Growth calculator

Baby percentile calculator: weight, length and centiles

Enter your baby's age and measurements and see instantly where they sit on the official WHO growth chart, in plain language.

  • Official WHO growth standards
  • Calculated on your device
  • Done in 30 seconds

Your baby's details

In completed months: a 6-week-old counts as 1 month.
For babies born before 37 weeks, care teams use corrected age (age minus the weeks early) and special preterm charts. This calculator can't reliably reproduce that. Your health visitor or GP follows your baby's growth on the right chart.
Your baby's details never leave your device and are not stored.

Where your baby sits on the WHO chart

One measurement is a snapshot. The trend across several measurements says far more than a single point. A baby growing steadily along their own line is the best sign there is.
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What do centiles mean?

A centile compares your baby with 100 healthy babies of the same age and sex in the WHO standard. The 50th centile is the exact middle: half measure more, half less. The 20th or 85th is just as healthy as the 50th. It's not about scoring high, but about growing steadily along their own line.

Babies sometimes shift lines in the first year, especially around growth spurts. Only when weight clearly drops or climbs away from their own line across several measurements is it worth having someone take a look.

This calculator uses the official WHO growth standards. UK clinicians use the UK-WHO charts (centiles, with special handling for preterm babies), so small differences with your red book are possible.
SOURCES1. WHO Child Growth Standards (LMS-tabellen, audit-set 2026-07-09)  ·  2. RCPCH: UK-WHO growth charts  ·  3. NCJ/TNO: Groeidiagrammen JGZ
This result is a guide, not a diagnosis. Your health visitor or GP assesses your baby's growth in context.

How the baby percentile calculator works

The calculator converts your baby's measurements into a position on the official WHO growth chart, using the same formula professionals use. For each measurement you see the centile: your baby's place among 100 healthy babies of the same age and sex.

Average weight and length by month

What's average differs by month and by sex. A 6-month-old boy on the middle of the WHO chart weighs about 7.9 kg; a 12-month-old girl about 8.9 kg. The spread is wide: anything between the 3rd and 97th centile is within the typical range. Enter your baby's details for the real answer rather than a table average.

What do the lines on a growth chart mean?

The lines on a growth chart are centile lines: 3rd, 15th, 50th, 85th and 97th. They show how weight and length are distributed among healthy children. Your baby doesn't need to sit on the 50th. What matters is following their own line, and weight and length roughly matching each other.

My baby weighs less or more than average

Less or more than average is usually simply healthy: half of all babies are below average by definition. It's worth having someone look when a measurement sits below the 3rd or above the 97th centile, or when the trend clearly moves away from their own line. Your health visitor weighs and measures for free.

Frequently asked questions

What is a growth chart?
A chart comparing your baby's weight, length or head circumference with healthy babies of the same age. The lines are centiles: the 50th is the middle, the 3rd and 97th mark the edges of the typical range.
What percentile is normal for a baby?
Anything between the 3rd and 97th centile is within the typical range. The 20th is just as healthy as the 80th. More important than the number is a steady trend along your baby's own line.
Should I worry if my baby is in a low percentile?
Usually not: half of all babies weigh less than average by definition. Watch the trend across measurements. If weight sits below the 3rd centile or clearly drops away from their own line, have your health visitor take a look.
Is it normal for my baby's percentile to change?
Small shifts happen, especially in the first year and around growth spurts. If your baby drops or climbs two or more centile lines across measurements, that's a good moment to discuss it with your health visitor.
Do premature babies use different growth charts?
Yes. For babies born before 37 weeks, care teams use corrected age and dedicated preterm charts. This calculator isn't built for that; your health visitor follows growth on the right chart.
Do breastfed and formula-fed babies grow differently?
The WHO standard is based on healthy breastfed children. Formula-fed babies can gain a little faster in the second half of the first year. Both patterns are healthy as long as the trend is steady.
How often should my baby be weighed?
Weighing more often than the routine check-ups is usually unnecessary and mainly feeds worry. If your baby feeds well and grows well, the standard weigh-ins are enough.
Why does the result differ from my red book?
This calculator uses the WHO standard, while UK clinicians use the UK-WHO charts, and measuring at a different moment gives slightly different values too. Small differences are normal. Your health visitor's assessment leads.
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