Birth Weight Percentile Calculator

Created by Dominika Śmiałek, MD, PhD candidate
Reviewed by Dominik Czernia, PhD and Jack Bowater
Based on research by
Villar J, Cheikh Ismail L, Victora CG, Ohuma EO, Bertino E, Altman DG, Lambert A, Papageorghiou AT, Carvalho M, Jaffer YA, Gravett MG, Purwar M, Frederick IO, Noble AJ, Pang R, Barros FC, Chumlea C, Bhutta ZA, Kennedy SH International Fetal and Newborn Growth Consortium for the 21st Century (INTERGROWTH-21st). International standards for newborn weight, length, and head circumference by gestational age and sex: the Newborn Cross-Sectional Study of the INTERGROWTH-21st Project Lancet (September 2014)See 2 more sources
Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology (October 1991)Nicolaides KH, Wright D, Syngelaki A, Wright A, Akolekar R. Fetal Medicine Foundation fetal and neonatal population weight charts Ultrasound Obstet Gynecol. (July 2018)
Last updated: Sep 29, 2023


Are you looking for a tool to estimate your baby's weight at birth? Use this birth weight percentile calculator to find out where among their peers your child places. It not only shows the average weight of a newborn baby, but this calculator also adjusts their weight according to the week of pregnancy in which the baby was delivered (take a look at the due date calculator).

In the article below, you can find some information on the average newborn weight, according to the weight percentile calculator, the average newborn weight classification, and an explanation of how to use this birth weight percentile calculator.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. All information on this website is for informational purposes only and is not intended to serve as a substitute for medical consultation. Always consult your results with a health care provider.

Average baby weight by a week of delivery ⏳

The weight of a baby at birth differs depending on their age when they are delivered. We need to know the adjusted age of a baby to correctly assess its development and milestones. To calculate the adjusted age, use the adjusted age calculator.

Because of such a wide range of possible pregnancy duration, the average weight of a newborn baby may also vary.

Average weight of newborn baby 👶

The average birth weight for newborns born full-term is between 5.5 lb (2.5 kg) and 8.8 lb (4 kg).

What is the classification?

  1. Extremely low birth weight (ELBW) is a term used for infants who weigh less than 1,000 g at birth (2.2 lb).
  2. Very low birth weight (VLBW) describes babies born weighing less than 1,500 grams (3.3 lb).
  3. Low birth weight (LBW) is when a baby's weight is less than 2,500 g (5.5 lb) at birth.
  4. Normal weight (NBW) is if the baby weight is between 2,500 and 4,000 g (5.5 - 8.8 lb).
  5. High birth weight (HBW) is for babies born weighing more than 4 kg (8.8 lb).

Abnormal newborn weight causes

A preterm birth or slow prenatal growth may cause low birth weight. There are however other causes, such as genetic factors, maternal health issues (check out pregnancy weight gain calculator), or problems with the placenta. 👩

Birth weight percentile calculator ⚖️

To estimate the newborn weight percentile, you need to know two values. The first one is the gestational age of the baby at birth. The second one is their weight at birth.

Once you put in the data, the result - the percentile - will appear. It is an approximation based on the "Fetal Medicine Foundation fetal and neonatal population weight charts" study by Nicolaides et al.

Below the calculator you can see a birth weight percentile chart with percentiles marked in different colors. The 50th percentile means that 50% of babies born at that age weigh more and 50% weigh less. Your result is a dark dot, found somewhere between the percentile lines. The X values are the gestational age at which the baby may be born (between 20 and 42 weeks), and the Y values are weight. The color lines are percentile lines to help you read where your result is.

Newborn weight percentile calculator in practice

Let's work on an example of a newborn weight percentile calculation. Lucy, born yesterday, was born in the 35th week and 5th day of pregnancy. The neonatologist wants to calculate the percentile she's in according to her weight. The physician knows that preterm birth is a cause of low weight relative to children born on their due date. She uses this birth weight percentile calculator to estimate how Lucy's weight compares with the other children born at the same age.

She weighs 2500 g (5.512 lb). According to our calculator, that puts her between the 25th and 50th percentile (approximately the 27th percentile).

FAQ

How do we call babies with birth weight lower than the average?

Such a baby can be classified as either:

  • Low birth weight (LBW): 1500–2500 grams;
  • Very low birth weight (VLBW): 1000–1500 grams;
  • Extremely low birth weight (ELBW): 750–1000 grams; or
  • Incredibly low birth weight (ILBW) (which is very rare): 500–750 grams.

What do we need to know to estimate the newborn weight percentile?

The tables for birth weight percentiles require these data points:

  • Gestational age at the time of birth;
  • Gender of a baby; and
  • Current weight of a baby.

How to estimate the birth weight percentile?

We estimate the newborn percentile using the birth weight percentile curves or tables. Such graphs require us to know the gestational age, gender and weight of a baby. All these points of information allow us to find the specific place on a curve and compare your baby's growth to that of the general population.

Why is my baby losing weight after birth?

It is normal for a baby to lose around 7–10% of their weight soon after birth. The weight loss is primarily due to the excretion of their first urine and stool. The baby on average, regains the lost weight during the next 2 weeks.

Dominika Śmiałek, MD, PhD candidate
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