How Much of Height Is Written in the Genes?

The companion calculator predicts a child's adult height from the parents' heights and gives a wide range around it. The width of that range is the interesting part. It reflects a genuine scientific truth: height is strongly inherited, yet far from fully determined by parents, and several forces pull a child's eventual height away from a simple average.

Strongly Genetic, But Not Only Genetic

Within a well-nourished population, studies of families and twins suggest height is highly heritable, often estimated around 80 percent. That is why a formula based on parental heights works at all. But heritability is a statement about variation within a population under given conditions, not a guarantee for one child, and the remaining share left to environment, especially nutrition and health during growth, is large enough to move a real child meaningfully. Hundreds of genes each contribute a little, which is why height varies so smoothly rather than in a few discrete outcomes.

The Secular Trend: Whole Populations Got Taller

The clearest proof that environment matters is history. Across much of the world, average adult height rose substantially over the last century or so, a shift called the secular trend, driven largely by better childhood nutrition, reduced disease burden, and improved living conditions. The Netherlands is the textbook case, its population going from around average to among the tallest in the world within a few generations, far too fast to be genetic change. Genes did not rewrite themselves; the environment that genes express themselves in improved.

Why Very Tall Parents Often Have Shorter Children

A statistical effect called regression to the mean shapes these predictions.

The reason is that extreme height reflects both a genetic push and a dose of chance factors that are unlikely to line up as favorably in the next generation. This is not a flaw in the prediction; it is built into why the estimate carries such a wide range rather than a precise number.

The Actual Clock: Growth Plates and Bone Age

Beneath all of this, a child's real growth timeline runs on their bones. Long bones lengthen at zones called growth plates, which stay open through childhood and close after puberty, at which point height is essentially final. Doctors can assess a child's "bone age" from an X-ray to see how much growth potential remains, a far more direct read on an individual than any parent-based formula. Two children the same age can have quite different amounts of growing left.

Holding the Prediction Loosely

Use this calculator as a fun, rough midpoint with a deliberately wide range, not a forecast. A child's actual path depends on their own genetic mix, their nutrition and health, and their individual growth timing. If there are real concerns about a child growing much faster or slower than expected, a pediatrician can assess growth trajectory and, when warranted, bone age, which tells far more than parental heights ever could.