How Child Height Predictions Work — Measuring, Growth Charts and Parents' Heights

This one started at home. While measuring my kids' heights, two questions came up right away: where do they stand compared with other children their age, and roughly how tall might they end up? I suspected plenty of parents wonder the same thing, so I built a small calculator for Webooro. This article explains what goes into an estimate like that — and, just as important, what it can't tell you.

Step 1 — Measure height the same way every time

A prediction is only as good as the number you type in. A few habits make home measurements far more reliable:

Same time of day. Height is usually a little lower in the evening than right after waking, often by around a centimetre, because the spine compresses during the day. Pick one time and stick to it.
Barefoot, heels together. Heels, buttocks, shoulder blades and the back of the head should touch the wall.
Eyes level. Ask your child to look straight ahead, not up at you.
Use a right angle. Press a hardcover book flat on the head and against the wall, mark the bottom edge, then measure the mark.
Write it down with the date. One number says little. Several numbers over months show the growth rate, which matters more.

Step 2 — Find the percentile on a growth chart

A growth chart shows how tall children of a given age and sex usually are. Webooro's calculator uses the 2017 Korean National Growth Charts published by the Korea Disease Control and Prevention Agency and the Korean Pediatric Society, because they reflect Korean children. Other tools may use WHO or US CDC charts, which is one reason results differ between sites.

The percentile is a rank, not a grade. The 30th percentile means that if 100 children of the same age and sex lined up by height, your child would be around 30th from the shortest. Anything between the 3rd and 97th percentile is generally treated as within the normal range.

For reference, the 50th-percentile height at age 18 in the 2017 charts is about 173.6 cm for boys and 160.6 cm for girls.

Step 3 — Add the parents' heights

Height is strongly inherited, so paediatricians often use the mid-parental height as a target:

Boys: (father's height + mother's height + 13) ÷ 2
Girls: (father's height + mother's height − 13) ÷ 2

The 13 cm adjusts for the average difference between adult men and women. With a 175 cm father and a 162 cm mother, the target is 175 cm for a son and 162 cm for a daughter. Most children end up within roughly 5 cm either side of this number.

Webooro blends this target with the "current trend" (the height your child would reach by keeping today's percentile). For young children the parents' heights get more weight, because a toddler's percentile can still shift a lot; as children get older, the current trend counts for more.

What a home estimate can't tell you

Bone age. Doctors check an X-ray of the hand to see how much growth is left. Two children of the same age and height can have very different bone ages.
Timing of puberty. Early puberty can make a child look tall now but end growth sooner; late puberty can do the opposite.
Health conditions. Nutrition, sleep, chronic illness and hormones all play a part.

When to talk to a doctor

Whatever a calculator says, consider seeing a paediatrician if:

your child is below the 3rd percentile, or clearly below the mid-parental target;
a child who hasn't started puberty grows less than about 4 cm in a year;
signs of puberty appear before age 8 in girls or before age 9 in boys.

The growth report on Webooro is currently in Korean only, since it is built on the Korean charts. The method above works with any growth chart, though — and the habit that helps most is simply measuring the same way a few times a year and keeping the record.