1. What the mid-parental method is
The mid-parental method (sometimes called the Tanner method, after the paediatrician James Tanner who formalised it) estimates a child’s adult height from the heights of the child’s two biological parents. It is the standard first step in paediatric growth assessment: a doctor plots a child’s height against the parents’ heights to judge whether the child is growing as expected for their family.
The method needs only three inputs — father’s height, mother’s height, and the child’s sex — and it deliberately does not use the child’s current height. That is both its strength and its limit: it works for a newborn as well as for a teenager, because it estimates the genetic target rather than extrapolating from today’s measurement. For an older child, a doctor would combine it with the child’s own growth curve; this tool gives the family-target part alone, and says so.
2. How the calculation works
First the parents’ heights are averaged: add father’s and mother’s heights and divide by two. Then adjust for sex, because average adult height differs between men and women by about 13 cm (about 5 inches): add 13 cm for a boy, subtract 13 cm for a girl, then divide by two. In full: a boy’s mid-parental height is (father + mother + 13 cm) ÷ 2, and a girl’s is (father + mother − 13 cm) ÷ 2.
The 13 cm adjustment is an average sex difference, not a rule about any individual family — in some families the parents differ by much more or much less than 13 cm, and the formula still applies the same average adjustment. That is one reason the result is a range, not a point: the formula captures the population average, and individual families vary around it.
For the same parents, the boy’s and girl’s estimates always differ by exactly 13 cm, because the only difference between the two formulas is +13 versus −13, halved twice over. The calculator shows both, so one set of parents’ heights answers the question for any child in the family.
3. What the ±8.5 cm range means
The headline number is the centre of a range, and the range is the honest answer. The standard target range is the mid-parental height ± 8.5 cm (about ±3.3 inches), a total width of 17 cm. Most children reach an adult height inside that range. A child landing near the top or the bottom of it is not unusual — that is what the range is for.
It helps to read the range as a probability, not a promise. If the estimate is 179 cm with a range of 170.5–187.5 cm, the child is most likely to end up somewhere in that span, more likely near the middle than at the edges, and possibly outside it. Nothing in the calculation can narrow it further from parents’ heights alone — narrowing it needs the child’s own growth data, which is a doctor’s job, not a calculator’s.
Two common misreadings are worth avoiding. First, the range is not an error bar on the calculator — it is the biology: children of the same parents genuinely vary by this much. Second, landing outside the range is not by itself a problem, but it is the point at which a paediatrician would want to look closer, which is why the range exists.
4. Worked examples, verified by the calculator
Father 180 cm, mother 165 cm. For a boy: (180 + 165 + 13) ÷ 2 = 179.0 cm (5 ft 10.5 in), with a range of 170.5–187.5 cm. For a girl: (180 + 165 − 13) ÷ 2 = 166.0 cm (5 ft 5.5 in), with a range of 157.5–174.5 cm.
Father 175 cm, mother 160 cm. For a boy: (175 + 160 + 13) ÷ 2 = 174.0 cm (5 ft 8.5 in), with a range of 165.5–182.5 cm. For a girl: (175 + 160 − 13) ÷ 2 = 161.0 cm (5 ft 3.5 in), with a range of 152.5–169.5 cm.
Notice the pattern in both examples: the boy’s estimate is exactly 13 cm above the girl’s, and each range is exactly 17 cm wide. Those two constants — 13 cm between the sexes, 17 cm of range — hold for every pair of parents’ heights, because they are built into the formulas.
5. What moves a child within — or outside — the range
Genetics accounts for most of the variation in height: studies of twins put the genetic share at roughly 80%, which is why parents’ heights predict as well as they do. But “most” leaves room. Nutrition in childhood, chronic illness, sleep, and the timing of puberty all shift where inside — or outside — the range a child lands.
Puberty timing matters more than most parents expect. A child who enters puberty early tends to be taller than peers for a while and then stop growing sooner; a late bloomer tends to be shorter for longer and then catch up. Both can end up at the same adult height, inside the same range, by different routes — which is why a single childhood measurement never settles the question.
What the range cannot absorb is a sustained problem: long-term undernutrition, untreated hormone deficiency, or a chronic condition that suppresses growth will push a child below the range, not just to its bottom edge. That distinction — inside the range versus below it — is the practical use of the whole calculation.
6. When to ask a paediatrician
This tool is general information, not medical advice, but three situations are worth a conversation with a paediatrician rather than another calculation. First, a child tracking well below the range, or dropping across growth percentiles over time. Second, growth that has slowed markedly or stopped well before puberty is complete. Third, puberty that starts very early or very late, or a child whose height is a source of distress.
