Child Height Predictor

About the Child Height Predictor

"How tall will they be?" is one of the oldest questions parents ask, and there is a standard, well-tested way to answer it — with a range rather than a single number, because genes set likelihoods, not outcomes.

The method doctors use is the mid-parental height, devised by the British paediatrician James Tanner. It averages the parents' heights after adjusting for the average difference between men and women, and it comes with a target range that covers about 95% of children of those parents.

This child height predictor gives the mid-parental height and the target range in centimetres or feet and inches. If you know your child's height at two years (for a boy) or eighteen months (for a girl), it also shows the popular doubling rule alongside, for comparison.

How to Use the Child Height Predictor

Choose metric or imperial units and your child's sex.

Enter the father's and mother's heights. Measured heights are better than remembered ones — people tend to overestimate their own height by a centimetre or two.

Optionally, enter your child's height at 2 years (boys) or 18 months (girls) to see the doubling rule.

How the Mid-Parental Height Works

  Boys:   (father's height + mother's height + 13 cm) ÷ 2
  Girls:  (father's height + mother's height − 13 cm) ÷ 2
  Target range:  mid-parental height ± 8.5 cm

The 13 cm is the average difference in adult height between men and women. Adding it to the mother's height converts her height to a "male equivalent" before averaging for a boy; subtracting it from the father's converts his to a "female equivalent" for a girl. That puts both parents on the same scale.

In inches, the adjustment is about 5 inches and the range about ±3.3 inches.

Step-by-Step Example

Father 178 cm, mother 165 cm, boy.

  (178 + 165 + 13) ÷ 2  =  178 cm   (5′ 10″)
  Target range:           170 to 187 cm   (5′ 7″ to 6′ 1″)

For a daughter of the same parents:

  (178 + 165 − 13) ÷ 2  =  165 cm   (5′ 5″)
  Target range:           157 to 174 cm

The brother and sister would be expected to differ by about 13 cm — the same as the average difference between men and women.

Why the Range Matters More Than the Number

Children inherit a mix of genes from each parent, and which mix they get varies. That is why siblings with the same parents can differ by 10 cm or more, and why the mid-parental height is only the middle of a band.

About 95% of children end up within roughly 8.5 cm either side of the mid-parental figure. So for the example above, a son finishing anywhere between 170 and 187 cm would be entirely in keeping with his family.

Doctors use this range not to promise an outcome but to check growth. A child whose growth curve is heading well outside the family's range may be worth a closer look, while a child who is small but tracking within it is usually just following their genes.

The Doubling Rule

Many parents have heard that you can double a toddler's height to predict their adult height: a boy's height at 2 years, or a girl's at 18 months (girls reach the equivalent point in growth earlier).

It works surprisingly often, but it is a rule of thumb rather than a validated method, and it comes with no stated margin of error. The calculator shows it beside the mid-parental figure; if the two roughly agree, that is reassuring, and if they differ widely, neither should be taken too literally.

What Else Affects Adult Height

Genes set the likely range, but where a child ends up within it depends on more:

Nutrition throughout childhood, especially in the early years.

General health — chronic illness, untreated coeliac disease or hormone problems can all hold growth back.

Sleep, when growth hormone is mostly released.

The timing of puberty — early developers often stop growing earlier, late developers later; both usually end up in the same place.

Average heights have also risen over generations in many countries as nutrition and health improved, which is one reason children often end up slightly taller than their parents' figures predict.

Understanding Your Result

Predicted adult height is the mid-parental height.

Likely range is the target range covering about 95% of children of those parents.

In both units gives the figures in centimetres and feet and inches.

Doubling rule shows the rule-of-thumb estimate if you entered an early height.

Worth knowing explains how to read the result and when growth is worth checking.

When Should You Use This Calculator?

Out of curiosity, which is how most people use it.

To put a child's height in family context — a small child of short parents is usually just following their genes.

Before talking to a doctor about growth, to have the family range to hand.

Not to diagnose a growth problem. Measurements over time on a growth chart do that.

Common Mistakes

Treating the prediction as exact. The range is ±8.5 cm.

Using remembered rather than measured heights. Self-reported heights run high.

Forgetting the sex adjustment. Plain averaging underestimates boys and overestimates girls.

Worrying about one measurement. Growth over time is what matters.

Relying on the doubling rule alone. It is a rough rule of thumb.

If you are concerned about your child's growth, your doctor or health visitor can measure and chart it properly. Nothing here is medical advice.