Growth Percentile Calculator

When your health visitor weighs and measures your baby, they plot the results on UK-WHO growth charts — the same charts used in your baby's Red Book. These charts show how your little one compares to thousands of other children of the same age and sex across the UK.

How percentiles work

A percentile tells you where your baby sits compared to others. If your baby is on the 50th percentile for weight, half of babies their age weigh less and half weigh more. A baby on the 25th percentile is lighter than average, while one on the 75th is heavier — but both are perfectly healthy.

There's no "good" or "bad" percentile. The 9th percentile is just as normal as the 91st. What matters most is that your baby follows their own curve over time, rather than jumping between lines.

When to use this calculator

This tool is handy for checking your baby's measurements between health visitor appointments. Perhaps you've had them weighed at a baby clinic and want to see where they fall, or you're simply curious after a growth spurt. It uses the exact same UK-WHO data that the NHS uses, so the percentiles you see here should match your Red Book.

When to speak to your health visitor or GP

Most babies are healthy regardless of their percentile, but it's worth having a chat with your health visitor if:

• Your baby's weight or length crosses two or more percentile 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 — adjusted age calculations may apply

Remember, this calculator gives you a snapshot. Your health visitor looks at the bigger picture — how your baby is feeding, developing, and thriving overall. Trust your instincts, and never hesitate to ask for support.