To estimate a child's adult height, add the parents' heights in inches, subtract 5 inches for a girl or add 5 inches for a boy, and divide by 2. Alternately, multiply a boy's height at age 24 months or a girl's height at 18 months by 2.
While there is no way to predict a child's final height with absolute certainty, height is largely genetic and these formulas provide a reasonable estimate. Most children will grow to within 4 inches of these estimates by adulthood. People have different growth rates, and not all children will reach their final height by their teenage years.