Baby Growth Percentile Calculator
Check your baby's weight, length or head-circumference percentile on the WHO (under 2) and CDC (2+) growth charts — the same standards clinicians use. Add gestational weeks to correct for prematurity. A percentile is a reference point for a conversation with your doctor, never a pass or fail.
How it works
- Choose your baby's sex and the measurement (weight, length or head).
- Enter their age, then the measured value in your preferred units.
- If your baby was born before 40 weeks, add the gestational weeks to use corrected age.
- Read the percentile and z-score — then bring your baby's trend over time to your clinician for interpretation.
Related reading
Growth & percentiles
Weight gain, charts and corrected age explained.
What is a growth percentile?
A plain-language definition.
What is corrected age?
Adjusting age for prematurity.
Cocoon works this out from your logs automatically
Log feeds, sleep and growth in seconds and Cocoon surfaces wake windows, feeding rhythm and WHO/CDC percentiles for you — free, offline-first, no subscription.
Common questions
What does a baby's growth percentile mean?
A percentile compares your baby to a reference population of the same age and sex. The 60th percentile for weight means about 60 in 100 babies weigh the same or less. There is no 'good' or 'bad' percentile — the 5th and 95th can both be perfectly healthy.
Which charts does this use?
It uses the WHO Child Growth Standards for babies under 24 months and the CDC 2000 charts from 24 months, following AAP guidance. Head circumference uses the WHO chart throughout.
What is corrected age for prematurity?
For babies born early, growth is assessed using corrected (adjusted) age — chronological age minus the weeks born before 40 weeks — usually until about 2 years. Enter gestational weeks and the tool corrects the age for you.
Should I worry if the percentile changes?
A single number matters less than the trend. Babies often settle onto their own curve in the early months. A sudden, sustained jump or drop across percentile lines is worth discussing with your doctor or child-health nurse — but only they can interpret it.
This tool gives typical ranges for general information only. Every baby is different — it is not medical advice and not a medical device. For anything about your baby's health, growth or feeding, talk with your doctor or child-health nurse.