Adult height is strongly heritable — twin and family studies put the genetic contribution at around 80 per cent in well-nourished populations — so parental heights carry real predictive information. The mid-parental method used in paediatric clinics averages the two parents' heights and then adds or subtracts 6.5 centimetres depending on the child's sex.
The 13 centimetre gap being split in half comes from the average adult height difference between men and women. The Tanner formulation adds 6.5 cm to the mid-parental height for a boy and subtracts 6.5 cm for a girl. An equivalent phrasing adds 13 cm to the mother's height before averaging for a boy, or subtracts 13 cm from the father's for a girl, which reaches the same number.
A father of 178 cm and a mother of 165 cm have a mid-parental height of 171.5 cm. A son would be predicted at 178.0 cm and a daughter at 165.0 cm. The confidence interval is wide: about 8.5 centimetres either side covers roughly the middle 95 per cent of outcomes, so the prediction is a centre of gravity rather than a target.
The formula
Mid-parental height- The average of both parents' heights
±6.5 cm- Added for a boy, subtracted for a girl
13 cm- The average adult height difference between the sexes
±8.5 cm- Roughly the 95 per cent range around the prediction
How it works, step by step
- Enter the biological father's adult height.
- Enter the biological mother's adult height.
- Select the child's sex.
- The gauge shows the predicted adult height, with the range below.
Worked examples
Father 178 cm, mother 165 cm — a son
Mid-parental height is (178 + 165) ÷ 2 = 171.5 cm. Adding 6.5 gives a prediction of 178.0 cm, with a likely range of 169.5 to 186.5 cm.
The same parents — a daughter
Subtracting 6.5 from 171.5 gives 165.0 cm, which is 13 cm shorter than the prediction for a brother. That 13 cm gap is the average adult difference between the sexes, not anything about these parents.
How to read your score
Frequently asked questions
How accurate is this?
The 95 per cent range is about 8.5 centimetres either side, which is wide — a 17 centimetre span. It is genuinely useful for spotting a child growing far outside their genetic potential, and not useful for predicting an exact adult height.
Why 6.5 centimetres?
It is half of the 13 centimetre average adult height difference between men and women. Adding half for a boy and subtracting half for a girl centres each prediction on the right distribution.
Are there better methods?
Yes, for clinical use. The Khamis-Roche method uses the child's current height, weight and parental heights, and bone age assessment with the Bayley-Pinneau or Tanner-Whitehouse methods is more accurate again. All need measured data rather than just parental heights.
What if a child is far from the prediction?
Growing consistently along a percentile line matters more than matching the prediction. A child crossing percentile lines downwards, or sitting far below their mid-parental prediction, is worth raising with a GP so growth can be plotted properly.
Does nutrition change adult height?
Substantially, at a population level — average heights rose across Europe through the twentieth century as childhood nutrition improved. Within a well-nourished population, genetics dominates. It is an estimate derived from a population equation, not a diagnosis, and it is not a substitute for assessment by a qualified clinician who can see your full history and examine you.
Mid-parental predictions
| Father | Mother | Mid-parental | Boy | Girl |
|---|---|---|---|---|
| 165 cm | 155 cm | 160.0 cm | 166.5 cm | 153.5 cm |
| 170 cm | 158 cm | 164.0 cm | 170.5 cm | 157.5 cm |
| 175 cm | 162 cm | 168.5 cm | 175.0 cm | 162.0 cm |
| 178 cm | 165 cm | 171.5 cm | 178.0 cm | 165.0 cm |
| 183 cm | 170 cm | 176.5 cm | 183.0 cm | 170.0 cm |
| 188 cm | 175 cm | 181.5 cm | 188.0 cm | 175.0 cm |
| 193 cm | 180 cm | 186.5 cm | 193.0 cm | 180.0 cm |
Add or subtract 8.5 cm for the approximate 95 per cent range.