Birth Weight Percentile Calculator

A birth weight percentile compares a baby's weight with other babies of the same sex born at the same point in pregnancy. At the 40th percentile, a baby weighed more than about 40 out of 100 babies born at that week. The week matters as much as the weight, because babies still gain about 110 to 200 g a week between 36 and 41 weeks¹.

What Birth Weight Percentile Measures

It measures size at birth for gestational age (how far along the pregnancy was) and sex. It is not a health score and does not track growth after birth. For weight checks after birth, use the Child Growth Percentile Calculator.

The Formula

This calculator uses the INTERGROWTH-21st newborn standards. They were built from 20,486 healthy, low-risk pregnancies in eight urban populations, dated by early ultrasound, with babies weighed within 12 hours of birth¹. From 24 to 32 weeks it uses a matching very preterm reference from the same project².

Percentile = 100 x the share of reference babies (same sex, same week and day) who weighed less than your baby

24+0 to 32+6 weeks: a separate curve for each day and sex from the very preterm reference²

33+0 to 42+6 weeks: a skewed curve (skew t) for each day and sex from the newborn standard¹

SGA: below the 10th percentile. AGA: 10th to 90th. LGA: above the 90th.

How This Calculator Computes Your Result

Choose boy or girl. Enter gestational age at birth in weeks and days (for example 38 weeks 4 days), or enter your due date and your baby's birth date and the tool counts back from 40 weeks. Then enter the weight in pounds and ounces or in grams.

You get a whole-number percentile, the SGA, AGA or LGA label, a z-score, and the 10th, median and 90th percentile weights for that day and sex. The label uses the exact value, so 9.8 shows as the 9th percentile and SGA. Far from the middle, it says "below the 1st" or "above the 99th". It covers 24 weeks 0 days to 42 weeks 6 days. Below 28 weeks it adds a note, because few babies that early were measured².

What Your Result Means

These are the usual cut-offs for SGA and LGA³ ⁴. Because the standard was built from healthy pregnancies, about 1 in 10 of those babies sit below the 10th percentile and 1 in 10 above the 90th. A label is a reason to look closer, not a diagnosis.

Published INTERGROWTH-21st weights at the start of each week, rounded to 10 g¹ ²:

The calculator works from the curves behind these tables, so its reference weights can differ from the table by up to about 15 g.

Same Weight, Different Week

Take a girl who weighs 2,600 g (5 lb 11.7 oz). Born at 35 weeks 0 days, she is at the 72nd percentile. At 37 weeks 0 days she is at the 29th, and at 39 weeks 0 days she is at the 8th, which is SGA¹.

SGA is also not the same as low birth weight. The WHO defines low birth weight as under 2,500 g at any gestational age, caused by growth restriction, early birth or both⁵. So a 2,300 g girl born at 35 weeks has low birth weight but sits near the 42nd percentile, while the 2,600 g girl born at 39 weeks is SGA without low birth weight.

Which Chart Is Used Changes the Answer

Charts can label the same baby differently. In 1,475,457 single births at 33 to 42 weeks in 15 European countries, national charts labeled about 10% of babies SGA and 10% LGA in every country⁶. With INTERGROWTH-21st, SGA ranged from 3.0% to 10.1% and LGA from 8.0% to 24.9%, depending on the country⁶. Hospitals may use Fenton, Olsen, national or customized charts, so the percentile in your baby's notes can differ from this one.