Mid-Parental Height: The Formula Behind Every Height Calculator

Nearly every height calculator on the internet, including this one, is built on a single piece of arithmetic developed for paediatric clinics. It is worth understanding directly — both because it takes thirty seconds to do by hand, and because knowing what it does not account for tells you how much weight any prediction deserves.

The formula

The output is called the target height or mid-parental height. Both parents must be measured in the same units, and both should be biological parents — this is a purely genetic calculation.

Why 6.5 centimetres?

Across most populations, adult men average roughly 13 cm taller than adult women. Averaging a mother's and a father's height produces a figure that belongs to neither sex — a family midpoint. Adding half the 13 cm gap converts that midpoint into a male-scale estimate; subtracting half converts it to a female one. The constant is not magic, it is just half of an observed population difference.

Worked examples

Notice the pattern in the last two rows: the formula simply shifts the family midpoint up or down. It has no mechanism for a child who breaks the family pattern, because it never looks at the child.

The target range is much wider than people expect

Presenting mid-parental height as a single number is where most online calculators mislead. In clinical use it is drawn as a band, conventionally about 8.5 cm either side of the target — roughly 17 cm from top to bottom.

For the 178 cm / 165 cm couple above, a son's target range runs from about 169.5 cm to 186.5 cm. That is the honest genetic expectation: somewhere in a seventeen-centimetre window. Most children finish inside it. The formula cannot say where inside it, because it has been given no information about the individual child — not their current height, not their age, not whether puberty has started.

Narrowing the band: what the calculator adds

The way to tighten the estimate is to bring in information the formula ignores — data about the child themselves.

• Current height for age. At any age, children have completed a fairly predictable share of their eventual growth. Dividing a child's height today by that share gives an independent adult estimate that owes nothing to the parents.
• Pubertal stage. Two 13-year-olds of identical height can have very different amounts of growing left depending on whether the spurt has started, is peaking, or has finished. This is the largest source of error in age-only methods.
• The disagreement between methods. When the parental estimate and the child's own trajectory point to the same figure, confidence is high and the range narrows. When they diverge sharply, that gap is itself information — and it is a reasonable thing to mention at a check-up.

The calculator on this page blends the parental estimate with the child's own projection and widens or narrows the reported interval according to age, pubertal stage and how much the two methods disagree. Running it takes about a minute, and the result is shown without an account.

Other methods you may come across

Mid-parental height is the most widely used approach, but it is not the only one.

• Khamis-Roche. Uses the child's current height and weight alongside mid-parental height, without requiring an X-ray. It is generally more accurate than the parental formula alone for children aged four to seventeen, at the cost of needing reliable weight data.
• Bayley-Pinneau. Uses bone age from a hand and wrist X-ray to estimate the fraction of growth already completed. More accurate again, but it requires imaging and clinical interpretation, so it belongs in a paediatric endocrinology clinic rather than a website.
• Growth-curve extrapolation. Following a child's own percentile line forward. Simple and surprisingly effective for children who have tracked steadily, and useless for children who have not.

Every one of these produces a range, not a certainty. Anyone offering a guaranteed final height is selling something.

Mistakes that skew the result

• Using remembered or self-reported parental heights. Adults consistently round themselves upward, and height declines slowly from middle age. Measure both parents rather than trusting a driving licence.
• Mixing units. Feeding one parent in centimetres and the other in inches produces confident nonsense. Convert first.
• Shoes on. A two-centimetre error on each parent moves the child's target by two centimetres.
• Applying it too young. Below about age four the formula's assumptions have not settled; before then, a child's size still reflects the pregnancy as much as their genetics.
• Treating it as a ceiling or a floor. It is the centre of a wide distribution. Children finishing above or below their target range are common enough to be unremarkable on their own.

Frequently asked questions

This page is general information for parents and carers, not medical advice, diagnosis or treatment. Height estimates are statistical, not predictions about an individual. If you are concerned about your child's growth, speak to a paediatrician or family doctor.