ReckonBoxHow the height calculator works
The mid-parental method predicts a child's adult height by taking the midpoint of the two parents' heights, then adding 6.5 cm for a boy or subtracting 6.5 cm for a girl. With a mother of 165 cm and a father of 180 cm the midpoint is 172.5 cm, so a son predicts to 179 cm and a daughter to 166 cm. Written as one step, that's the parents' heights plus 13 cm over two for a boy, and minus 13 cm over two for a girl.
The method traces back to Tanner, Goldstein and Whitehouse in 1970, and it's the calculation almost every consumer height predictor runs. What most of them leave out is how wide the answer really is.
How accurate is a height prediction?
A mid-parental prediction carries a spread of roughly 8.5 cm either side, about one standard deviation, so around two children in three finish inside that band. A prediction of 179 cm honestly means somewhere near 170.5 cm to 187.5 cm, a range 17 cm wide. WebMD says the same thing in imperial: predictions are usually within 4 inches of the height a child actually reaches.
That band is the number the top-ranked page never prints. calculator.net offers the calculation with no margin of error, no confidence band and no FAQ, and it doesn't publish the formula for the Khamis-Roche method it runs.
| Method | What it uses | Stated margin |
|---|---|---|
| Mid-parental | Both parents' heights | About 8.5 cm, or 4 inches |
| Khamis-Roche | Parents' heights plus the child's height, weight and age | 2.1 inches for boys, 1.7 inches for girls |
Why tall parents get shorter children than the formula says
Regression to the mean means children of very tall parents tend to finish shorter than the mid-parental figure predicts, and children of very short parents tend to finish taller. Francis Galton described the pattern more than a century ago, and a 2024 study in the journal Children measured the correlation at minus 0.47 across 303 adult children from 23 large families.
So the formula is at its weakest exactly where people are most curious about it: at the extremes. WebMD hints at this, noting estimates are less accurate when parents' heights sit far from average, without saying why. The same 2024 paper goes further and argues the constant 13 cm sex correction is itself imperfect, proposing a multiplicative factor of 1.08 instead, which cut the prediction bias in its data from 2.7 cm to 0.14 cm.
When is a prediction worth trusting?
Age changes the picture. Before about 8 the prediction is at its widest, because the timing of puberty is still unknown and a child's growth percentile can still move; after about 12 estimates firm up. Genetics accounts for somewhere between 60% and 80% of adult height, and the 2024 study put heritability at 80% under its improved model, which leaves a real share to nutrition, sleep, chronic illness and hormones.
One caveat on the population behind the numbers. The Khamis-Roche method was developed on a cohort of Caucasian children, calculator.net restricts its own version to Caucasian children aged 4 to 9, and the University of Saskatchewan tool says outright that its equations came from a white Caucasian population and may carry error for others.
What this calculator does not do
It runs the mid-parental method and stops there. Khamis-Roche is the more accurate calculation, and this page does not reproduce it, because its coefficient tables appear on none of the ten sources consulted and calculator.net runs it without printing the formula. Guessing at coefficients for a paediatric growth estimate would mean inventing medicine, so we implement the method whose formula is published and name the other one honestly.
Bone-age methods sit outside too, since they need a hand radiograph read by a clinician. This is an educational estimate from two numbers, and it cannot diagnose anything. Growth is assessed properly by plotting a child on a growth chart over time, so any concern about how a child is growing belongs with a paediatrician rather than a web form. For other everyday body measures, the BMI calculator and ideal weight calculator sit alongside this one.
Frequently asked questions
How do you predict a child adult height? The mid-parental method takes the midpoint of the two parents heights, then adds 6.5 cm for a boy or subtracts 6.5 cm for a girl. With a mother of 165 cm and a father of 180 cm the midpoint is 172.5 cm, so a son comes out at 179 cm and a daughter at 166 cm. It is an estimate of a range, not a measurement.
How accurate is the mid-parental height prediction? The spread is about 8.5 cm either side of the figure, which is roughly one standard deviation, so about two children in three finish inside that 17 cm band. WebMD puts the same idea in inches, saying predictions are usually within 4 inches of the eventual adult height. A prediction of 179 cm really means somewhere around 170.5 cm to 187.5 cm.
Why do very tall parents often have shorter children than predicted? Because of regression to the mean, a pattern Francis Galton described more than a century ago and which a 2024 study in the journal Children confirmed with a correlation of minus 0.47. Children of very tall parents tend to finish shorter than the mid-parental figure suggests, and children of very short parents tend to finish taller, so the formula overshoots at one end and undershoots at the other.
What is the difference between mid-parental height and the Khamis-Roche method? Mid-parental height uses only the two parents heights, while Khamis-Roche also uses the child current height, weight and age, which makes it more accurate: Omni Calculator gives its margin as 2.1 inches for boys and 1.7 inches for girls. This page implements the mid-parental method because its formula is published, and does not reproduce Khamis-Roche because its coefficients appear on none of the sources consulted.
At what age does a height prediction become reliable? Predictions are widest before about age 8, because the timing of puberty is still unknown and a child growth percentile can still shift, and they firm up after about age 12. Estimates also weaken when the parents heights sit far from average, which is where regression to the mean bites hardest.
Does the mid-parental method work for every population? The underlying standards come from Tanner, Goldstein and Whitehouse in 1970, and the related Khamis-Roche method was developed on a cohort of Caucasian children, which the sources say may make it less accurate for other groups. Nutrition, chronic illness, sleep and hormones all shift final height too, so treat the number as a rough family-level guide.
Is this a medical assessment of my child growth? No. This is an educational estimate from two numbers, and it cannot diagnose anything or replace a clinical growth assessment, which uses growth charts plotted over time and sometimes a bone-age radiograph. Any concern about how a child is growing belongs with a paediatrician.