The formula, and where the 13 comes from
Mid-parental height is the average of the two parents with a correction for the sex of the child: add 13 centimetres for a boy, subtract 13 for a girl, then halve the total. Written in inches the constant is usually rounded to five, which is why the same method appears in two apparently different forms. Thirteen centimetres is approximately the average adult height difference between men and women in the populations these charts were built from, and it is doing nothing more sophisticated than putting both parents onto a common scale before averaging them.
A father of 5 ft 10 and a mother of 5 ft 4 give 178 and 163 centimetres. For a boy: 178 plus 163 plus 13 is 354, halved is 177 centimetres, a little under 5 ft 10. For a girl the same pair gives 164, about 5 ft 5.
Reading the range instead of the number
The estimate is conventionally quoted with a margin of about 8.5 centimetres either side, which is roughly two standard deviations of the observed scatter of adult heights around the mid-parental value. That makes the interval about 6.7 inches wide, and about one child in twenty ends up outside it altogether.
An interval that wide contains most of the answers anyone actually wants. A midpoint of 5 ft 10 covers everything from about 5 ft 6 to 6 ft 1. That is the honest content of the calculation, and quoting the midpoint alone — which is what most versions of this calculator do — converts a broad statistical statement into a specific-looking claim it cannot support.
What the formula does not contain
| Factor | Why it is missing |
|---|---|
| Nutrition through childhood | Not an input; population height has shifted several inches within a century on this factor alone |
| Chronic illness | Conditions affecting absorption, hormones or inflammation change growth and are invisible here |
| Timing of puberty | Early puberty closes the growth plates sooner; late puberty extends growth. Neither is an input |
| Sleep and physical activity | Associated with growth hormone secretion; not measurable from two parent heights |
| The child | Nothing about their current height, growth rate or bone age enters the calculation at all |
The last row is the important one. Every input is about the parents. A child already tracking well above or below the family range carries information that this formula throws away, which is precisely why clinicians use it as one reference line on a chart rather than as an answer.
How the estimate is used properly
In a paediatric growth assessment, the mid-parental height is marked on the chart and the question asked is whether the child's own curve is heading towards that range. A child at the 10th percentile whose parents are short is consistent; a child at the 10th percentile whose mid-parental estimate sits at the 60th is a mismatch worth understanding. That comparison needs both the family range and a series of the child's own measurements, which is what the growth percentile calculator covers for the first two years.
Where an actual projection is needed, it comes from a bone age radiograph. A hand and wrist film shows how much growth plate remains, and combining skeletal maturity with current height gives a far tighter estimate than parental heights can. That is a clinic procedure with a clinical indication behind it, and it is worth understanding that nothing typed into a web form is a substitute for it.
Questions people ask
How accurate is mid-parental height?
As a point estimate, not very. It centres the range correctly on average across many families, which is what it was designed to do, but for any individual child the observed scatter is around 8.5 centimetres either side and roughly five percent finish outside even that. Studies comparing predicted with achieved adult height consistently find errors of several centimetres as the norm rather than the exception. It is a useful reference line for a clinician reading a growth chart and a poor basis for any expectation about a specific child.
Both parents are short. Does that settle it?
It lowers the midpoint and it does not settle anything. The range is the same width regardless of where it sits, so a child of two short parents can still finish several inches above the estimate. There is an additional consideration worth knowing: adult height reflects the nutritional and health conditions of the generation that grew up in it, so parents who grew up under worse conditions than their children may be shorter than their own genetic potential. In that situation children commonly exceed the mid-parental estimate, and populations have gained several inches in a century for exactly this reason.
Do supplements or growth products increase adult height?
For a child who is not deficient in anything and has no medical cause of short stature, there is no good evidence that any over-the-counter product changes final adult height. Correcting a genuine nutritional deficiency does matter, which is a different claim. Where short stature has a medical cause — growth hormone deficiency, thyroid disease, coeliac disease, chronic kidney disease and others — treatment can affect the outcome substantially, and identifying that is a clinical process, not a purchasing decision.
When should short stature be looked at by a doctor?
The commonly cited triggers are height below the 3rd percentile for age, a growth rate under about two inches a year in mid-childhood, a growth curve crossing downward through percentile lines, or a substantial mismatch between the child's percentile and the mid-parental range. Signs of puberty before about age eight in girls or nine in boys are also a reason to be seen, because early puberty can shorten the total growing period. Timing matters here more than in most things, since intervention is only possible while the growth plates are open.