Growth Percentile Calculator — BMI & Head Circumference

Find your child's CDC BMI-for-age or head-circumference-for-age growth percentile using the official LMS method. Boys and girls, birth to 20 years.

July 14, 2026

Sex

Age (2-20 years)

BMI Input Method

BMI (kg/m²)

BMI-for-Age Percentile

73th percentile

Healthy weight

BMI

18.5

Z-Score

0.62

Age Used

10.0 yr

CDC weight-status categories: Underweight (below 5th), Healthy weight (5th to below 85th), Overweight (85th to below 95th), Obesity (95th or above). This girl's result compares against same-age, same-sex girls.

Calculated using the LMS (Box-Cox power-exponential) method — the same formula as our baby percentile calculator, applied to CDC 2000 BMI-for-age reference data (ages 2-20). This tool is for general reference only and does not replace a pediatrician's clinical growth assessment.

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Worked Examples

10-Year-Old Boy, BMI 18.5

Sex: Boy
Age: 10 years
Metric: BMI-for-age
BMI: 18.5 kg/m^2

78th percentile (Z = 0.78) — Healthy weight

At 10 years the CDC LMS values for boys are L=-2.774, M=16.625, S=0.120. Z = ((18.5/16.625)^-2.774 - 1) / (-2.774 x 0.120) = 0.78, which converts to the 78th percentile. Since 78 falls between the 5th and 85th percentile, this lands in the Healthy weight category.

6-Year-Old Girl, BMI 17.8

Sex: Girl
Age: 6 years
Metric: BMI-for-age
BMI: 17.8 kg/m^2

91st percentile (Z = 1.32) — Overweight

At 6 years the CDC LMS values for girls are L=-3.236, M=15.211, S=0.093. Z = ((17.8/15.211)^-3.236 - 1) / (-3.236 x 0.093) = 1.32, which converts to the 91st percentile — inside the 85th-to-<95th band, so the CDC weight-status category is Overweight.

14-Year-Old Boy, BMI 26.0

Sex: Boy
Age: 14 years
Metric: BMI-for-age
BMI: 26.0 kg/m^2

95th percentile (Z = 1.65) — Obesity

At 14 years the CDC LMS values for boys are L=-2.231, M=19.129, S=0.135. Z = ((26.0/19.129)^-2.231 - 1) / (-2.231 x 0.135) = 1.65, which converts to the 95th percentile — right at the Obesity cutoff (95th percentile or greater).

6-Month-Old Boy, Head Circumference 44.5 cm

Sex: Boy
Age: 6 months
Metric: Head circumference-for-age
Head circumference: 44.5 cm

72nd percentile (Z = 0.59) — Within typical range

At 6 months the CDC LMS values for boys are L=1.465, M=43.720, S=0.031. Z = ((44.5/43.720)^1.465 - 1) / (1.465 x 0.031) = 0.59, which converts to the 72nd percentile — comfortably inside the typical 3rd-97th percentile band.

18-Month-Old Girl, Head Circumference 42.5 cm

Sex: Girl
Age: 18 months
Metric: Head circumference-for-age
Head circumference: 42.5 cm

0.1st percentile (Z = -3.02) — Outside typical range, discuss with pediatrician

At 18 months the CDC LMS values for girls are L=1.013, M=46.513, S=0.029. Z = ((42.5/46.513)^1.013 - 1) / (1.013 x 0.029) = -3.02, which converts to roughly the 0.1st percentile — well below the 3rd percentile cutoff, flagged as a screening concern (possible microcephaly) that warrants a pediatrician's evaluation rather than a diagnosis on its own.

How to Use This Calculator

1

Choose a tab

BMI-for-Age covers ages 2-20 years; Head Circumference-for-Age covers birth to 36 months.
2

Select sex

Boy or girl — each has its own growth curve.
3

Enter age

Exact age in years (BMI tab) or months/years (head circumference tab).
4

Enter the measurement

BMI tab: type a known BMI directly, or switch to Height & Weight to have it calculated for you. Head circumference tab: enter the measurement in cm or inches.
5

Read the percentile, Z-score, and category

BMI results include the CDC weight-status category (Underweight/Healthy weight/Overweight/Obesity); head-circumference results include a plain-English typical-range flag.

What Each Value Means

BMI-for-age percentile (percent): How a child's Body Mass Index (weight divided by height squared) compares to same-age, same-sex children in the CDC 2000 reference population, expressed as a percentage.
Head-circumference-for-age percentile (percent): How a child's head circumference compares to same-age, same-sex children in the CDC 2000 reference population, used clinically as a screening measure for brain growth.
Z-score (standard deviations): The number of standard deviations a measurement sits from the reference median for that exact age and sex, calculated from the LMS parameters before conversion to a percentile.

Frequently Asked Questions

How is this different from the site's baby percentile calculator?

The baby percentile calculator covers weight-for-age and height/length-for-age. This calculator covers two different metrics entirely: BMI-for-age (ages 2-20, which factors in both height and weight together) and head-circumference-for-age (birth to 36 months, used to screen head growth rather than body size). Both tools use the same underlying LMS statistical method and the same CDC/WHO data family — they just measure different things, so use whichever matches the measurement you actually have.

What do the BMI weight-status categories mean?

The CDC classifies child and teen BMI-for-age into four categories: Underweight (below the 5th percentile), Healthy weight (5th to below the 85th percentile), Overweight (85th to below the 95th percentile), and Obesity (95th percentile or above). Unlike adult BMI, which uses fixed cutoffs like 25 or 30, child BMI is always judged relative to same-age, same-sex peers because a healthy BMI value changes substantially as children grow.

What does an unusual head circumference percentile actually mean?

It's a screening flag, not a diagnosis. A head circumference below the 3rd percentile can be an early sign of microcephaly, and above the 97th percentile can be an early sign of macrocephaly — both of which sometimes point to an underlying condition worth investigating. But plenty of children with head circumference outside this range are completely healthy, especially if a parent also has an unusually small or large head. Pediatricians look at the trend across visits and the whole clinical picture, not a single measurement.

Why does BMI-for-age use height and weight together instead of just weight?

Weight alone doesn't distinguish between a child who is taller and proportionally heavier from one who is carrying excess weight for their height. BMI (weight divided by height squared) adjusts for that, which is why it's the standard screening tool pediatricians use for weight status — though it's still just a screening number, not a diagnostic one, since it doesn't distinguish muscle from fat.

Can I use head circumference percentile after 36 months?

This calculator's head-circumference data stops at 36 months because that's the standard age range for the CDC's clinical head-circumference growth chart — head growth slows dramatically after age 3, and routine head-circumference screening at well-child visits typically stops around then too. For growth tracking past 36 months, weight and height percentiles (via our baby percentile calculator) and BMI-for-age (this calculator's other tab) are the metrics pediatricians continue to use.