Predict the adult height target
The simple genetics behind the formula
Adult height is one of the most heritable human traits: twin and family studies attribute roughly 80 percent of its variation to genes. The mid-parental height method, associated with the growth researcher James Tanner, turns that heritability into bedside arithmetic. Average the parents' heights, then adjust for the sex of the child: add 13 cm for a boy, subtract 13 cm for a girl, reflecting the average adult height difference between men and women.
So for a father of 180 cm and a mother of 165 cm, the parental average is 172.5 cm. A son's target is (180 + 165 + 13) / 2 = 179 cm. A daughter's target is (180 + 165 - 13) / 2 = 166 cm. The expected range extends 8.5 cm either way, so the son is expected to land between 170.5 and 187.5 cm and the daughter between 157.5 and 174.5 cm. About 95 percent of children do.
The method assumes the parents themselves reached their genetic potential, which is not always true: a parent who was malnourished or chronically ill in childhood may be shorter than their genes intended, which drags the prediction down. It also assumes average puberty timing; very early or very late puberty shifts the outcome within the range.
What the target is used for in clinic
The target height is not a party trick; it is a clinical tool with two main jobs. First, it sets expectations: a child tracking toward 160 cm whose target is 178 cm is a different case from one whose target is 162 cm. Second, it converts an absolute height into a meaningful deviation. Paediatric endocrinologists express this as the difference between the child's height z-score and the target height z-score: a large gap suggests the child is not reaching their potential and deserves investigation.
In practice, a child whose projected adult height falls well below the target range, or whose current growth curve points away from it, gets worked up for the treatable causes of poor growth: hypothyroidism, coeliac disease, growth hormone deficiency, chronic systemic disease. A child growing happily toward the target is usually left alone, even if the absolute centile looks low.
Worked examples
Example 1. Father 180 cm, mother 165 cm, boy. Target = (180 + 165 + 13) / 2 = 179.0 cm; range 170.5 to 187.5 cm.
Example 2. Father 175 cm, mother 160 cm, girl. Target = (175 + 160 - 13) / 2 = 161.0 cm; range 152.5 to 169.5 cm.
Example 3. Father 170 cm, mother 170 cm, boy. Target = (170 + 170 + 13) / 2 = 176.5 cm; range 168.0 to 185.0 cm. Note the method still adds the 13 cm sex adjustment even when the parents are the same height.
Limits and common misunderstandings
The prediction is probabilistic, not personal: it says where children like this one usually end up, not where this one will. Five percent fall outside the range through normal variation alone. Secular trends add a further wrinkle: each generation in well-nourished populations has grown slightly taller than the last, so targets based on parents measured decades ago can underestimate slightly.
It also cannot account for the child's own medical history. A child who had prolonged undernutrition, chronic disease or high-dose steroids will not necessarily reach the target, and a child with precocious puberty may stop growing early and finish below it. The target describes potential; the growth chart and the history describe whether potential is being realised. When the two disagree, believe the chart and investigate.
Using the target through childhood
The target is most useful as a moving reference rather than a one-off number. At each visit, the clinician can ask a simple question: is this child's growth curve heading toward the target range? A 6-year-old girl on the 10th centile whose target range centres on the 15th is roughly on track; the same centile in a girl whose target centres on the 75th suggests she is falling short of her potential even though the absolute centile is not alarming.
This comparison becomes formal in endocrinology as the difference between height z-score and target height z-score, but the intuition works without any statistics. Parents often find the target reassuring in both directions: it explains why a healthy short child of short parents needs no investigation, and it gives a concrete reason to investigate a tall family's child who is not keeping up. Keep the target written in the growth chart so every future measurement is read against it.
One caveat for the early years: do not over-interpret the gap between a toddler's centile and the adult target. Infant and toddler growth is noisy, influenced by feeding transitions and early illness, and centiles commonly shift in the first two years before settling onto the child's true curve. The target comparison becomes meaningful once the child has settled, usually from around age 3 onwards.
Key takeaways
- Target = (father + mother + 13) / 2 for boys, minus 13 for girls; the range is target plus or minus 8.5 cm.
- About 95 percent of children reach adult height inside the target range.
- The target sets expectations and helps judge whether a child is reaching their genetic potential.
- Parental illness or undernutrition in childhood can make the prediction less reliable.
- A child growing away from their target deserves assessment even if the absolute centile looks fine.
Frequently asked questions
Why 13 cm? Men and women differ by more than that in some countries.
The 13 cm is the average adult sex difference in the reference populations Tanner worked with, and it has held up well as a clinical convention. Real populations vary, but changing the constant would require revalidating the whole method. For clinical use, consistency matters more than the exact centimetre.
Should I use the parents' current heights or their young-adult heights?
Their young-adult heights, before age-related shrinkage. A 65-year-old father may have lost 2 to 3 cm from his peak; using the shrunken height underestimates the target. Ask what they measured in their twenties if possible.
My child is already taller than the target range. Is that a problem?
Not usually. Being above the range is far less concerning than being below it, and tall parents often have children who overshoot slightly. It becomes worth discussing only if growth is accelerating abnormally, puberty is very early, or there are syndromic features suggesting an overgrowth condition.
Does nutrition really change adult height within families?
Yes. The target assumes optimal conditions; siblings can differ by several centimetres when one had a prolonged illness or period of undernutrition and the other did not. Population data show the same effect at scale: well-nourished generations outgrow their parents' generation.
Can the target predict exactly how tall my child will be?
No. It gives the centre and range of the likely outcome, not a single destiny. Think of it as the most probable neighbourhood rather than a street address. The child's own growth curve, measured over time, is always the better guide to what is actually happening.