Growth Percentile Calculator
Check your term baby's approximate growth percentile for weight, length and head circumference from 2 weeks to 24 months.
Health visitors plot measurements on UK-WHO growth charts in the Red Book. The charts combine UK birth data with WHO standards based on healthy breastfed children from several countries.
How growth percentiles work
Growth percentiles show how a measurement compares with babies of the same age and sex. The NHS calls these centiles. A measurement near the 50th centile is close to the middle of the reference range; it is not a score or diagnosis.
There is no single “best” centile. The pattern across accurate measurements matters more than one result, alongside feeding, development and wellbeing.
When to use this calculator
Use this as a broad guide between appointments for a baby born at 37 weeks or later. It estimates a centile band from WHO reference values and will not exactly reproduce professional Red Book plotting, which uses precise dates and specialist UK-WHO calculations.
When to speak to your health visitor or GP
Speak to your health visitor or GP if:
- Your baby's weight crosses two centile lines up or down
- Their head circumference is growing unusually fast or slow
- You have any concerns about feeding, development, or wellbeing
- Your baby was premature — corrected age or gestation-specific plotting may be needed
Babies under 2 weeks and babies born before 37 weeks need professional plotting. Do not delay asking for help because of a result from this guide.
Approximate growth guide
What does this result mean?
The result places each measurement near or between the nine UK centile lines: 0.4, 2, 9, 25, 50, 75, 91, 98 and 99.6.
It is an estimate, not a diagnosis. Use professionally plotted Red Book measurements to assess change over time.
A single measurement is only a snapshot. Measurement technique and the pattern across several visits both matter.
When to ask for advice
Speak to your health visitor or GP if you are worried, if weight crosses two centile lines, or if head growth changes unexpectedly. For babies born before 37 weeks, ask for corrected-age plotting.