The mid-parental target height method, with the prediction band shown honestly — it is wider than most calculators admit.
Average the two parents’ heights, then adjust for the child’s sex. Adult men average about 13 cm taller than adult women, so the convention adds half that — 6.5 cm — for a boy and subtracts it for a girl.
Written out: boys = (father + mother + 13) ÷ 2, and girls = (father + mother − 13) ÷ 2. In imperial the adjustment is 5 inches rather than 13 cm.
It is simple, it has been in clinical use for decades, and it is roughly as good as any short formula gets. What it is not is precise.
Father 178 cm, mother 165 cm. The mid-parental height is 171.5 cm. For a boy, the target is 178 cm; for a girl, 165 cm. The likely range around each is ±8.5 cm — so the boy’s estimate spans 169.5 to 186.5 cm, and the girl’s spans 156.5 to 173.5 cm.
That band is 17 cm wide, and it is not padding. It represents roughly the 3rd to 97th percentile of where children with those parents actually end up. Any calculator that hands you a single number to the nearest centimetre is overstating what the method can do.
Height is polygenic — influenced by hundreds of genetic variants rather than a few. Each parent passes on half their genes, but which half is a lottery, and the combination can express differently. Siblings from the same two parents routinely differ by 10 cm or more as adults, which is the clearest demonstration that the parental average sets a centre rather than a destination.
Twin studies put the heritability of height at roughly 80% in well-nourished populations. That leaves a fifth of the variation to everything else, and "everything else" includes some things that matter a great deal.
Nutrition. The single largest non-genetic factor, and the reason average heights across whole countries have risen dramatically within living memory. Dutch men gained around 20 cm in a century — far too fast for genetic change, so it was food, and the reduction of childhood infection.
Timing of puberty. The growth spurt is the last major contribution, and it ends when the growth plates close. Early puberty means an earlier spurt and an earlier stop, which can leave a child who was tall for their age ending up average. Late puberty does the reverse — which is why a short adolescent is often not a short adult.
Chronic illness and sleep. Untreated coeliac disease, poorly managed asthma, and hormone disorders can all suppress growth. Growth hormone is secreted predominantly during deep sleep, which is the practical reason sleep matters here rather than a folk belief.
The useful signal is not height itself but the trajectory. A child who has consistently tracked along the 10th percentile is very likely just a small child. A child who has been on the 50th and drops to the 10th over a couple of years is a different situation, and worth investigating.
Other things worth raising: a child much shorter than the mid-parental range predicts, growth that has visibly stalled, signs of puberty before 8 in girls or 9 in boys, or no signs by 13 and 14 respectively. A paediatrician can order a bone age X-ray, which compares skeletal maturity against chronological age and gives a considerably better prediction than any parental-height formula.
It gives a reasonable centre and a wide band. The commonly used prediction interval is ±8.5 cm, which spans roughly the 3rd to 97th percentile of outcomes — a 17 cm range. Published estimates of the interval vary from about ±5 cm to ±10 cm depending on the population studied. Treat the single target number as the midpoint of a broad range rather than a forecast, and be sceptical of any calculator that reports it to the nearest centimetre without a range.
Easily, and it is common. Height is influenced by hundreds of genes, and each parent passes on a random half of theirs — a child can inherit a combination that expresses taller than either parent does. Better nutrition than the parents had as children also pushes in that direction, which is why generational height gains have been so pronounced in countries where childhood nutrition improved rapidly. Siblings from the same parents often differ by 10 cm or more.
Typically around 14 to 16 for girls and 16 to 18 for boys, though there is wide individual variation. Growth ends when the epiphyseal plates — the growth plates near the ends of the long bones — fuse, a process driven by sex hormones during puberty. Girls generally finish earlier because puberty starts earlier. Some boys continue adding a small amount into their early twenties.
Often, yes. Later puberty means the growth plates stay open longer, so there is more time growing at pre-pubertal rates before the final spurt. This is the mechanism behind constitutional delay of growth and puberty, sometimes called being a "late bloomer" — the child is short through adolescence and then catches up substantially. It frequently runs in families, so a parent who was a late developer is informative.
Nothing beyond making sure nothing is holding them back. Adequate nutrition, enough sleep, treatment of chronic illness and normal physical activity let a child reach their genetic potential; none of them push past it. Supplements, stretching and specific sports do not add height — basketball players are tall because tall people succeed at basketball, not the other way round. Growth hormone therapy is a genuine medical treatment for diagnosed deficiency and specific conditions, not something to seek for a healthy short child.
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