Kids BMI Calculator
Find your child's BMI-for-age percentile using the same CDC growth chart method pediatricians use, for ages 2 through 19. Unlike the fixed adult BMI scale, a child's healthy range depends on their exact age and sex.
Your result will appear here
Enter your child's age, sex, height, and weight, then click calculate to see their BMI percentile.
CalcClearly Health & Fitness Team
Content reviewed against CDC growth chart methodology
This page is researched and maintained by CalcClearly's editorial team, using the CDC's published BMI-for-age reference data and methodology. It's meant as general education, not a substitute for advice from your child's pediatrician.
Why a Child’s BMI Number Means Nothing on Its Own
An adult BMI of 19.0 is an adult BMI of 19.0 — one number, one category, at any age. For children that is simply not how it works, and the difference is larger than most people expect.
Here is the same BMI of 19.0, read against the CDC reference for boys at different ages:
| Age | BMI 19.0 sits at |
|---|---|
| 6 years | 97th percentile |
| 8 years | 92nd percentile |
| 10 years | 83rd percentile |
| 12 years | 68th percentile |
| 14 years | 48th percentile |
| 16 years | 27th percentile |
| 18 years | 11th percentile |
One number, running from near the top of the chart to near the bottom, purely because of age. That is why looking up a child’s BMI on an adult chart gives an answer that is not merely imprecise but can point in the opposite direction.
The reason is that typical BMI is not flat through childhood. It falls through the preschool years, reaches its lowest point around age 5 or 6, then rises steadily through adolescence — the median for boys runs about 15.4 at age 6 and about 21.8 at 18. The percentile compares your child to that moving curve, not to a fixed threshold.
Reading the Result
The CDC sorts BMI-for-age percentiles into four bands. These are the CDC’s own category names, and they describe where a measurement sits in a reference population — nothing more:
- Below the 5th percentile — underweight
- 5th to 84th percentile — healthy weight
- 85th to 94th percentile — overweight
- 95th percentile and above — obesity
A percentile is a ranking, not a score out of a hundred and not a grade. The 60th percentile means the child’s BMI is higher than about 60% of children of the same age and sex in the CDC reference population. Half of all healthy children are, by definition, above the 50th.
What a single reading cannot tell you is why. BMI is calculated from height and weight alone, so it does not distinguish muscle from fat, does not account for build or frame, and does not know where a child is in the timing of their growth spurt — which can move the figure substantially in either direction over a year. A child who is measured shortly before a growth spurt and again shortly after can produce two quite different percentiles without anything having changed about their health.
This is why a pediatrician plots the measurements over time. The shape of the line across several visits carries information that no single point does.
Quick Answer
This calculator compares your child's BMI to the CDC's BMI-for-age growth chart reference data for their exact age (in months) and sex, and returns a percentile: the 5th-84th percentile is a healthy weight, below the 5th is underweight, the 85th-94th is overweight, and the 95th and above is classified as obesity.
How It Works: Formula & Variables
BMI = weight (kg) / height (m)²
Z = ((BMI / M)^L − 1) / (L × S)
Percentile = normal distribution CDF(Z) × 100
L, M, and S are reference values published by the CDC for each age (in months) and sex, describing the skew, median, and spread of BMI in the reference population. This calculator interpolates between the CDC's published values at 6-month age intervals.
Worked Examples
Example 1: 8-year-old boy
A boy aged 8 years 0 months, 50 inches tall (127 cm), weighing 55 lb (25 kg), has a BMI of about 15.5, which lands at roughly the 43rd percentile for his age, a healthy weight.
Example 2: 12-year-old girl
A girl aged 12 years 0 months, 60 inches tall (152 cm), weighing 119 lb (54 kg), has a BMI of about 23.4, which lands at roughly the 91st percentile for her age, in the overweight category.
Key Concepts
Percentile, not a fixed score: A child's BMI percentile changes as they grow even if their BMI number stays exactly the same, since the reference curve itself shifts with age.
Sex-specific reference data: Boys and girls have different typical growth patterns, so this calculator uses separate CDC reference tables for each.
A screening tool, not a diagnosis: Like adult BMI, a child's percentile doesn't measure body fat directly or account for build, muscle mass, or growth timing. A pediatrician considers it alongside growth history and other factors.
