The mid-parental height method combines the biological mother’s and father’s adult heights, then applies a sex-based adjustment. The calculation is simple enough to check with a pencil. Its simplicity is also a reason to read the result cautiously: it produces a rough family estimate rather than a personalized growth forecast.
The calculation, one step at a time
Use the same unit for both parents. Height Forecast follows the five-inch adjustment in the American Academy of Pediatrics’ published formula:
- For a boy: (mother’s height + father’s height + 5 inches) ÷ 2.
- For a girl: (mother’s height + father’s height − 5 inches) ÷ 2.
In centimeters, replace five inches with 12.7 cm. Add or subtract that amount before dividing the total by two. Equivalently, average the parents’ heights first, then add or subtract 6.35 cm. Both approaches give the same result.
The boy/girl selection identifies the adjustment used by this published method. It is a limited model input, not a complete account of a child’s biology or identity.
A worked example in centimeters
Suppose the biological mother measures 165 cm and the biological father measures 180 cm. Their combined height is 345 cm.
- For the boy calculation, add 12.7: 345 + 12.7 = 357.7 cm.
- Divide by two: 357.7 ÷ 2 = 178.85 cm.
- For the girl calculation, subtract instead: (345 − 12.7) ÷ 2 = 166.15 cm.
Rounded to one decimal place, the estimates are 178.9 cm and 166.2 cm. These are examples of the arithmetic, not predictions for a real child. The extra digits in the working help you check the math; they do not imply extra biological precision.
Why this site uses 12.7 rather than 13
One inch equals exactly 2.54 centimeters, so five inches equals 12.7 cm. Some published metric versions round the adjustment to 13 cm. Height Forecast keeps the exact conversion so that switching units does not introduce a second formula.
The difference between those adjustments is 0.3 cm before division, or 0.15 cm in the result. That small arithmetic difference should be considered alongside the much broader uncertainty of the method.
Put the broad range beside the answer
Our reference band extends roughly four inches, or 10.16 cm, in either direction. For the 178.85 cm example, that gives endpoints of 168.69 and 189.01 cm before display rounding.
This is a rough family reference range, not an individual confidence interval or a growth projection. Read how accurate height predictors are before interpreting a single number as an expected outcome.
Use inputs you can stand behind
Use measured adult heights where possible. In feet and inches, enter the feet and the remaining inches separately: five feet six inches is not 5.6 feet. If either biological parent’s height is unknown, this calculator cannot provide the family estimate without inventing an input.
You can still keep dated measurements without calculating an adult-height estimate. Our measurement guide explains the home setup, and How it works describes the calculator and tracker. Neither tool determines whether a child’s growth is healthy.