Baby Growth Percentile Calculator

The baby growth percentile calculator compares your baby's weight, length and head circumference with the WHO Child Growth Standards for the same sex and age (birth to 24 months). Enter any or all three measurements to get the percentile and z-score for each, the weight-for-length percentile and the median and normal range for your baby's age.

How to use the Baby Growth Percentile Calculator

1. Select your baby's sex and enter age in months plus extra days (0 to 24 months). Use adjusted age if your baby was born early.
2. Enter any of weight, length and head circumference. Leave a box empty to skip it. For weight in pounds and ounces, put pounds in the weight box and ounces in the ounces box.
3. Choose the units for each measurement.
4. Press Calculate to see the percentile and z-score for each measurement, plus the weight-for-length percentile when both weight and length are entered.

Formula and method

The WHO publishes three numbers for each sex at each age: L (skewness), M (median) and S (coefficient of variation). The calculator interpolates them to your baby's exact age in months and days and computes z = ((measurement / M)^L − 1) / (L × S), or ln(measurement / M) / S when L is 0. The percentile is the normal distribution function of z times 100. A z-score of 0 is the 50th percentile, −2 is about the 2nd and +2 is about the 98th.

The WHO tables used here are the monthly LMS values from birth to 24 months. Between whole months the calculator interpolates linearly, so results can differ from the WHO daily tables by a small fraction of a percentile point.

Baby weight by age: boys (WHO)

Weight at key percentiles for boys from birth to 24 months, from the WHO standards. The median column also shows pounds.

Baby weight by age: girls (WHO)

The same percentiles for girls.

Baby length and head circumference by age

Length (in the table above) and head circumference (below) at the 5th, 50th and 95th percentiles.

Weight-for-length: is weight in proportion to length?

Weight-for-length compares weight with babies of the same length, whatever their age. It is a way to see if a baby is in proportion. Values from the WHO tables:

How percentiles work for babies

A percentile is not a grade. A baby at the 25th percentile weighs the same as or more than 25 out of 100 babies of the same age and sex in the WHO sample. Healthy babies fall everywhere between the 3rd and 97th percentile. A steady path along the same percentile channel, whichever one, is a reassuring sign. A sudden drop across two major lines is the thing a doctor will want to look at.

WHO standards and breastfed versus formula-fed babies

The WHO standards describe how healthy, mostly breastfed children grew in six countries under good conditions. The CDC and the American Academy of Pediatrics recommend them for children younger than 2 in the United States. Breastfed babies often gain faster than average in the first 2 to 3 months and then more slowly, so a breastfed baby's percentile may drift down a little in the first year when compared with older CDC charts; the WHO standards reflect that pattern.

Premature babies and adjusted age

Babies born before 37 weeks are usually plotted by adjusted (corrected) age until about 24 months. Adjusted age is the age since birth minus the number of weeks early. Enter the adjusted age in the age boxes and ask your baby's doctor how they interpret the result, because very preterm babies often follow their own curves. The Adjusted Age Calculator works out the adjusted age for you.

Newborn weight loss and the first weeks

Most newborns lose some weight in the first few days and regain birth weight by about two weeks. That early dip is expected and does not mean the baby is off the curve. Your baby's doctor will compare weight at each check with the birth weight and the percentile line.

How to measure your baby accurately

Weigh a baby undressed on the same scale, ideally a pediatric scale. Measure length lying down on a flat surface with the legs straight and a second person holding the head against a fixed board, since length varies by about a centimeter depending on position. For head circumference, wrap a flexible tape just above the eyebrows and ears around the widest part of the back of the head, and take the largest of three readings.

When to call the doctor

Call your baby's doctor or nurse if a measurement is below the 3rd or above the 97th percentile, if a percentile has crossed two major lines (for example from the 50th to the 10th), if your baby is not regaining birth weight by about two weeks, is feeding poorly, has fewer wet diapers than usual, seems unusually sleepy or fussy, or if you are worried for any reason. Do not wait for the next routine visit if something seems wrong.

Frequently asked questions

What is a good percentile for a baby?

Any percentile from the 3rd to the 97th is within the usual range. The 50th is the median, not a goal. A steady line over time matters more than the number.

My baby is in the 5th percentile. Is that bad?

Not by itself. Five percent of healthy babies are at or below that line. Doctors look at the trend, feeding, development and the parents' size.

How accurate is this percentile calculator?

It uses the WHO LMS tables and interpolates between months, which gives results within a fraction of a percentile point of the daily WHO tables. Measurement error is usually bigger than this.

Should I use CDC or WHO charts for my baby?

The CDC and AAP recommend WHO standards for birth to 24 months and CDC charts from age 2.

Does weight percentile matter more than height?

No single number matters most. Doctors look at weight-for-age, length-for-age, head circumference and weight-for-length together with how the baby is feeding and developing.

What if my baby crosses percentile lines?

Some movement is normal in the first months. If a baby crosses two major lines, your doctor will want to review feeding and health.

Sources

• WHO Child Growth Standards
• CDC: WHO Growth Charts for children from birth to 24 months
• CDC: Growth chart percentile data files with LMS values

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