Measure Height 
How the prediction works
This is percentile tracking, the same thing a pediatrician does when they run a finger along the growth chart. It takes the child's height at their age, works out where that sits on the CDC 2000 stature-for-age charts for boys or girls, and reads off the height at age 20 on that same percentile.
The calculation uses the CDC's own LMS parameters, the three numbers per month of age that the published charts are drawn from, so the percentile is exact rather than eyeballed between printed lines. A child on the 42nd percentile at age 8 is projected to the 42nd percentile of adult height.
If you add both parents' heights, the tool also shows the mid-parental target (Tanner's method: average the parents, add 6.5 cm for a boy or subtract 6.5 cm for a girl, plus or minus 8.5 cm) and tells you whether the projection lands inside that range.
Worked examples
Each row is a real run of the calculator, rounded for the table.
| Child | Height now | Percentile | Projected at 20 |
|---|---|---|---|
| Boy, 8 y | 127 cm (4'2") | 42nd | 175.5 cm (5'9") |
| Girl, 10 y | 145 cm (4'9") | 84th | 169.7 cm (5'7") |
| Boy, 5 y | 104 cm (3'5") | 13th | 168.9 cm (5'6.5") |
| Girl, 3 y | 92 cm (3'0") | 29th | 159.7 cm (5'3") |
| Boy, 13 y | 150 cm (4'11") | 21st | 171.0 cm (5'7.5") |
Note the 13-year-old boy. At the 21st percentile he projects to 5'7.5", but at 13 a late bloomer has not started his growth spurt yet and sits low on the chart for that reason alone. That row is the least trustworthy of the five.
How far to trust it
Percentile tracking assumes a child stays on their line. Most stay close; plenty do not. Three cases where the projection is weakest:
- Under about 3. Babies often move up or down a percentile band or two as they settle toward their genetic size.
- Around puberty, 10 to 15. Early maturers read tall and finish average; late maturers read short and catch up.
- Any recent illness or a big change in eating. The chart reflects the last few months, not the child's potential.
Nothing that runs on a phone beats this by much. The more precise clinical methods need an X-ray of the hand to read bone age (Bayley-Pinneau), and Khamis-Roche, the best-known no-X-ray formula, adds weight and parents' heights but still carries error bars of a few centimetres. If you need a prediction for a medical reason, that is a job for a pediatric endocrinologist.
What about "double their height at two"?
The old rule says a boy's adult height is twice his height at 24 months and a girl's is twice her height at 18 months. Run it on the median child and it holds up: the WHO median boy at 24 months is 87.8 cm, doubled 175.6 cm, against a CDC adult median of 176.8 cm. The median girl at 18 months is 80.7 cm, doubled 161.4 cm, against 163.3 cm. So it lands about an inch short for an average child and inherits every weakness of early percentiles for everyone else.
Questions
How accurate is a height predictor?
For a child who keeps growing along the same percentile, very. The problem is that many children do not. The biggest moves happen in the first two years and around puberty, so a projection made at age 6 or 7 is usually steadier than one made at 2 or at 12. Treat the result as the most likely outcome, not a promise.
Which age gives the best prediction?
Mid-childhood, roughly 4 to 9 years. The early catch-up or slow-down after birth has settled and puberty has not started. Between about 10 and 15 an early maturer looks tall and a late maturer looks short, and both tend to drift back.
Why do I need the exact age in months?
Children grow fast enough that a few months matter. A boy who is 127 cm at exactly 8 is a little below average; the same 127 cm at 7 and a half is a little above it. The calculator uses the age in whole months, the way the CDC tables are built.
What if the projection and the parents' target disagree?
That is common and usually not a problem. The projection says where the child is heading on the current percentile; the parents' range says where genetics tends to put children of these parents. A child well outside the range, or one who has dropped across two major percentile lines, is worth mentioning to a pediatrician.
Does it work for adults or for babies?
No. It covers ages 2 to 17, because that is the span of the CDC stature charts it uses. After growth plates close there is nothing left to predict, and under 2 the WHO standards apply and the percentile is much less stable.
Source: CDC 2000 growth charts, stature-for-age LMS data (Kuczmarski et al., Vital Health Stat 11(246), 2002). Target height method: Tanner, Goldstein and Whitehouse, Arch Dis Child 1970. This tool is an estimate and not medical advice.