Where your range comes from

Add your mum’s height and your dad’s height and halve it. Boys usually end up about 6.5 cm taller than that, girls about 6.5 cm shorter. That’s because boys and girls grow differently.

Most people finish within 8.5 cm of their middle. So your range is that middle ±8.5 cm. This is the Tanner method, from 1970. It uses only your parents’ heights, so it’s wide.

middle = (mum + dad ± 13 cm) ÷ 2
             (+13 for the boys’ chart, −13 for the girls’ chart)

range  = middle − 8.5 cm  to  middle + 8.5 cm

Adding 13 cm before halving is the same as adding 6.5 cm after. It’s written this way because that’s how the app does it. Heights outside 100 cm to 230 cm are refused, not guessed.

The ±8.5 cm width covers roughly the 3rd to the 97th centile. That’s about 95 people in 100. Tanner later widened it to about ±9 cm for girls and ±10 cm for boys. Centile keeps the original width, so the true range may be wider.

Plus uses four things: your age, your height now, your weight and your parents’ middle. Each one is multiplied by a number. That number fits your age and the chart you picked. Added together, they give your number.

Nine in ten boys finish within 5.3 cm of their number. For girls it’s 4.3 cm. That distance either side is your Plus range. This is the Khamis–Roche method, from 1994.

This is the regression, which works in inches and pounds:

predicted adult height (inches)
    = intercept
    + (height coefficient  × current height in inches)
    + (weight coefficient  × current weight in pounds)
    + (parent coefficient  × mean parent height in inches)

The four numbers change with age and with the chart. We use the last half-year age band you have fully reached. We don’t round up. We convert to inches and pounds, work it out at full precision, then convert back. We use the corrected 1995 tables.