Doubling the height of a 2-year-old boy or an 18-month-old girl is a simple way to predict adult height, according to HealthyChildren.org. However, there is no way to predict adult height with complete accuracy, says Mayo Clinic.
Doubling a toddler's height to obtain a final height prediction is reasonably accurate because children experience an initial growth spurt between infant and toddler ages, says HealthyChildren.org. This first growth spurt actually makes up nearly half of the child's adult height. Because girls grow and develop more quickly, their height must be doubled earlier than a boy's.
A second at-home prediction method uses the parents' heights, reports Mayo Clinic. For this method, the parents' heights are added together in inches or centimeters. If the child in question is a boy, 5 inches or 13 centimeters are added and the result is divided by two. For a girl, the same number of inches or centimeters are subtracted before dividing.
The most accurate predictor of a child's adult height is a child's bone age, obtained from an X-ray of the left wrist, hand and fingers, say researchers Arkadiusz Gertych et.al. A bone age assessment shows any discrepancies between a person's skeletal age and their age in years to predict how much growth is still expected of the individual.
A person's final height depends upon genetics, nutrition, hormones, medications and preexisting health conditions, says HealthyChildren.org. Children also begin growth spurts at different ages, making any method of prediction potentially faulty.