Child height prediction and age calculations
6 min read · Updated 2026-10-12 · FitCalc
Two small questions parents ask constantly: how tall will my child end up, and how old exactly are they today. Both have standard methods, and both are far more approximate than the arithmetic suggests — which is worth knowing before treating a number as a target.
The mid-parental method and its assumptions
The mid-parental method adds the parents' heights, adjusts for the sex of the child, and averages. It works because height is strongly heritable and because the average child of two average parents lands near the average population. Its accuracy is roughly plus or minus 8 to 10 centimetres for a single prediction.
The error is not the method's fault — it is that growth is genuinely variable. If one parent is far from the population average, the average-of-two-parents assumption breaks down, and a projection from the child's current growth curve is often more informative.
Why children grow the way they do
Children do not grow steadily. In the first year they may add twenty-five centimetres, and in the year after that only ten or so, before a steady but modest childhood rate of five to six centimetres a year. Then puberty brings a growth spurt whose size and timing are largely genetic.
This is why predicting from current height alone goes wrong for younger children: a three-year-old has a long way to grow, and their position on the growth chart says more about how they are tracking than about their adult height. Percentile curves are a better tool than arithmetic at that age.
Exact age, and why people calculate it
An age in years, months and days looks like precision but is really just a re-expression of a single date. It matters for anything with an age threshold — school intake, competitive age categories, medical dosing, insurance bands — where two people of the same nominal age can be on opposite sides of a cutoff.
The calculation itself is straightforward once the month lengths are handled properly, and the total number of days is a useful figure for anyone thinking in weeks of pregnancy or days of habit.
When a projection is worth revisiting
A child tracking well along their own percentile curve is more reassuring than any projection from parental height. A crossing of percentiles, or a growth velocity that has clearly flattened, is worth raising with a paediatrician regardless of what the arithmetic says.
And a predicted adult height from a formula is not a target. It is a population average with an error bar, and treating it as a promise sets up disappointment that the method never intended.
Run your own numbers
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Common questions
How accurate is predicting a child's adult height?
The mid-parental method is typically accurate to within about 8 to 10 centimetres for a single prediction, and less reliable when one parent is far from the average height.
At what age can adult height be predicted?
Predictions are weak in early childhood because so much growth remains. A projection from the child's own growth curve becomes more informative in the teenage years, around puberty.
Why does my child grow in jumps rather than steadily?
Growth follows a normal pattern: fast in the first year, slower through early childhood at about 5 to 6 cm a year, then a pubertal spurt whose size and timing are largely genetic.
FitCalc provides general information only and is not medical advice, diagnosis or treatment. Figures are illustrative and depend on your own circumstances. Always consult a qualified health professional about your situation. See our full disclaimer.