Data and Method
This page explains, once for the whole site, where the height reference values come from, how the figures on the height chart pages are calculated, and how the height predictor on the home page works. The chart pages link here instead of repeating it.
Reference tables used
| Ages | Source | File | Measurement | Rows used |
|---|---|---|---|---|
| Birth to 13 weeks | WHO Child Growth Standards (2006), length-for-age | boys, girls | Length, lying down | Week=0 to Week=13 |
| Birth to 24 months | WHO Child Growth Standards (2006), length-for-age | boys, girls | Length, lying down | Month=0 to Month=24 |
| 2 to 20 years | CDC 2000 Growth Charts, stature-for-age | statage.csv | Height, standing | Agemos = completed months + 0.5 (15 years uses 180.5); 240 for age 20 |
| Adults, 20 and older | NCHS, NHANES Anthropometric Reference Data, United States, August 2021–August 2023 | report, Tables 7 and 8 | Height, standing | Seven age groups, 20–29 to 80 and older |
Every value on the chart pages is read from one of these tables. File names link to the official downloads.
The five WHO and CDC files are stored with the scripts that build the chart pages and are checked against fixed SHA-256 checksums on every rebuild; if a file has changed, the build stops. The adult values were copied cell by cell from the NCHS report tables. Values are shown rounded to 0.1 cm. Nothing is interpolated: an age that is not a row in the source table gets no value.
Under 2 years the site shows WHO lengths, measured lying down; from 2 years it shows CDC heights, measured standing. They are separate references measured in different positions, so no chart page compares a value from one table with a value from the other.
How to read a percentile
A percentile is a position in the reference distribution for children of the same sex and age. At the 25th percentile, about 25 in 100 children in the reference are shorter. The 50th percentile is the median: half are shorter and half are taller. The 3rd and 97th percentiles bound the middle 94% of the reference.
A percentile is not a forecast, a target, a diagnosis or a measure of a child's health. The measuring method and the reference chart used can change the reported position. For one child's measurement, the guide to reading growth-chart percentiles goes further.
How the figures on the age pages are calculated
Each age page adds figures that the source tables do not print directly. All of them come from rows of the same table, for the same sex unless stated otherwise:
- Change in the median over 6 or 12 months. The 50th-percentile value in a later row minus the value in the page's row. It describes how the reference distribution shifts with age; it is not a measured growth rate for any one child.
- Movement of the 3rd and 97th percentile lines. The same subtraction for those two columns. When the 3rd-percentile value rises more than the 97th, the range between them narrows.
- Spread. The 97th-percentile value minus the 3rd (the middle 94%), and the 75th minus the 25th (the middle half).
- Largest and smallest yearly gains. Every 12-month change in the median is computed from each monthly row of the CDC table. The pages report the largest one starting at age 6 or later, the smallest one starting between ages 4 and 10, and the first age from which every later 12-month change is under 1 cm.
- Height for age. For the 3rd and 97th percentile values, the age whose median is closest, searching the monthly rows of the same table. Because the match is to the nearest row, the two values are close but not always identical; both are shown.
- Share of the age-20 median. The median at the page's age divided by the median at 240 months.
- Boys and girls. The same row of the same table for the other sex.
Differences are calculated from the rounded values as displayed, so the figures in the prose add up against the tables on the same page.
The percentile checker on the age pages
The checker uses the L, M and S parameters printed on the same row of the source table (the LMS method). For a height x, z = ((x ÷ M)L − 1) ÷ (L × S), and the percentile is the share of a standard normal distribution below z. It accepts values from z = −4 to z = 4 and runs in the browser; the value you type is not sent anywhere.
How the height predictor works
The height predictor estimates adult height for a child or teen, and is designed for roughly ages 8 to 20. It combines two estimates:
- Mid-parental target. The average of both parents' heights, plus 6.5 cm for a boy or minus 6.5 cm for a girl. The mid-parental height page explains the formula.
- Current-height projection. Current height divided by the share of adult height usually reached at that age and sex, from a table for ages 8 to 20 approximated from CDC growth-chart medians (for example 91% for a 14-year-old boy and 97% for a 14-year-old girl). A reported puberty stage moves that share by up to 6 percentage points; "not started" and "just started" move it only from age 13 for girls and 14 for boys.
When both are available, the estimate is 55% of the mid-parental target plus 45% of the projection. If that comes out below the current height plus 60% of the growth the projection implies, it is raised to that level. When the table shows growth as complete, the estimate is the current height.
The result is a range. Its half-width starts at 3 cm and grows by 0.6 cm for each year of age under 16, by 2.5 cm when current height is not given, and by 15% of the gap between the two estimates, up to 4 cm. When growth is complete the range only extends 2 cm above current height.
Published target-height methods land within 8.5 cm (3.3 in) of adult height for about 95% of people (Tanner et al., 1970). This calculator has not been checked against measured adult heights, so no accuracy figure is claimed for it.
When to see a doctor
No table or calculator can find a medical cause of slow growth. The site's guides list these as reasons to speak to a pediatrician or family doctor:
- crossing downward through percentile lines over repeated measurements;
- growth under about 4 cm a year between age 4 and puberty;
- a height far below what the parents' heights suggest, with no family explanation;
- no signs of puberty by 13 in girls or 14 in boys, or puberty starting before 8 in girls or 9 in boys;
- slow growth together with symptoms such as fatigue, weight loss or digestive problems.
The guide late bloomer or growth problem? covers what a doctor checks.
Pages that were merged or withdrawn
Earlier versions of the site had a separate page for every age and a set of weight-for-age pages. Most single-age height pages are now rows in one table per sex (boys, girls), the adult age groups are tables on the height chart overview, and the weight pages were withdrawn. Old addresses redirect to the closest table.
Corrections
If a value on the site does not match its source file, please write in with the page address and the row you checked. Who runs the site and why is on the about page.
Reference information only. Nothing on this site is medical advice, diagnosis or treatment.