Percentile chartsSpec 2.7
In short
A percentile chart is used to monitor the growth of a baby or child by comparing their measurements, such as mass, with those of many children of the same age and sex. On the 50th percentile, half of children have a lower value and half a higher value. A child who moves across to a different curve may need investigating.
A percentile chart is used to monitor the growth of a baby or child. It shows curves for the measurements of a large number of children of the same age and sex, such as mass. A child's measurement is plotted on the chart.
- The 50th percentile line means 50% of children have a lower value and 50% have a higher value.
- A child on the 25th percentile has a value higher than 25% of children and lower than 75% of children.
- Plotting a child's measurements over time shows whether they follow a curve or move across to a different one. A large change may need to be investigated.
The chart is a graph. Use it to turn a number into a percentile, or the reverse.
Percentage gain in mass
A baby has a mass of 3.2 kg at birth and 4.0 kg at 8 weeks. Calculate the percentage gain in mass.
- change in mass = 4.0 − 3.2 = 0.8 kg
- percentage gain = 0.8 ÷ 3.2 × 100
Answer: 25%
For percentage loss, use the same equation with the decrease in mass. Always divide by the original mass, not the final mass.
Use an estimate to check your answer is sensible. Round 3.2 to 3: 0.8 ÷ 3 is about 0.27, so the answer should be a little under 27%.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026