A doctor will do what this tool cannot: measure the child accurately, plot the growth curve against standard charts, check the parents’ heights (reported heights are often overstated by a centimetre or two), and, if warranted, look for medical causes. Bring the mid-parental range to that appointment — it is the number the doctor will compute first anyway.
7. What this predictor cannot do
The formula needs the biological parents’ heights. For an adopted child, the adoptive parents’ heights do not carry the genetic information the method relies on, and the tool will give a misleading answer if used with them. Where a biological parent’s height is unknown, the method cannot be completed — the calculator does not guess or substitute an average, because a guessed input would produce a confident-looking but baseless result.
The method also says nothing about when growth happens, only where it is likely to end. It does not predict height at any particular age, does not account for medical conditions or treatments, and does not adjust for ethnicity or population differences beyond the average 13 cm sex adjustment. Two children of the same parents can differ by more than the range if a medical factor is involved — the range describes typical variation, not a guarantee.
Finally, the result is an estimate for one child, not a family average. Siblings share the same mid-parental height and range, but each child lands somewhere in the range independently — which is why brothers and sisters routinely differ by several centimetres.
9. Frequently Asked Questions
How accurate is a child height predictor?
It gives a range, not an exact height. Most children land within ±8.5 cm of the mid-parental height. The centre is the best single estimate from parents’ heights alone; the range is the honest answer.
What is mid-parental height?
The average of the two biological parents’ heights, adjusted for sex: (father + mother + 13 cm) ÷ 2 for a boy, (father + mother − 13 cm) ÷ 2 for a girl.
Why add 13 cm for boys and subtract it for girls?
Because average adult height differs between men and women by about 13 cm (about 5 inches). The adjustment centres the estimate on the child’s sex.
What does the ±8.5 cm range mean?
It is the standard target range: most children reach an adult height within 8.5 cm above or below the mid-parental height, a total span of 17 cm.
Can I use it for a baby or toddler?
Yes. The method needs only the parents’ heights, not the child’s current height, so it works at any age. For older children a doctor would also use the child’s own growth curve.
What if my child is adopted?
The method needs the biological parents’ heights. Using adoptive parents’ heights would give a misleading result, so the tool should not be used that way.
What if one parent’s height is unknown?
The method cannot be completed. The calculator does not guess or substitute an average — a guessed input would produce a baseless result.
Why is the boy’s estimate exactly 13 cm above the girl’s?
Because the two formulas differ only in the sex adjustment (+13 versus −13), and halving that difference twice over leaves exactly 13 cm.
Can nutrition change my child’s adult height?
Within limits. Good nutrition supports reaching the genetic range; sustained undernutrition or illness can push a child below it. Normal variation in diet moves a child within the range, not beyond it.
When should I see a doctor about my child’s height?
If the child tracks well below the range, drops across growth percentiles, stops growing early, or has very early or very late puberty. Bring the mid-parental range to the appointment.
Can I enter heights in feet and inches?
Yes. Choose feet/inches, enter feet and inches for each parent, and the result is shown in both centimetres and feet/inches.
10. Methodology & Data Integrity
Method: the engine applies the mid-parental (Tanner) formulas — boy: (father + mother + 13 cm) ÷ 2; girl: (father + mother − 13 cm) ÷ 2 — with the standard target range of ±8.5 cm around the mid-parental height. Heights are computed in centimetres; 1 inch = 2.54 cm exactly. Results are shown to one decimal place in centimetres and to the nearest half inch in feet/inches.
Verification: the engine was tested against hand-computed cases (father 180 cm, mother 165 cm → boy 179.0 cm, range 170.5–187.5 cm; girl 166.0 cm, range 157.5–174.5 cm; father 175 cm, mother 160 cm → boy 174.0 cm, girl 161.0 cm), against the invariants that the boy–girl gap is always exactly 13 cm and each range exactly 17 cm wide, and against exact unit conversions (70 in = 177.8 cm; 5 ft 9 in = 175.26 cm) — all matched exactly.
Worked examples in this page were computed by the same engine and are stated with their ranges, never as exact predictions. The range is disclosed on the results panel, in the facts card and in the FAQ, per the standing rule for this tool.
Limits: biological parents’ heights only; no guessing for unknown parents; no medical conditions, treatments, puberty timing or population adjustments; no height-at-age prediction; general information only, not medical advice — see a paediatrician for growth concerns.
