Birth Weight Percentile Calculator

The percentile for birth weight compares the weight of a newborn with other babies of the same sex and gestational age. The 50th percentile means that half of all babies weigh more than this baby, while the other half weighs less. This calculator converts weight, gestational age, and sex into percentiles and z-scores and plots the results on standard growth charts.

What your birth weight percentile means:

Weight alone is often not meaningful without additional information. The expected birth weight of a baby born at 32 weeks will be much less than the expected birth weight of a baby born at 40 weeks. Percentiles provide this additional information. By comparing a baby's weight to a reference group of babies of similar age and sex, we can see that a baby who weighs 2,000 grams may be in the normal range for 33 weeks gestation but would likely be considered very small at 40 weeks gestation.

Doctors divide the results into three areas: A percentile below the 10th percentile indicates a small for gestational age (SGA) baby. The area between the 10th and 90th percentiles is considered average weight for gestational age (AGA). A percentile above the 90th percentile indicates large for gestational age (LGA). The 3rd and 97th percentiles show extreme values.

How to use this calculator:

Select the baby's gender, enter your last gestational week and additional days, then enter their birth weight in grams, kilograms, pounds or ounces. The percentiles, z-scores and body classifications will be displayed immediately.

If the sex is unknown or not to be specified, select "Unisex". This option will calculate the average of male and female curves and give a value similar to the gender-neutral chart used in many simple tools. The results area also shows the median weight as well as the 10th-90th percentile range for each gestational week so you can see a typical range.

Reference values and calculation formulas

This tool uses international reference values called the INTERGROWTH-21st newborn body size standards based on data from healthy pregnancies across different countries. These standards show weight distributions for each sex and gestational week, so individual weights can be converted into z-scores and percentiles.

The z-score indicates how many standard deviations the weight is away from the median. Once the z-score is known, then the percentile can be determined using the standard normal cumulative probability.

where z is the z-score and Φ is the standard normal cumulative distribution function. A z-score of 0 corresponds to the 50th percentile, a score of about −1.28 corresponds to the 10th percentile and a score of about +1.28 corresponds to the 90th percentile. The following example shows a male who was born at 40 weeks and 0 days gestational age and had a weight of 2,880 g.

This table shows the inputs and example sentences for each value.

Birth weight classification

Unlike percentile-based classification which depends on the gestational age, absolute weight-based classification is independent of gestational age. Extremely low birth weight is less than 1000 g. Very low birth weight is between 1000 and 1500 g. Low birth weight is between 1500 and 2500 g. The normal range for birth weight is between 2500 and 4000 g. Above 4000 g is considered to be macrosomia, also known as "big baby".

Even if a baby is born small for gestational age, they can still be of average weight for their preterm week. So percentiles and absolute weights answer different questions, and this calculator gives both at the same time.

How to interpret the results:

Percentiles in the middle of the range are a reassuring sign. Percentiles at either end should be discussed with your doctor, but both small and large babies are often perfectly healthy. At very low percentiles, your doctor may investigate fetal growth retardation, and at very high percentiles he or she might test for gestational diabetes. Trends over time are more important than individual values.

This calculator is for general educational purposes only and does not constitute medical advice or replace the judgment of a qualified healthcare professional. Always consult your pediatrician or midwife regarding your baby's growth.