
Enter the child's age, sex, weight, and height, and the tool instantly computes the BMI and the estimated percentile.
How to Use
- Enter the child's Age in whole years. These charts are designed for children and teens from 2 to 19 years.
- Choose the child's Sex — the CDC publishes separate growth charts for boys and girls.
- Type the child's Weight in pounds. Use a recent, accurate measurement taken on a bathroom scale rather than a guess.
- Type the child's Height in inches, measured standing up without shoes.
- Read the result card: the Child's BMI, the estimated BMI-for-Age Percentile, the z-score, and the Weight Status label.
What the weight-status number means. The result card returns a small number that the page maps to a plain label: 0 = Underweight (below the 5th percentile), 1 = Healthy weight (5th to below the 85th), 2 = Overweight (85th to below the 95th), and 3 = Obese (at or above the 95th). The percentile is shown as a number followed by a percent sign; the z-score tells you how many standard deviations above (+) or below (−) the median your child sits.
The BMI itself is a simple ratio of weight to height: BMI = weight in pounds ÷ (height in inches × height in inches) × 703. The ×703 factor converts the pounds-and-inches units into the standard BMI scale.
Why Age Matters More Than a Flat Number
For adults, the same cutoff applies to everyone: below 18.5 is underweight, 18.5–24.9 is healthy, 25–29.9 is overweight, and 30 or above is obese. Children are still growing, so a BMI that is perfectly normal at age 4 would be considered underweight at age 14. The CDC growth charts solve this by comparing the child against other children of the same sex and age, expressed as a percentile.
The four CDC weight-status categories are:
- Underweight — BMI below the 5th percentile.
- Healthy weight — BMI from the 5th percentile to below the 85th percentile.
- Overweight — BMI from the 85th percentile to below the 95th percentile.
- Obese — BMI at or above the 95th percentile.
This calculator gives you the raw BMI number, an estimated percentile, and a weight-status label, so you can see where your child sits on the growth curve. For a definitive reading, your pediatrician plots the number on the official CDC chart across many visits.
Examples
Example 1 — A typical 8-year-old boy. An 8-year-old boy weighs 56 lb and is 50 in tall. His BMI is 56 ÷ (50 × 50) × 703 = 15.7. On the boys' BMI-for-age curve at 8 years, a BMI of 15.7 sits at about the 55th percentile, which falls in the healthy weight band (status 1).
Example 2 — A 10-year-old girl. A 10-year-old girl weighs 70 lb and is 54 in tall. Her BMI is 70 ÷ (54 × 54) × 703 = 16.9. At age 10 for girls, 16.9 is around the 51st percentile — squarely in the middle of the healthy range.
Example 3 — Recognizing overweight early. A 9-year-old boy who is 52 in tall but weighs 90 lb has a BMI of 90 ÷ (52 × 52) × 703 ≈ 23.4. At 9 years, that is well above the 95th percentile, placing him in the Obese band (status 3, percentile about 99.9). This is the kind of flag that prompts a conversation with the pediatrician about activity, sleep, and diet.
How a Percentile Is Read
If your child is at the 60th percentile, it means 60% of children of the same age and sex weigh less, and 40% weigh more. Being high on the chart is not automatically a problem, and being low is not automatically a problem — the percentile only matters once it crosses one of the threshold lines (5%, 85%, 95%). Pediatricians also track the trend over years: a child who steadily climbs percentile lines, even while staying "healthy," is worth discussing with a doctor.
FAQ
What BMI percentile is healthy for a child?
A healthy weight is the 5th to below 85th percentile. Below the 5th percentile is underweight; the 85th to below 95th percentile is overweight; and at or above the 95th percentile is obese. These bands apply to both boys and girls, but each sex uses its own chart.
At what ages does this calculator work?
The CDC BMI-for-age charts are intended for children and teens aged 2 to 19 years. For children under 2, pediatricians use weight-for-length and other WHO charts rather than BMI-for-age.
My child is at the 90th percentile — is that obesity?
No. The 90th percentile falls in the overweight range (85th to below 95th), not obesity. Obesity begins at or above the 95th percentile. Being overweight is an early warning sign worth addressing with family eating and activity habits.
Can I use this for a teenager?
Yes, the same method applies through age 19. Teens go through growth spurts at different times, so the percentile naturally shifts during puberty.
Should I be worried by one measurement?
One BMI is a screening snapshot, not a diagnosis. Body composition varies, and puberty changes results quickly. What matters most is the trend across annual checkups. Bring the numbers to your pediatrician, who can also consider growth history, diet, and activity.
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Disclaimer
This child BMI calculator is provided for informational purposes only and is not medical advice. It computes BMI from the weight and height you enter using the standard formula. It is not a substitute for professional medical evaluation, diagnosis, or treatment. Percentile placement must be confirmed by a pediatrician using the official CDC growth charts. Always seek the advice of your child's physician with any questions about growth, weight, or health.