Baby Growth Percentile Calculator
How this is calculated
This calculator is based on the WHO Child Growth Standards, the growth reference most pediatric organizations recommend for children under 2, and commonly used through age 5. It approximates the WHO's median weight, length/height and head circumference for each age in months, then estimates how far your child's measurement sits from that median to produce a percentile.
A percentile isn't a grade or a score to maximize — it simply describes where a measurement falls compared to a reference population of healthy children of the same age and sex. Being at the 40th percentile for weight doesn't mean "below average" in any concerning sense; it means 40% of children in the reference group weigh less and 60% weigh more, and plenty of perfectly healthy children sit anywhere in that range.
What tends to matter more to pediatricians than any single percentile is the trend over time — whether a child is tracking steadily along roughly the same curve at each visit, rather than one isolated number.
Common edge cases and things to know
This tool does not adjust for prematurity. Babies born early are typically tracked using their corrected (adjusted) age — their age counted from their original due date rather than their birth date — usually until around 24 months old, since comparing a preterm baby's raw age against full-term reference curves can understate their growth. If your baby was born early, ask your pediatrician about using corrected age instead of the calendar age entered here.
Head circumference is only calculated up to 36 months here, in line with the age range these estimates cover well. A single measurement can also be affected by things like recent illness, feeding changes, or simple measurement variation — which is another reason a growth trend across visits is more informative than one number in isolation.
Formula-fed and breastfed babies also tend to gain weight at somewhat different average paces in the first year, which is part of why the WHO standards (based largely on breastfed infants as the healthy growth pattern) can look different from some older, formula-era reference charts. A percentile that crosses two major lines on a growth chart between visits, or one that sits consistently below the 3rd or above the 97th percentile, is generally what prompts a closer look from a pediatrician — a single percentile in the middle of the range, on its own, usually isn't a cause for concern.