Birthweight Percentile Calculator

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Health

Estimate where a baby's birth weight falls on a growth chart. Enter the birth weight along with the reference mean and standard deviation for the baby's gestational age and sex, and this calculator returns the z-score and the corresponding percentile.

Rates and fees change frequently. Always confirm the current figures with the official authority before relying on this result.

Formula

z = (birth weight − mean) ÷ standard deviation; percentile = Φ(z)
  • The z-score measures how many standard deviations the birth weight is above or below the reference mean.
  • The percentile is the area under the normal curve up to that z-score — the share of babies expected to weigh less.
  • Use mean and standard deviation values from a growth reference matched to the baby's gestational age and sex (e.g. WHO, Fenton, or INTERGROWTH-21st).
  • Below the 10th percentile is often labelled small for gestational age (SGA); above the 90th is large for gestational age (LGA).
  • This is an educational estimate, not a clinical assessment. A clinician interprets growth using validated charts and the full clinical picture.

3300 g, mean 3500 g, SD 450 g

Inputs
  • Birth Weight: 3300 g
  • Reference Mean Weight: 3500 g
  • Reference Standard Deviation: 450 g

z = (3300 − 3500) / 450 ≈ −0.44, which corresponds to about the 33rd percentile — within the normal range.

Frequently asked questions

What is a birth weight percentile?
It tells you what fraction of babies of the same gestational age and sex weigh less than this baby. The 50th percentile is the median; the 10th and 90th mark the usual lower and upper bounds.
Where do I get the mean and standard deviation?
From a published growth reference (WHO, Fenton, or INTERGROWTH-21st) for the matching gestational age and sex. The accuracy of the result depends entirely on using the right reference values.
What do SGA and LGA mean?
Small for gestational age (SGA) usually means below the 10th percentile; large for gestational age (LGA) means above the 90th. Both can warrant closer monitoring.
Is a low percentile a problem?
Not necessarily — many small babies are perfectly healthy. Percentiles are a screening signal, and a clinician evaluates them alongside other findings.
Can I use this instead of a clinical chart?
No. This is an educational estimate. Always rely on a healthcare professional and validated growth charts for medical decisions.