BMI Calculator for Kids & Teens

A BMI calculator for kids does not stop at the BMI number. It converts that number into a BMI-for-age percentile against CDC growth-chart data for children of the same age and sex. A 10-year-old boy who is 4 ft 6 in and 80 lb has a BMI of 19.3, which lands at the 84th percentile and counts as healthy weight, just under the 85th-percentile overweight line.

This page is the child and teen version of the adult tool. Use the BMI Calculator from the 20th birthday on, where the fixed 18.5 / 25 / 30 cutoffs apply, and use this page for ages 2 through 19, where the cutoffs move with every month of age. Two more health tools sit nearby: the Child Height Predictor estimates adult height from parents' heights, and the Ideal Weight Calculator gives an adult target weight from height and frame. For a fat measurement rather than a weight-for-height screen, the Body Fat Calculator works from tape measurements.

How the BMI number is calculated

The arithmetic is the same at every age:

In US units the equivalent form is BMI = 703 × weight (lb) ÷ height (in)². This calculator converts pounds to kilograms (× 0.45359237) and inches to meters (× 0.0254) and then applies the metric definition, which is the official one.

How the percentile is calculated

CDC publishes the growth charts as a table of three numbers, called LMS parameters, for every half-month of age from 24 to 240.5 months, separately for boys and girls. L is a skewness (Box-Cox) power, M is the median BMI at that age, and S is a coefficient of variation. The child's BMI becomes a z-score with:

When L is exactly 0 the formula becomes Z = ln(BMI ÷ M) ÷ S. The percentile is then the standard normal cumulative probability of that z-score. Running the formula backwards gives the BMI at any percentile: BMI = M × (1 + L × S × Z)1/L, which is how the calculator reports the 5th, 50th, 85th and 95th percentile cut points for your child's exact age.

Worked example: a 10-year-old boy

Take a boy aged exactly 10 years 0 months (120 months), 4 ft 6 in tall and 80 lb.

1. Convert: 54 inches × 0.0254 = 1.3716 m; 80 lb × 0.45359237 = 36.287 kg.
2. BMI = 36.287 ÷ 1.3716² = 19.29.
3. CDC LMS values for boys at 120 months: L = -2.7737, M = 16.6246, S = 0.1198.
4. Z = ((19.29 ÷ 16.6246)^-2.7737 − 1) ÷ (-2.7737 × 0.1198) = +1.02.
5. The normal probability at z = +1.02 is 0.845, so the BMI-for-age percentile is 84.5.

That is the healthy weight category, because the overweight band does not start until the 85th percentile. Running the formula in reverse for the same age gives the cut points: the 5th percentile is a BMI of 14.2 (58.9 lb at this height), the median is 16.6 (69.0 lb), the 85th is 19.4 (80.3 lb), and the 95th is 22.1 (91.7 lb). In other words this boy is roughly a quarter of a pound below the overweight line, which is exactly the kind of result that should be re-checked at the next visit rather than acted on today.

Second worked example: a 14-year-old girl

A girl aged 14 years 0 months (168 months), 5 ft 3 in and 135 lb has a BMI of 23.91. The CDC LMS values for girls at 168 months are L = -1.9754, M = 19.3275, S = 0.1513, giving a z-score of +1.15 and a percentile of 87.5. That falls in the overweight band (85th to below 95th). The 85th-percentile BMI at this age is 23.32, which is 131.6 lb at 5 ft 3 in, so the reading sits about 3.4 lb above the line. The 95th-percentile BMI is 27.22, or 153.7 lb. Note how far away the obesity threshold still is: the same category label covers a 22-pound span at this height.

CDC BMI cut points by age and sex

These are the BMI values at the three category boundaries, computed from the CDC LMS parameters at each exact birthday. Read across your child's age row to see where their BMI number falls before the percentile is even mentioned.

Two patterns stand out. The healthy band is narrow in early childhood and widens steadily through the teen years: for boys it spans 13.7 to 17.0 at age 6 and 18.2 to 25.6 at age 18. And the boy and girl curves separate after age 9, with the girls' upper cut points running above the boys' from puberty onward.

The same BMI, a different verdict at each age

This is the whole reason a child needs a percentile rather than a fixed cutoff. Below is one single BMI value, 19.3, run through the CDC curves at seven different ages. Nothing about the child changed except the birthday.

A BMI of 19.3 is an obesity-range reading for a 6-year-old and a comfortably below-average reading for an 18-year-old. Anyone who applies the adult threshold of 25 to a young child will miss the first case entirely.

Healthy weight range by height: a 10-year-old boy

Percentiles are easier to act on once they are converted back into pounds. The table takes the 5th, 85th and 95th percentile BMI values for a boy at 120 months and turns each into a weight at a range of heights.

A single foot of height moves the healthy ceiling from 63 lb to 99 lb at the same age, which is why a weight number on its own tells a parent almost nothing. The calculator above builds this same band for your child's exact age, sex and height.

The four CDC categories, and the fifth band

CDC defines the weight status categories for ages 2 through 19 purely by percentile:

• Underweight: less than the 5th percentile.
• Healthy weight: the 5th percentile to less than the 85th percentile.
• Overweight: the 85th percentile to less than the 95th percentile.
• Obesity: the 95th percentile or greater.
• Severe obesity: 120% of the 95th-percentile value or greater, or a BMI of 35 or greater.

That last band exists because the ordinary percentile scale runs out of room. The 2000 growth charts do not extend past the 97th percentile, so above it the curves crowd together and small BMI changes produce almost no percentile movement. Expressing the reading as a percentage of the 95th percentile keeps it readable. For a 10-year-old boy the 95th percentile is a BMI of 22.1, so 120% of it is a BMI of 26.5.

