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Birth Weight Percentile Calculator

Health

Estimate a newborn's birth weight percentile for gestational age using reference growth curve data. See if a birth weight is average, high, or low for the week born.

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3005,500

Birth Weight Percentile

40.8
Reference Median Weight for Gestational Age
3,400
z-score
-0.23

This calculator computes your Birth Weight Percentile, Reference Median Weight for Gestational Age, z-score from the values you enter.

Inputs
Gestational Age at BirthBirth Weight
Outputs
Birth Weight PercentileReference Median Weight for Gestational Agez-score

What is a Birth Weight Percentile?

The Birth Weight Percentile Calculator estimates where a newborn's birth weight ranks compared to reference data for babies born at the same gestational age. Because expected birth weight rises steadily through pregnancy, this calculator anchors the comparison to gestational age in weeks rather than a single population-wide average.

It pairs naturally with the Fetal Weight Percentile Calculator for prenatal estimates and the Baby Percentile Calculator for tracking growth after birth.

How to use this Birth Weight Percentile calculator

  1. Enter the gestational age at birth in completed weeks.
  2. Enter the birth weight in grams.
  3. Read the Birth Weight Percentile result instantly.
  4. Compare against the Reference Median Weight shown for that gestational age.
  5. Use the copy button to save the result for a health record or discussion with a pediatrician.

Formula & Methodology

The calculator interpolates a reference median birth weight and standard deviation (SD) for the entered gestational age from a reference growth curve, then computes:

z-score = (Birth weight โˆ’ Reference median) รท Reference SD

Percentile = ฮฆ(z-score) ร— 100, where ฮฆ is the cumulative standard normal distribution.

Worked example โ€” a baby born at 38 weeks weighing 3,100 grams:

Reference median at 38 weeks โ‰ˆ 3,190 g, SD โ‰ˆ 400 g (interpolated between the 36 and 40-week reference points)

z-score = (3,100 โˆ’ 3,190) รท 400 = โˆ’0.23

Percentile โ‰ˆ ฮฆ(โˆ’0.23) ร— 100 = 41st percentile

This means the birth weight is within the typical range, close to but slightly below the reference median for 38 weeks.

Frequently Asked Questions

Birth weight percentile compares a newborn's weight against reference data for babies born at the same gestational age, expressing the result as the percentage of babies born at that gestational age who weighed less. It's used to gauge whether a birth weight is typical, low, or high for the pregnancy duration.
Birth weights below the 10th percentile for gestational age are often described as small for gestational age (SGA), while those above the 90th percentile are described as large for gestational age (LGA); these are population reference terms and don't by themselves diagnose a medical condition.
The calculator compares the entered birth weight to a reference median weight for the same gestational age in weeks, computes a z-score based on reference standard deviation data, and converts that z-score to a percentile using the normal distribution.
A baby born at 32 weeks is expected to weigh much less than one born at 40 weeks simply because there was less time for growth in the womb, so gestational age is the essential reference point rather than comparing all newborns against a single average.
The [Fetal Weight Percentile Calculator](/fetal-weight-percentile-calculator/) estimates percentile from a prenatal ultrasound weight estimate before birth, while this calculator uses the actual weight measured at birth.
Full term is generally considered 39 to 40 weeks of gestation, though babies born anywhere from 37 to 42 weeks are typically categorized within the broader term range by most reference growth curves.
No โ€” birth weight percentile is a population comparison, and factors like parental size, sex, and normal biological variation mean plenty of babies below the 10th percentile are perfectly healthy; percentile is one data point among many a clinician considers.
This calculator's reference data reflects singleton pregnancies; twins and other multiples are typically smaller on average at any given gestational age than singletons, so results for multiples should be interpreted with that context in mind.
This tool covers gestational ages from 22 to 42 weeks, spanning very preterm through post-term births.
This calculator accepts birth weight in grams; 1 pound equals approximately 453.6 grams, so a birth weight of 3,300 grams is roughly 7 lb 4 oz.
Also known as
birth weight for gestational ageSGA calculatorLGA calculatornewborn weight percentile