BMI for Kids Calculator

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Walk-through

How to Use This Calculator

3 steps

1

Enter your child's sex, age, weight, and height

Choose Boy or Girl, then enter age in years and months (2 years 0 months through 19 years 11 months), current weight, and current height. Switch between metric (cm/kg) and imperial (in/lb) units at any time — the calculator converts automatically.

2

Read the BMI-for-age percentile

The result card shows the BMI-for-age percentile — where your child's BMI ranks compared to 100 same-age, same-sex children in the CDC reference population — plus the weight-status category (Underweight, Healthy weight, Overweight, or Obesity) that percentile falls into.

3

Check the BMI and Category tabs for context

The BMI tab shows the raw BMI value the percentile was calculated from, and the Category tab lists all four CDC weight-status bands with your child's band highlighted, so you can see how close the result is to a neighboring category.

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Reference

Formula & Methodology

2 formulas

Raw BMI

BMI = weight (kg) / height (m)²

The same BMI formula used for adults. Weight in kilograms divided by height in metres, squared. On its own this number means little for a child, since a healthy BMI changes substantially with age — a BMI of 17 is normal for a 6-year-old but underweight for a 16-year-old.

BMI-for-age percentile (CDC LMS method)

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

The CDC Growth Charts 2000 publish three parameters — L (a Box-Cox power that corrects skew), M (the median BMI), and S (the coefficient of variation) — for every whole month of age from 24 to 240 months, separately for boys and girls. The raw BMI is converted to a z-score using those parameters, then to a percentile via the standard normal cumulative distribution function (Φ). This is the identical calculation the CDC's own official BMI percentile calculator performs, and it is the same CDC BMI-for-age dataset used by this site's multi-metric Growth Percentile calculator.

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YMYL

Trust, Methodology & Sources

Reviewed by Calculover Editorial Team · Updated 2026-07-23 · 2 sources

Editorial accountability

Author: Calculover Editorial Team - Editor

Owner: Calculover Editorial Team - Editorial owner

Reviewer: Calculover Editorial Team - Clinical reviewer

Last reviewed: 2026-07-23

Last verified: 2026-07-23

Data effective: 2026-07-23

Methodology

The BMI for Kids Calculator computes raw BMI as weight (kg) / height (m)^2, then converts it to a BMI-for-age percentile using the CDC Growth Charts 2000 LMS (Lambda-Mu-Sigma) reference method: z = ((BMI/M)^L - 1) / (L*S), followed by percentile = Phi(z) * 100 using the standard normal cumulative distribution function. The L, M, and S parameters are looked up for the child's exact age in whole months (24-239, i.e. ages 2 through 19) and sex from the official CDC Growth Charts 2000 dataset. The weight-status category (Underweight, Healthy weight, Overweight, Obesity) is read from the CDC's published percentile cutoffs: below the 5th, 5th to below the 85th, 85th to below the 95th, and 95th and above respectively. This is the same underlying CDC dataset and method used by this site's multi-metric Growth Percentile calculator.

Assumption: Weight and height are measured accurately for the child in question, with the correct unit system selected before reading the result.

Assumption: The entered age is accurate to the month, since the CDC reference curve is defined at whole-month resolution and shifts meaningfully across childhood and adolescence.

Assumption: The user wants a screening estimate of weight status relative to CDC reference data, not a diagnosis or a full clinical growth assessment.

Limitations & guidance

This calculator only covers ages 2 years 0 months through 19 years 11 months, matching the CDC's published BMI-for-age reference range; it does not support infants under 2 or adults 20 and older.

BMI is a screening measure computed from weight and height only; it does not distinguish fat mass from muscle mass, so unusually muscular or unusually lean children may be classified in a way that does not reflect their actual body composition or health.

A single BMI-for-age percentile is one data point. Pediatric growth assessment properly considers trends across multiple visits, family history, puberty timing, and other clinical factors that this calculator does not have access to.

Professional guidance: The BMI for Kids Calculator is for health education and screening only. It is a screening estimate, not a diagnosis, and it is not medical advice or a substitute for assessment by your child's pediatrician or another qualified healthcare professional. Do not make medical decisions based on its output alone.

Primary sources

Centers for Disease Control and Prevention, National Center for Health Statistics. CDC Growth Charts: United States, 2000 — BMI-for-age, ages 2-20 years. - CDC

Centers for Disease Control and Prevention. About Child & Teen BMI. - CDC

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Glossary

Key Terms Explained

6 terms

BMI-for-age A child's raw BMI (weight/height²) expressed as a percentile against the CDC reference population for their exact age (in months) and sex — the standard way pediatric BMI is reported, because a fixed BMI number does not mean the same thing at every age.

Percentile The share of same-age, same-sex children in the CDC reference population whose BMI is at or below your child's BMI. A 70th percentile means roughly 70 out of 100 similar children have a lower or equal BMI.

CDC growth chart A reference dataset published by the U.S. Centers for Disease Control and Prevention (CDC Growth Charts, 2000) describing the typical distribution of BMI, weight, height, and head circumference by age and sex for U.S. children.

Weight status category One of four bands the CDC defines from BMI-for-age percentile: Underweight (below the 5th percentile), Healthy weight (5th to below the 85th), Overweight (85th to below the 95th), and Obesity (95th percentile and above).

Pediatric BMI BMI calculated for a child or teen. Unlike adult BMI, it is always interpreted through an age- and sex-specific percentile rather than a single fixed cutoff, because children's healthy body composition changes continuously as they grow.

