Baby Weight Percentile Calculator
Estimate a baby’s weight percentile for age and sex against WHO growth references for the first two years.
How it works
z = (weight − median-for-age) ÷ SD · SD ≈ 12% of median · percentile = Φ(z) × 100
Percentiles compare one baby to a reference population: the 60th percentile means about 60% of same-age, same-sex babies weigh less. This tool takes the WHO median weight for the baby’s age and sex, assumes a spread of roughly 12% of the median, and converts the gap into a percentile using the normal distribution. It is a close approximation of the official LMS growth charts — good for orientation, not a substitute for the chart your pediatrician plots at each visit.
Worked example
A 6-month-old boy weighing 8 kg: the WHO median at 6 months is 7.9 kg, so SD ≈ 0.95 kg and z = (8 − 7.9) ÷ 0.95 ≈ 0.11 — about the 54th percentile, right around average.
Sources: WHO Child Growth Standards
Frequently asked questions
What is a “good” percentile for my baby?
There is no single good number — the 5th and the 95th percentile can both be perfectly healthy. What matters most is that a baby tracks steadily along their own curve over time rather than jumping or dropping across percentiles.
Why does my result differ slightly from the doctor’s chart?
Official charts use full LMS tables that capture the exact skew of weight at each age. This calculator uses a normal approximation with a fixed spread, so it can be off by a few percentile points near the extremes.
Does this work for premature babies?
Not directly. For a premature baby you should use corrected (adjusted) age and specialised preterm charts — ask your pediatrician.
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This is a general educational estimate, not medical advice. Every child grows, feeds and responds to medicine differently — for anything about your child’s health, growth, nutrition or medication, follow the guidance of your pediatrician.