Common Mistakes
Applying adult BMI categories to kids: A BMI of 22 might be a healthy weight for one 10-year-old and overweight for another, depending on age and sex. There's no fixed "healthy" BMI number for children the way there is for adults.
Reacting to a single reading: Pediatricians look at growth trends over multiple visits, not one measurement in isolation. A single percentile is a data point, not a verdict.
Using this for children under 2: Infants and toddlers under 24 months are tracked on separate WHO growth charts, not the CDC BMI-for-age charts this calculator uses.
Frequently Asked Questions
Adults are compared to fixed cutoffs (18.5, 25, 30), but a child's healthy weight range changes constantly as they grow. Instead of a fixed number, a child's BMI is compared to thousands of other children of the same age and sex, using CDC growth charts, and expressed as a percentile rather than a category.
The CDC considers the 5th to 84th percentile the healthy weight range for children and teens. Below the 5th percentile is classified as underweight, the 85th to 94th percentile as overweight, and the 95th percentile and above as obesity.
It means their BMI is higher than about 75% of children of the same age and sex in the CDC reference population, and lower than about 25%. It's a relative ranking, not a pass/fail grade, and the 75th percentile still falls inside the healthy weight range (5th-84th percentile).
This calculator covers ages 2 through 19 years (24 to 240 months), matching the CDC's BMI-for-age growth charts. For children under 2, growth is tracked using WHO weight-for-length charts instead, which use a different reference population and aren't covered here.
It uses the official CDC BMI-for-age reference data (the L, M, and S parameters published by the CDC), sampled at 6-month age intervals and interpolated in between. That's a close approximation of the CDC's exact calculation, which uses slightly finer age resolution, so results may differ by a percentile point or two from a pediatrician's exact software.
Yes, and it is the whole reason children are not measured against the adult scale. A BMI of 19.0 sits at about the 97th percentile for a 6-year-old boy but only the 11th percentile for an 18-year-old, because the reference curve for BMI dips through early childhood and climbs again through adolescence. The same is true across sexes at the same age. A child's BMI number on its own tells you almost nothing until it is placed against their age and sex.
It can. The reference curve moves with age, so a child whose BMI stays exactly the same can drift up or down the percentiles simply because the typical BMI for their age has changed. This is normal and is one reason a single reading means less than a trend. Pediatricians plot the points over several visits rather than reacting to one.
A single BMI percentile reading isn't a diagnosis. Growth patterns, family history, puberty timing, and overall health all matter alongside this number. Bring the result to your child's pediatrician, who can track it over time alongside other measurements rather than react to one data point.
Related Calculators
What this page is, and what it is not
This is a screening calculation, not an assessment of your child’s health, and it is not a diagnosis of anything. It applies the CDC’s published BMI-for-age reference data to the height, weight, age and sex you enter, and reports where that lands. It cannot see your child, their growth history, their family background, their stage of puberty, or anything else that a clinician would weigh alongside this number.
The content here is written and maintained by CalcClearly’s editorial team and checked against the CDC’s published methodology. It has not been reviewed by a pediatrician or other clinician, which is why this page deliberately explains how the measurement works rather than offering guidance on what to do about a result. If a result concerns you, that conversation belongs with your child’s doctor.
Sources
- Reference data. CDC BMI-for-age growth charts, the published L, M and S parameters for ages 24–240 months, sampled at six-month intervals and interpolated between: CDC, Growth Charts – Percentile Data Files. Because the CDC’s own software uses finer age resolution, a result here can differ from a clinic’s by a percentile point or two.
- Category thresholds. The 5th, 85th and 95th percentile cut-offs are the CDC’s: CDC, BMI Calculator for Child and Teen.
- Age range. Ages 2 through 19 only. Children under 24 months are tracked on WHO weight-for-length charts, which use a different reference population and are not covered by this tool.
Measuring an adult instead?
From age 20 the fixed adult scale applies and percentiles are no longer used, so a different tool is needed: the adult BMI calculator uses the standard 18.5 / 25 / 30 thresholds. For a 19-year-old the two approaches sit side by side and can disagree, which is expected rather than a fault in either.