How the prediction is calculated
The calculator uses the mid-parental height, also called target height, described by the British paediatrician James Tanner and colleagues in 1970. It is still the method doctors use to see whether a child's growth fits their family.
The idea is simple. Children tend to end up somewhere around the average of their two biological parents. Because men are on average about 13 cm taller than women, the formula first moves one parent's height by 13 cm so both are on the same scale:
- Boy: (mother's height + father's height + 13 cm) ÷ 2
- Girl: (mother's height + father's height − 13 cm) ÷ 2
The result is the target height. Around it sits a target range of 8.5 cm either way, where most children end up as adults.
Why it asks for the mother and the father
The formula is built on biological parents, because it is the genes they pass on that shape height. It also needs to know which height belongs to the mother and which to the father, because that decides which way the 13 cm correction goes. That is the only reason the calculator uses those words: it is not an assumption about who is raising the child. Step-parents, adoptive parents and partners matter for a child in every other way, but they do not change this sum.
A worked example
Say the biological mother is 165 cm and the father is 180 cm. Together that is 345 cm.
- For a boy: (345 + 13) ÷ 2 = 179 cm, with a range from 170.5 to 187.5 cm.
- For a girl: (345 − 13) ÷ 2 = 166 cm, with a range from 157.5 to 174.5 cm.
A brother and sister in this family would be about 13 cm apart on average, though real siblings often differ by more or less than that.
What the result means
The target is the middle of what is likely, not a promise. Two children of the same parents can easily end up 10 cm apart. Doctors use the target alongside a growth chart: a child who stays on a steady line near the family target is reassuring, while a child who drifts far away from it, especially downwards, may need a closer look.
The prediction is weaker at the extremes. Wright and Cheetham showed in 1999 that children of very tall parents tend to end up a little shorter than the formula says, and children of very short parents a little taller. Geneticists call this regression to the mean.
What else affects height
Genes explain most of the difference in height between people, but not all of it. Good nutrition, sleep and general health in childhood help a child reach their potential, and long-term illness can hold growth back. In many countries each generation has also been a little taller than the last, so children can end up slightly above the parents' average.
The timing of puberty changes when a child grows, not much how tall they end up. Early developers are often tall for their age and then stop sooner, while late developers catch up later.
Other ways to estimate adult height
Mid-parental height only looks at the parents. Once a child is a few years old, their own growth adds information: most children follow roughly the same centile line on a growth chart from the age of 2 or 3 until puberty. When a doctor needs a closer estimate, they can also take an X-ray of the hand and wrist to judge bone age, which shows how much growing time is left.
Limits of this calculator
This is a simplified estimate, and real inheritance is more complex: hundreds of genes each add a little to height. The formula does not apply to children with conditions that affect growth, and it cannot be used without both biological parents' heights. If you have any concern about how your child is growing, your doctor or health visitor can measure them and plot the result on a growth chart.