How to use this calculator

1. Pick the sex. Boys and girls have separate CDC curves, and the difference grows through puberty, so this is not a cosmetic choice.
2. Enter the age in years and months. Months matter: the reference curve shifts with every month, so rounding a 10-year-8-month-old to "10" changes the answer.
3. Choose your units. US units (feet, inches, pounds) are the default; the metric toggle switches to centimeters and kilograms.
4. Enter height carefully. Shoes off, heels and back against a wall, eyes looking straight ahead, and read to the nearest half inch or better.
5. Enter weight. Ideally on a home scale in light clothing, at a consistent time of day.
6. Read the percentile, then the category. The card also shows the BMI, the z-score, and the pound-for-pound cut points at your child's height, which is usually the most useful part.

Who this calculator is for

• Parents who saw a percentile written on a well-visit summary and want to understand what it means.
• Teens who tried an adult BMI calculator and got a category that did not fit their age.
• School nurses and coaches doing routine screening who need the age-and-sex-adjusted number rather than a raw BMI.
• Caregivers tracking a trend across several measurements, where the direction of travel matters more than any single reading.
• Anyone converting a percentile into pounds, since the weight cut points are what actually guide day-to-day decisions.

Key terms explained

• BMI-for-age percentile: the share of children of the same age and sex in the CDC reference population with a lower BMI. The 84th percentile means 84 out of 100 reference children have a lower BMI.
• LMS parameters: the three numbers (skewness L, median M, coefficient of variation S) that describe the BMI distribution at each age and sex. CDC publishes them so any calculator can reproduce the charts exactly.
• Z-score: the percentile in standard deviations. Zero is the median, +1.04 is the 85th percentile, +1.64 is the 95th. Clinicians prefer it because it keeps resolving readings far out on the curve.
• Growth chart: the printed curves showing the 3rd through 97th percentile lines against age; the percentile from this calculator is the point where a child sits on the BMI-for-age version.
• Weight status category: the CDC label (underweight, healthy weight, overweight, obesity) that the percentile falls into. It is a screening label, not a diagnosis.

What changes the result the most

• Height accuracy. BMI divides by height squared, so a half-inch error moves the result more than most people expect. For the 10-year-old boy above, 53.5 inches instead of 54 pushes the percentile from 84.5 to 86.8 and flips the category.
• Age in months. Around ages 9 to 12 the curve rises steeply. Entering the wrong number of months can shift the percentile by several points on its own.
• Sex. After puberty starts, the boy and girl curves diverge by more than a full BMI point at the 95th percentile.
• Weight. Perhaps surprisingly, it is the least sensitive input in percentile terms for older children: one pound moves that 10-year-old boy from 84.5 to 86.1, but the same pound would barely register for a 17-year-old.
• Timing. Weight varies through the day. Measuring at a consistent time makes a trend across months readable.

Practical tips for reading the number

• Watch the line, not the dot. A child who tracks along the 60th percentile for three years is doing something quite different from one who moves from the 60th to the 90th in a year, even though today's readings might match.
• Expect the dip. BMI normally falls from about age 2 to around age 5 or 6 and then rises again. That rise is called the adiposity rebound and is a normal part of growth.
• Do not diet a growing child. Children need consistent nutrition to grow. CDC and pediatricians usually start with family habits (meals, sleep, screen time, activity) rather than restriction.
• Take height seriously. Half the calculation is height, and it is the half most often measured casually at home.
• Bring the printout to the visit. A home reading plus the clinic's own history is more useful than either alone.

Limitations and assumptions

• BMI does not measure body fat. A muscular child and a child with excess fat can produce identical readings, so the percentile can flag one and miss the other.
• The 2000 CDC growth charts are a reference, built from national survey data collected between 1963 and 1994. They describe how US children were, not how they should be.
• The charts do not extend past the 97th percentile. Very high readings are better described by the percent-of-the-95th-percentile figure than by a percentile.
• They apply to ages 2 through 19 only. Under 2, weight-for-length charts are used; from 20, the adult categories apply.
• Timing of puberty varies widely, and an early or late developer can read high or low for reasons that have nothing to do with body composition.
• A single measurement carries measurement error in both height and weight. Two or three readings across months are far more informative.

Which calculator to use when

Ages 2 through 19 belong on this page, because the cutoffs move with age and sex. From the 20th birthday, switch to the BMI Calculator, which uses the fixed adult bands of 18.5, 25 and 30. If you want a fat estimate rather than a weight-for-height screen, the Body Fat Calculator works from tape measurements, and the Body Surface Area Calculator gives the figure clinicians use for pediatric drug dosing. To estimate how tall a child will end up, use the Height Calculator, and for an adult goal weight rather than a child percentile, use the Ideal Weight Calculator. Daily calorie needs for a grown household member come from the Calorie Calculator and the BMR Calculator, neither of which is validated for children.

Sources

• Centers for Disease Control and Prevention (CDC) - CDC Growth Charts.
• CDC / National Center for Health Statistics - Percentile Data Files with LMS Values, and the BMI-for-age data file bmiagerev.csv used verbatim by this calculator.
• CDC - Child and Teen BMI Calculator, source of the weight status category definitions.
• CDC / National Center for Health Statistics - CDC Extended BMI-for-age Growth Charts, background on readings above the 97th percentile.
• CDC - About Body Mass Index (BMI).
• The BMI formula itself and the LMS z-score transformation are exact arithmetic and need no source beyond the CDC parameter file; unit conversions use the exact NIST definitions of 1 inch = 0.0254 m and 1 pound = 0.45359237 kg.