It is difficult to predict with absolute accuracy how tall a child will be when finished growing, which is typically around age 14 or 15 for a girl and age 16 for a boy. Adult height is influenced by various factors, including genetics, nutrition and overall health -- but even having the same genes and eating a similar diet while growing up do not mean two children will grow to be the same size. The most accurate method of height prediction comes from using a child's "bone age," determined by X-ray, but there are several methods you can use at home to get an idea of how tall your child will eventually be.
Double the Height
Children usually grow at a predictable rate, adding about 14 inches over the first two years and then slowing into a steady rate of about 2 1/2 inches per year until puberty, at which point growth accelerates again to a rate of 3 to 5 inches per year. The earliest growth spurt, from baby to toddler, actually accounts for roughly half of your child's eventual height, so the simplest way of predicting adult height is by doubling the height a boy reaches by age 2. Girls develop more quickly, so you can double their height at 18 months to come up with an estimate of how tall they will be as adults.
Look to the Parents
A slightly more complex method of predicting your child's height is known as the "mid-parental method." To use this method, you need to determine the height of both parents in inches, then add the two numbers and divide them by two. To this number, add 2 1/2 inches if your child is a boy, or subtract 2 1/2 inches if you have a girl. You can expect a margin of error of about four inches up or down -- for example, if you calculate your child's adult height to be 5 feet 10 inches, he could grow to be anywhere from 5 feet 6 inches to 6 feet 2 inches.
A still more complex, yet more accurate, method of predicting a child's adult height involves the use of factors such as weight and age as well as the child's and parents' heights. One widely used type of calculator is based on the Khamis-Roche method, developed by doctors Harry Khamis and Alex Roche. Another calculator, developed by the Saskatchewan Childhood Growth and Development Research Group, uses leg length to predict a child's rate of growth and eventual adult height. This calculator, called the Prediction of Age of Peak Height Velocity, is best used with girls ages 9 to 13 and boys ages 12 to 16 -- although by that age, you may already have a pretty good idea of just how tall your child will be.
Diet and Drugs Matter
While all of these methods can be used to determine your child's potential height, there are a number of reasons he may not reach his maximum growth potential. Children who do not eat a balanced diet may not be as tall as they could be if properly nourished during their formative years. Childhood arthritis and hypothyroidism can also cause children to be short in stature, and growth hormone abnormalities can result in children who are either shorter or taller than would be expected. Certain drugs also have the potential to restrict growth, including corticosteroids taken for conditions such as asthma or Crohn’s disease. A 2012 study by the Liggins Institute of the University of Auckland has even determined that children conceived with the aid of fertility drugs may turn out to be shorter than their peers. A 2010 study, also by the Liggins Institute, found that children conceived by in vitro fertilization, are usually taller than children who were conceived naturally.