Child Height Predictor

The mid-parental method

The most common prediction takes the average of both parents' heights and adjusts by sex: add 6.5 centimetres for a boy, subtract 6.5 for a girl.

It is simple, it captures the substantial genetic component of height, and it comes with a wide confidence interval: roughly plus or minus 8.5 centimetres, meaning the true adult height falls within a 17 centimetre range around the prediction for most children.

That interval is rarely quoted alongside the number, which encourages parents to treat a prediction as a forecast. A prediction of 178 centimetres genuinely means somewhere between about 170 and 187.

Other methods

The Khamis-Roche method uses the child's current height and weight along with mid-parental height and is more accurate, particularly for children between four and seventeen.

Bone age assessment, from a hand and wrist X-ray, is the most accurate non-invasive method and is used clinically. It assesses skeletal maturity, which indicates how much growth remains.

The doubling method for two year olds, doubling the height at age two for boys and at 18 months for girls, is a rough approximation with a wide error and is more folk wisdom than clinical tool.

What determines adult height

Genetics accounts for roughly 80% of the variation in height within a population, which is why parental height is such a strong predictor.

The remaining variation comes from nutrition, particularly protein and micronutrients in early childhood, from chronic illness, and from hormonal factors. Population average heights have risen substantially over the last century, which reflects environment rather than genetics.

Which means a well-nourished child in a stable environment will land close to their genetic potential, and the prediction is describing that potential rather than a certainty.

Growth patterns

Growth is not steady. Infancy is rapid, childhood is slower and relatively constant, and puberty brings a growth spurt that accounts for around 20% of adult height.

Timing of puberty varies considerably, and early or late development changes when the spurt occurs without necessarily changing the final height. A late developer may be the shortest in their class at fourteen and taller than most by eighteen.

Growth generally completes around 16 in girls and 18 in boys, when the growth plates fuse. That timing is what bone age assessment measures.

When to see a doctor

Growth charts matter more than predictions. A child tracking consistently along a centile is growing normally regardless of which centile it is.

What warrants attention is crossing centiles, particularly downwards, or a growth rate that slows markedly. That pattern can indicate a nutritional, hormonal or chronic health issue and is worth investigating.

Height well outside the range expected from parental heights, or a child who is very short or very tall relative to peers with no family explanation, is also reasonable to raise. This calculator produces a number from three inputs; a clinician looks at the whole growth history, which is a far better source of information.

It is worth being direct about how these predictions get used, because that is where the harm sits. A number produced from three inputs occasionally becomes something a child hears about themselves, and height is not something anyone chooses or can change. The prediction has a 17 centimetre range and describes potential rather than destiny, and it is a poor thing to attach expectations to. Where it is useful at all, it is as reassurance for a parent worried about a child at the short end of their class, and the growth chart at the surgery answers that question far better.