Basically, you take your child's age and look along the bottom line of the chart, then the height/weight along the left side of the chart. If these two meet on the green line marked "0" then your child is average height/weight for his/her age ie at the 50th percentile. If they meet above the green line they are above average and below it, they are below average. HTH!

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What is a percentile?

Percentiles are the most commonly used clinical indicator to assess the size and growth patterns of individual children in the United States. Percentiles rank the position of an individual by indicating what percent of the reference population the individual would equal or exceed. For example, on the weight-for-age growth charts, a 5-year-old girl whose weight is at the 25th percentile, weighs the same or more than 25 percent of the reference population of 5-year-old girls, and weighs less than 75 percent of the 5-year-old girls in the reference population.

What is a z-score?

A z-score is the deviation of the value for an individual from the mean value of the reference population divided by the standard deviation for the reference population. Because z-scores have a direct relationship with percentiles, a conversion can occur in either direction using a standard normal distribution table. Therefore, for every z-score there is a corresponding percentile and vice versa.