Your son's height at age 12 is not the number that predicts his adult height. Your family's math is. A boy at the 25th percentile today could still hit 5'11" as an adult if his family says he will. And a boy at the 60th percentile at 12 could top out at 5'8" if his family says that is where he lands.
The mid-parental height formula is how pediatricians make that prediction. It has been used for decades. It is not perfect, but it is the single best predictor of a child's adult height that you can compute at your kitchen table, without a bone age X-ray, without a genetic test, and without paying anyone anything.
The formula (for boys)
Target height = (father's height + mother's height + 5 inches) ÷ 2
The "plus 5 inches" adjusts for the fact that boys, on average, end up about 5 inches taller than girls. So we shift mom's height up by 5 inches before averaging.
The predicted range is roughly plus or minus 4 inches from the target. That is a wide range, but it captures about 95% of outcomes.
Worked example
Dad is 5'8" (68 inches). Mom is 5'4" (64 inches).
- Target: (68 + 64 + 5) ÷ 2 = 137 ÷ 2 = 68.5 inches = 5'8.5"
- Likely range: 64.5 to 72.5 inches = 5'4.5" to 6'0.5"
Their son will most likely land somewhere between 5'4.5" and 6'0.5", with 5'8.5" being the statistical center.
The formula for girls
Target height = (father's height + mother's height - 5 inches) ÷ 2
Same idea in reverse: girls end up about 5 inches shorter than boys on average, so we shift dad's height down.
Why this matters more than his current percentile
Here is what most parents miss. The CDC growth chart shows you where your son ranks compared to all American boys his age. But "all American boys" includes the sons of NBA players and the sons of jockeys. It is a giant averaged bell curve that has almost nothing to do with your specific family's genetics.
Your son's adult height is going to be driven by his parents' heights, his grandparents' heights, and a bit of nutritional and environmental luck. The CDC chart is useful for tracking whether he is growing steadily. The mid-parental formula is what tells you where he is likely to end up.
When these two numbers agree, everything is fine. When they disagree, that is the interesting information.
How to read what the formula tells you
Case A: Current percentile roughly matches predicted adult height
He is at the 40th percentile now, family math predicts he lands around the 40th percentile as an adult. This is the boring case, and the boring case is almost always the right answer. He is on track.
Case B: Current percentile is LOWER than family math predicts
He is at the 15th percentile now, family math predicts he should be 50th percentile as an adult. Two possibilities: (1) he is a late bloomer and will catch up, (2) something is slowing his growth. Look at growth velocity to distinguish. Steady velocity plus family history of late blooming: probably a late bloomer. Slowed velocity or falling percentiles: worth a pediatrician conversation.
Case C: Current percentile is HIGHER than family math predicts
He is at the 75th percentile now, family math predicts he should be 30th percentile. Rare but happens. Usually early bloomer. He may finish earlier and end up closer to the family prediction than his current percentile would suggest.
Where the formula gets it wrong
The formula is a heuristic, not a prophecy. It misses in a few specific cases:
- Undiagnosed genetic conditions. Turner syndrome in girls, some rare pituitary conditions, Klinefelter syndrome. These change the outcome.
- Chronic malnutrition or serious untreated chronic illness. These slow growth in ways the formula cannot account for.
- Extremely tall or extremely short parents. The formula assumes normal-distribution genetics. At the tails, actual outcomes drift toward the population mean.
- Different parents than the birth parents. If a step-parent, adoptive parent, or otherwise unrelated adult raised your son, use the biological parents' heights.
For the vast majority of families, the formula gets within 2 inches of the actual adult height.
Run it against your son right now
The Growth Check on The Growth Path runs both calculations at once: his current CDC percentile and his mid-parental family range. In the same view. It takes 60 seconds and it costs nothing.
The answer you get is the same answer a pediatrician would compute for you, minus the co-pay and the wait.