Child Height Predictor from Parents (Mid-Parental)

The Mid-Parental Height Formula (Based on Parents' Heights), Step by Step

The formula behind this child height predictor from parents has two lines of arithmetic and one non-negotiable rule about how to report the result. Run the steps in order — the trace block below recomputes live from whatever heights you entered above, so you can verify every intermediate number by hand:

1. Measure both parents properly. Barefoot, morning, hard floor, wall behind the heels and shoulder blades, eyes level. Self-reported heights average 1–2 in above measured ones — the single largest error source in mid-parental math.
2. Add the two heights. Units do not matter as long as they match; the calculator normalizes to inches internally.
3. Apply the sex adjustment. Add 5 in (13 cm) for a boy, subtract 5 in for a girl. This is the average adult male–female gap compressed into one constant.
4. Divide by 2. The result is the target height — the family mean for that child's sex.
5. Always report ±3.5 in (±9 cm). The target is a distribution's center, not the child's destiny; roughly 9 in 10 children land inside the band.

$ midparental --mother 5′5.0″ --father 5′10.1″ --sex boy
> mother   65.0 in  (165.0 cm)
> father   70.1 in  (178.0 cm)
> sum      135.0 in
> sex adj  +5 in  →  140.0 in
> ÷ 2      70.0 in
= target   70.0 in  (5′10.0″ · 177.8 cm)
> range    66.5 – 73.5 in  (±3.5 in ≈ 90%)

Reading the trace: the sex-adjusted sum divided by two is the target; the final line is the honest answer. Swap the sex toggle above and watch step 4 flip the adjustment — the only difference between predicting a boy and a girl in this formula.

Three mistakes that corrupt mid-parental math

Beyond the Average: The Khamis-Roche Extension

Mid-parental math has one structural blind spot: it never looks at the child. Two siblings from the same parents carry identical mid-parental targets yet routinely finish 3–4 in apart, because each child's own growth trajectory carries real information. The Khamis-Roche method (1994) closes that gap by blending the child's current size-for-age with the parental target — no X-ray required. The accuracy jump is the largest single upgrade available without clinical equipment:

The practical rule: run mid-parental math for the family ceiling, then let it hand over to child-inclusive methods the moment the child has measurable data. The child height predictor calculator runs exactly that handover automatically from age 4 onward, and in "Child already born" mode above you can watch the blend respond to the child's height in real time.

Genetics Beyond MPH — The Grandparental Correction

Height is polygenic, and a family's genetic ceiling can hide in grandparents who never met their growth potential. The correction rule: when a grandparent's adult height sits below the 5th or above the 95th percentile for their sex, nudge the mid-parental target 0.5–1.0 in toward that extreme. Three worked examples show when the rule earns its keep — and when it double-counts:

Future Child Height: The Preconception Scenario

Parents-to-be are the natural users of a child height predictor from parents, because a pregnancy is the one situation where the parents' heights are the only data that exists. The Future-child mode above renders the full probability distribution rather than a single target: the 50th percentile row is the mid-parental target, and the 10th–90th rows trace the range that statistically covers 8 in 10 children. Two reading rules keep the output honest. First, the boy and girl rows differ by exactly the 5 in sex constant — there is no other boy-girl asymmetry in the math. Second, the distribution assumes a well-nourished pregnancy; significant maternal malnutrition or chronic disease during gestation shifts population outcomes, though individual in-utero effects on final height are smaller than folklore suggests.

Heritability Deep-Dive: Why the Formula Works, and Where It Breaks

Mid-parental math is only defensible because height is among the most heritable human traits: twin and family studies put additive genetics at roughly 80% of adult-height variance in well-nourished populations. The remaining 20% is why identical formulas produce different children — and why the ±3.5 in band is a feature, not a hedge. The decomposition below shows where the environmental variance actually lives:

The twin data makes the heritability concrete: identical twins reared together typically finish within about 1 inch of each other, while full siblings — who share only half their genes — differ by up to 4 inches or more. Both grew up in the same house with the same parents; the gap between 1 in and 4 in is what the genetic component alone explains. Five environmental factors do the rest of the shifting: chronic disease (the largest single mover — untreated celiac disease alone can cost inches), puberty timing (early maturation trades final height for earlier growth), sustained undernutrition, chronically short sleep during peak growth years, and the type of training load in adolescent athletes. None of these appear in the formula — which is exactly why the formula needs its band.

Six Family Scenarios — Which Path Applies to Yours

When You Don't Know Both Heights

Mother-only prediction (three fallbacks)

Single-parent families, donor-conceived children with an anonymous provider, and estranged co-parents all hit the same wall: one known height, one unknown. Three fallbacks, honestly ranked by confidence:

FAQ — Child Height Predictor from Parents