Percentile Charts

Why growth needs to be compared

Children do not all have the same height, mass or rate of growth. They also grow at different rates at different ages, so a measurement such as 18 kg means little by itself. To interpret it, the measurement must be compared with those of an appropriate reference population at the same age. Growth charts allow this comparison and help health professionals monitor health and wellbeing.

A percentile chart, also called a centile chart, has age on the horizontal axis and a body measurement—such as mass, height or head circumference—on the vertical axis. Its curved percentile lines are constructed from measurements collected from many children at different ages.

What a percentile means

The $N$th percentile is the measurement below which $N\%$ of the relevant reference population falls. For example, if a child's height is on the 25th percentile, about 25% of comparable children are shorter and about 75% are taller. It does not mean that the child has grown by 25% or achieved 25% of an expected height.

The 50th percentile is the median: half of the reference population has a lower measurement and half has a higher one. It is not an ideal value that every child should follow. Healthy children naturally occur on many different percentile lines. British growth charts can include lines such as the 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th and 99.6th percentiles.

How to use the chart

First select the appropriate chart and measurement. Find the child's age on the horizontal axis and the measured value on the vertical axis. Plot a point where these values meet, then identify the nearest percentile line. If the point lies between two lines, the child's percentile lies between those values; the graph does not justify a falsely precise answer.

Suppose a mass measurement lies on the 75th percentile. This means that about 75% of the reference population at that age has a lower mass and about 25% has a higher mass. It describes the child's position relative to others; it does not, by itself, establish whether the child is healthy.

Monitoring growth over time

Monitoring requires repeated measurements. Each new measurement is plotted at the child's new age and the points are considered as a pattern. A child may remain roughly within the same percentile region while the actual measurement increases substantially, because the measurements of the comparison population also increase with age.

A sequence that continues along a similar percentile track suggests steady growth relative to the reference population. A marked slowing or acceleration, especially if points move across several percentile spaces, may prompt further checks. Extreme or changing values can be signs of an underlying problem, but a growth chart is a monitoring tool rather than a diagnosis: measurement error, natural variation and other clinical information must also be considered.

Growth charts therefore turn isolated measurements into useful evidence. They show both the child's relative position at a particular age and, more importantly, how that position changes over time.

Read the chart

• Horizontal axis: age.
• Vertical axis: a measurement such as height, mass or head circumference.
• Plot the point where the age and measurement meet.
• Read the nearest percentile line.

Interpret the percentile

The $N$th percentile is the value below which $N\%$ of the reference population falls.

• 25th percentile: about 25% are below and 75% are above.
• 50th percentile: the median; half are below and half are above.
• A percentile is a relative position, not percentage growth.

Monitor growth

Use repeated measurements, not one point. Following a broadly similar percentile track indicates steady relative growth. A substantial change across percentile lines may justify further investigation, but the chart alone does not diagnose a disorder.