How Tall Will My Child Be? What the Prediction Can and Cannot Tell You

There is a simple formula for predicting a child’s adult height, it is the one paediatricians actually use, and it is far less precise than the confident single number suggests.

Both halves of that sentence matter.

The mid-parental method

boy:  (mother's height + father's height) ÷ 2 + 6.5 cm
girl: (mother's height + father's height) ÷ 2 − 6.5 cm

The 6.5 cm is half the average adult height difference between men and women, which is around 13 cm across most populations. Averaging the parents estimates the family’s genetic contribution; the adjustment shifts that onto the right distribution for the child.

For a mother of 164 cm and a father of 178 cm, the midpoint is 171. A son is predicted at 177.5 cm, a daughter at 164.5 cm.

An older version of this added or subtracted 13 cm to one parent’s height before averaging. It gives the same answer with more opportunity for confusion, which is why the version above has largely replaced it.

The range is 20 cm wide

Around 95% of children finish within about 10 cm either side of the mid-parental estimate.

That is a 20 cm window. For a predicted 177.5 cm, it means somewhere between roughly 167 and 187. The estimate cannot distinguish between a short man and a tall one.

Anyone quoting a prediction to the centimetre is overstating what the method can do.

The reason for the width is that height is not inherited as an average. It is polygenic, influenced by hundreds of variants, and a child receives a random half of each parent’s set. Two parents of identical height can produce children who differ substantially from one another.

There is also regression toward the mean: very tall parents tend to have children slightly shorter than themselves, and very short parents tend to have children slightly taller. The formula does not correct for this, so it drifts at the extremes.

This is, incidentally, where the statistical term “regression” comes from. Francis Galton coined it in the 1880s studying exactly this phenomenon in exactly this data.

What the environment contributes

Within a well-nourished population, roughly 80% of height variation is genetic. The remaining 20% is environment, and it is not evenly spread.

Nutrition, particularly protein and micronutrients during infancy and puberty. This is the largest single environmental factor and it is why average heights in many countries rose 10 cm or more over the twentieth century, far too fast for genetics to explain.

Sleep. Growth hormone is released in pulses, largely during deep sleep. Chronic sleep restriction through the growing years has a measurable effect.

Chronic illness. Untreated coeliac disease, inflammatory bowel disease, kidney disease and severe asthma can all suppress growth, and treating them frequently restores it.

Early-life health. Repeated infections in infancy divert energy from growth, which is much of why height correlates with childhood living conditions at a population level.

Notably absent: specific foods, supplements, stretching, hanging from bars, and playing basketball. There is no evidence that any of these increase final adult height in a healthy, well-fed child.

The pattern matters more than the number

A single height measurement tells you very little. The trajectory tells you almost everything.

Children grow along a percentile line, and which line they are on matters far less than whether they stay on it.

A child consistently on the tenth percentile is a small child in a family of small people. That is not a medical finding. A child who was on the fiftieth percentile at four and the tenth at eight has crossed two major lines, and that is worth investigating regardless of how tall they are in absolute terms.

Rough growth velocity through childhood:

• First year: about 25 cm
• Second year: about 12 cm
• Age three to puberty: 5 to 6 cm a year
• Peak of the pubertal spurt: 8 to 12 cm a year

Less than about 4 cm a year between age four and puberty is the usual threshold for a referral.

Puberty scrambles everything temporarily

Girls typically begin their growth spurt around 10 or 11 and finish growing between 14 and 16. Boys begin around 12 or 13 and finish between 16 and 18.

That two-year offset produces the familiar period between roughly 11 and 13 when the girls in a class are visibly taller than the boys. It reverses completely and it tells you nothing about final height.

Late developers are the group most often worried about unnecessarily. A boy who is short at 14 because puberty has not started has more growing time ahead of him than his peers, not less. Constitutional delay of growth and puberty usually runs in families and typically resolves with an entirely normal adult height.

If one parent was a late developer, that is relevant history and worth mentioning to a doctor, because it makes the benign explanation considerably more likely.

When to actually be concerned

Height alone is rarely the signal. What warrants a conversation:

• Crossing downward through two or more percentile lines
• Growth of less than 4 cm a year between four and puberty
• Height well below the mid-parental range with no family explanation
• Growth that has stalled entirely
• Height concerns alongside other symptoms: fatigue, digestive problems, delayed puberty

A growth chart covering the last few years is worth vastly more than any formula. A doctor can read one in about a minute and get more from it than any calculation provides.

What to do with the number

Treat it as a family expectation with wide error bars, and mostly as a curiosity.

It is genuinely useful for one thing: reassurance. Parents worried that a child is short often find the child is tracking exactly where the family arithmetic predicts, which is the answer they actually wanted.

It is not useful for planning, for sport selection, or for setting expectations that a child will then feel measured against. A prediction with a 20 cm range should not become something anyone has to live up to.