Growth reference The LMS (Lambda-Mu-Sigma) statistical model the CDC fits to its growth-chart data, allowing any raw measurement to be converted into a precise age- and sex-adjusted percentile or z-score.

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Scenarios

Real-World Examples

3 worked examples

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8-year-old boy

Typical grade-schooler

Age 8 years 0 months Weight 25 kg (55 lb) Height 128 cm (50.4 in)

BMI works out to about 15.3, which lands around the 37th percentile for an 8-year-old boy — squarely in the Healthy weight band (5th to below the 85th percentile). The raw BMI number alone looks low next to adult standards, but for this age it is normal — a reminder that pediatric BMI must always be read as a percentile, never as a fixed number.

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15-year-old girl

Adolescent above the reference curve

Age 15 years 0 months Weight 60 kg (132 lb) Height 150 cm (59 in)

BMI works out to about 26.7, which lands around the 93rd percentile for a 15-year-old girl — in the Overweight band (85th to below the 95th percentile). It is close to the 95th-percentile Obesity threshold, so the Category tab is useful here to see exactly how much room remains before the next band.

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Same BMI, two ages

Why percentile bands matter

Height/weight 130 cm, 28 kg (fixed) Compared at age 8 vs. age 13

Keeping height and weight fixed produces the exact same raw BMI (about 16.6) at both ages — but the percentile differs, because the CDC reference curve for what counts as typical shifts across childhood and puberty. The same BMI number can be a healthy-weight result at one age and an overweight signal at another, which is exactly why this calculator reports a percentile rather than a bare BMI value.

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Deep Dive

Understanding the BMI for Kids Calculator

3 chapters · 4 min read

Body mass index (BMI) is calculated the same way for children as for adults — weight divided by height squared — but a single BMI number cannot be read the same way for a child. Because a healthy BMI shifts continuously as kids grow, pediatricians compare a child's raw BMI against CDC reference data for their exact age and sex, producing a percentile. This calculator reproduces that CDC BMI-for-age percentile calculation for ages 2 through 19.

How the BMI for Kids Calculator works

The calculator first computes raw BMI the standard way: weight in kilograms divided by height in metres squared. It then looks up the CDC Growth Charts 2000 LMS reference values (L, M, and S) for the child's exact age in whole months and sex, and uses them to convert the raw BMI into a z-score and then a percentile via the standard normal distribution. This is the same LMS method — and the same underlying CDC dataset — that the CDC's own official child and teen BMI calculator uses, and that this site's multi-metric Growth Percentile calculator also draws on for its BMI-for-age track.

The weight-status category (Underweight, Healthy weight, Overweight, Obesity) is then read directly off that percentile using the CDC's published cutoffs: below the 5th percentile, 5th to below the 85th, 85th to below the 95th, and 95th and above.

Inputs and what they mean

Sex — boys and girls have separate CDC reference curves, since typical growth patterns and body composition differ by sex throughout childhood and adolescence.

Age — entered as years and months, because the CDC reference tables are defined at whole-month resolution; a percentile computed at age 8 years 0 months can differ meaningfully from one at 8 years 6 months, especially around growth spurts.

Weight and height — entered in either metric or imperial units. Small measurement errors matter less at older ages, where the reference curve is flatter, and more around early childhood, where it changes quickly — so measure as carefully as you can.

Limits and edge cases

This calculator only covers ages 2 years 0 months through 19 years 11 months, matching the CDC's own published reference range; outside that window the tool will not return a result. BMI itself is a screening measure, not a body-composition test — it does not distinguish fat mass from muscle mass, so an unusually muscular or unusually lean child can be classified in a category that does not reflect their actual health status. BMI-for-age percentile is also just one data point; pediatricians look at growth trends over multiple visits, family history, and other factors before drawing any conclusions. If a result surprises you or falls near a category boundary, discuss it with your child's pediatrician rather than acting on this tool alone.

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Questions

Frequently Asked Questions

6 questions

Why does this calculator show a percentile instead of just my child's BMI number?+

A raw BMI number means something different at every age — a BMI of 17 is normal for a young child but underweight for a teenager. Percentile compares your child's BMI to other children the same age and sex, which is the standard, clinically meaningful way pediatric BMI is reported.

What age range does this calculator cover?+

Ages 2 years 0 months through 19 years 11 months, matching the CDC's published BMI-for-age reference range. Outside that window (for example, under 2 or at 20 years and above), the calculator will not return a result — for children under 2, weight-for-length is used instead of BMI.

How accurate is this calculator?+

It uses the same CDC Growth Charts 2000 LMS reference data and the identical LMS statistical method (converting BMI to a z-score, then a percentile) that the CDC's own official child and teen BMI calculator uses. It is a precise implementation of that public methodology, but like any BMI-based tool it is a screening estimate, not a diagnosis — see a pediatrician for a full clinical assessment.

Does the calculator use different reference data for boys and girls?+

Yes. The CDC publishes separate LMS reference curves for boys and girls at every age, since typical body composition and growth patterns differ by sex throughout childhood and puberty. Select the correct sex before reading the result.

What units can I use?+

Weight in kilograms or pounds, and height in centimeters or inches. Use the unit toggle to switch — previously entered values convert automatically so you don't need to re-enter them.

Is this medical advice?+

No. This calculator is for general education only and is not a diagnosis or a substitute for assessment by your child's pediatrician. Weight-status category is one screening data point among many a doctor would consider — always discuss any concerns about your child's growth with a qualified healthcare professional.

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