Most centile charts have 7 centile lines on them, i.e. the 3rd, 10th, 25th, 50th, 75th, 90th and 97th centiles. Ninety seven percent of healthy children will fall below the 97th centile. Similarly 50% of healthy children will fall below the 50th centile and 3% below the 3rd centile. Therefore, each centile indicates what percentage of healthy children should have a size that falls below that line. This method enables one to compare the size of any child with the expected size of children of the same age.
If 100 healthy children of the same age were lined up from the tallest to the shortest, the height of the tallest child would be the 100th centile, that of the second tallest the 99th centile, and so on. The height of the child at the centre of the row would be the 50th centile. It is called a centile chart as it reflects the relative size of 100 normal children, and ‘centum’ is the Latin word for a hundred. International centile charts are used in all countries.