Measure Height 
How the calculation works
The method comes from J. M. Tanner and colleagues (Archives of Disease in Childhood, 1970) and it is still the one printed in pediatric references:
- Son: (father + mother + 13 cm) ÷ 2
- Daughter: (father + mother − 13 cm) ÷ 2
- Target range: the result plus or minus 8.5 cm, which Tanner set to cover the 3rd to 97th percentile of children of those parents.
In inches it is the same idea with 5 inches instead of 13 cm. The calculator works in centimetres internally and converts at the end, so switching units never changes the answer.
The percentile line under each result places the target on the CDC adult height distribution (the age-20 point of the CDC 2000 stature charts), so you can see whether the family is tall, average or short by US standards.
Worked examples
| Father / mother | Son target | Pct | Daughter target | Pct |
|---|---|---|---|---|
| 180 cm / 165 cm | 179.0 cm (5'10.5") | 62nd | 166.0 cm (5'5.5") | 66th |
| 5'10" / 5'4" | 176.7 cm (5'9.5") | 49th | 163.7 cm (5'4.5") | 52nd |
| 6'2" / 5'8" | 186.8 cm (6'1.5") | 92nd | 173.8 cm (5'8.5") | 95th |
| 170 cm / 155 cm | 169.0 cm (5'6.5") | 14th | 156.0 cm (5'1.5") | 13th |
Take the first row by hand: 180 + 165 = 345. Add 13 for a son, 358, halve it, 179 cm. The range runs from 170.5 to 187.5 cm. For a daughter, 345 − 13 = 332, halved 166 cm, range 157.5 to 174.5 cm.
What the target is good for, and what it is not
Pediatricians use this number as a sanity check on the growth chart. A child tracking the 10th percentile with parents whose target is also around the 10th is a small child from a small family, and nothing more. The same child with a target at the 75th percentile is worth a closer look.
It is not a prediction of the exact adult height. Two things work against it:
- Regression to the mean. Children of very tall parents tend to be tall but a little less tall than the formula says, and children of very short parents a little less short. The simple formula ignores this, so at the extremes it overshoots.
- The parents' heights are often wrong. Self-reported heights skew tall, especially for men. A father who says 6'0" and measures 5'10.5" moves the son's target by three quarters of an inch.
If you know the child's current height and age, the height predictor projects from where they actually are on the growth chart, and shows this target next to it.
Questions
What is the mid-parental height formula?
Add the father's and mother's heights. For a boy add 13 cm, for a girl subtract 13 cm, then divide by 2. In inches the adjustment is 5 inches. The normal range is the result plus or minus 8.5 cm (about 3.3 inches).
How accurate is mid-parental height?
It gives a range, not a number, and the range is wide on purpose: 17 cm from end to end. It is good at flagging a child growing far away from where their family would put them. It is poor at telling you whether a child will be 5'9" or 5'11".
Why add or subtract 13 cm?
Thirteen centimetres is roughly the average gap between adult men and women. The formula converts the opposite-sex parent to the child's sex before averaging. On the CDC charts the gap between the adult male and female medians is 13.5 cm, so the number still holds.
Can a child end up taller than both parents?
Yes. The target is the middle of a range, and a son of two average-height parents can easily land at the top of it. Nutrition and health in childhood also matter, which is one reason each generation in many countries ended up taller than the last.
Does it work for adopted children or with a step-parent?
Only biological parents count, since the formula is a stand-in for inherited height. With one biological parent unknown there is no reliable way to run it.
Sources: Tanner JM, Goldstein H, Whitehouse RH, Standards for children's height at ages 2 to 9 years allowing for height of parents, Arch Dis Child 1970. CDC 2000 growth charts, stature-for-age LMS data. An estimate, not medical advice.