TallPlanThe formula
Take both parents’ heights and adjust for sex. Grown men are on average about 13 cm (5 inches) taller than grown women, so a boy’s target sits half that gap above the parents’ average, and a girl’s sits half that gap below it.
For a boy: (father’s height + mother’s height + 13 cm) ÷ 2
For a girl: (father’s height + mother’s height − 13 cm) ÷ 2
In inches, use 5 instead of 13.
To skip the arithmetic, the height calculator does it for you, in centimetres or feet and inches.
The result is called the mid-parental height, or target height. James Tanner and his colleagues published it in 1970, and it is still widely used as a first reference point.
A worked example
Say the father is 180 cm and the mother is 165 cm.
| Step | Boy | Girl |
|---|---|---|
| Both parents together | 345 cm | 345 cm |
| Adjust for sex | +13 cm | −13 cm |
| Divide by 2 | 179 cm | 166 cm |
The same family in inches: a 71 in father and a 65 in mother give (71 + 65 + 5) ÷ 2 = 70.5 in for a son, about 5′10½″, and (71 + 65 − 5) ÷ 2 = 65.5 in for a daughter, about 5′5½″.
The number is the middle of a range
Mid-parental height tells you where a child’s adult height is most likely to land, not where it will land. The American Academy of Pediatrics gives a margin of error of about 10 cm (4 inches) either way. Brothers and sisters with the same parents can end up that far apart.
The range is wide because the formula knows very little:
- Each child gets a different mix of genes from the same two parents.
- It assumes a child takes equally after both parents.
- It knows nothing about the child: when puberty starts, how fast they are growing now, their health or what they eat.
- Very tall or very short parents tend to have children a little closer to average height than the formula suggests.
Getting a better number from it
- Measure both parents. Stand against a wall without shoes and mark the top of the head. Remembered heights are often a little generous.
- Use both parents. With one height unknown, a guess for that parent can move the result by several centimetres.
- Compare with the child’s own growth. On a growth chart, many children follow roughly the same percentile line through childhood. A child far from the line their family’s range points to is worth a closer look.
Other ways to estimate adult height
- Growth charts. Plotting height by age shows which percentile a child follows. The CDC growth charts are the standard reference in the US from age 2; under 2, the CDC recommends the WHO charts. Puberty timing can move a child off their line for a while.
- Methods that add the child’s own data. Some estimates, such as Khamis–Roche, combine the parents’ heights with the child’s current height and weight, which usually narrows the range.
- Bone age. An X-ray of the hand or wrist shows how much growing is left. Doctors use it when growth or puberty seems unusual; it isn’t a routine test.
How TallPlan uses it
TallPlan starts from the mid-parental height, then adjusts for age, sex, current height, tall relatives, puberty signs and how fast you are growing, and shows the result as a range. After about six months of weekly measurements it can use your measured growth speed, and the range can narrow. It is an estimate, not a clinical test, and it never promises extra height.
When to see a doctor
Any time something about growth or puberty worries you, ask a doctor. They can measure properly and plot it on a growth chart. Some common reasons to ask:
- A child is far outside their family’s range, or keeps dropping across percentile lines on a growth chart.
- Growth slows down a lot or stops before puberty is well under way.
- Signs of puberty start before age 8, or haven’t started by 13 in girls or 14 in boys.
- Worry about height is affecting mood, eating or sleep.
Sources
- Tanner JM, Goldstein H, Whitehouse RH. Standards for children’s height at ages 2–9 years allowing for heights of parents. Archives of Disease in Childhood, 1970.
- American Academy of Pediatrics, HealthyChildren.org. How tall will my child be? Predicting a child’s adult height.
- Wright CM, Cheetham TD. The strengths and limitations of parental heights as a predictor of attained height. Archives of Disease in Childhood, 1999.
- Khamis HJ, Roche AF. Predicting adult stature without using skeletal age: the Khamis-Roche method. Pediatrics, 1994.
- CDC. Growth charts.
- American Academy of Pediatrics, HealthyChildren.org. Delayed puberty in girls: information for parents.
- American Academy of Pediatrics, HealthyChildren.org. Delayed puberty in boys: information for parents.
