Birth Weight Percentile Calculator

Enter a baby's birth weight and a reference mean and standard deviation to see its percentile, z-score, and whether it is small, appropriate, or large for gestational age.

Quick Facts

Method
Percentile = Φ((weight - mean) / SD) x 100
A z-score is read off the normal distribution to give the percentile.
Categories
SGA below 10th, AGA 10th to 90th, LGA above 90th
Use a reference mean and SD for the baby's gestational age and sex.

Your Results

Calculated
Birth weight percentile
-
Percent of reference babies weighing less
Z-score
-
Standard deviations from the mean
Difference from mean
-
Weight minus reference mean
Size category
-
SGA, AGA, or LGA

Ready

Enter the birth weight and a reference mean and SD, then press Calculate.

About the Birth Weight Percentile

A birth weight percentile places a newborn's weight on a scale from 0 to 100 relative to a reference group of babies of the same gestational age and sex. A baby at the 60th percentile weighs more than about 60 percent of that reference group. Percentiles make it easy to see whether a weight is typical, on the light side, or on the heavy side for a baby's age, without needing to memorize the raw reference numbers.

The formula

The calculation is the standard statistical route from a measurement to a percentile. First it computes a z-score, the number of standard deviations the weight sits above or below the reference mean:

  • Z-score: z = (birth weight - reference mean) / reference standard deviation.
  • Percentile: the z-score is converted to a percentile using the cumulative normal distribution, percentile = Φ(z) x 100. A z of 0 is the 50th percentile, +1 is about the 84th, and -1 is about the 16th.

Because the reference mean and standard deviation change with gestational age and differ between boys and girls, you enter the mean and SD that match the baby's exact age and sex from a birth weight chart or table. The defaults shown are a rough stand-in for a term (about 40-week) newborn and should be replaced with values for your reference.

SGA, AGA, and LGA

Clinicians describe size for gestational age with three standard bands based on the percentile: SGA (small for gestational age) is below the 10th percentile, AGA (appropriate for gestational age) is from the 10th to the 90th, and LGA (large for gestational age) is above the 90th. The 10th and 90th percentiles correspond to z-scores of roughly -1.28 and +1.28.

How to get accurate results

  • Weigh the baby undressed on an accurate infant scale and record the weight in grams for the most precise input.
  • Match the reference mean and SD to the baby's exact completed weeks of gestation and to the correct sex, since both shift the reference substantially.
  • One weight is a single snapshot; a growth trajectory over time carries more information than any one percentile.

When to consult a professional

This calculator is a statistical estimate, not a diagnosis, and it does not account for a baby's medical history. For any result that concerns you, or for decisions about a newborn's care, consult a licensed healthcare provider who can interpret the percentile alongside the full clinical picture.

Frequently Asked Questions

What is a birth weight percentile?
A birth weight percentile ranks a newborn's weight against a reference group of babies of the same gestational age and sex. A 70th percentile means the baby weighs more than about 70 percent of that reference group. It is found from a z-score, z = (birth weight - reference mean) / reference standard deviation, then converted to a percentile with the normal distribution.
What do SGA, AGA, and LGA mean?
They are standard size-for-gestational-age categories. SGA (small for gestational age) is below the 10th percentile, AGA (appropriate for gestational age) is between the 10th and 90th, and LGA (large for gestational age) is above the 90th. They flag a weight that is unusual for the baby's age and may warrant closer review.
Where do the reference mean and standard deviation come from?
They come from a birth weight reference or growth chart for the baby's exact gestational age and sex. Enter the mean and standard deviation for that age and sex, and this calculator applies the standard normal z-score formula to turn the weight into a percentile.
Is a low or high percentile a diagnosis?
No. A percentile is a statistical position, not a diagnosis. Healthy babies span the whole range, and a single measurement does not describe a growth trend. A value outside the usual range is a prompt to discuss the result with a healthcare provider, not a conclusion on its own.

Related calculators