Baby Growth Percentile Calculator

The infant percentile calculator tool converts measurements into ranks. When you enter the gender and age along with a baby's weight, length or head circumference, it will show you what rank that measurement falls in among 100 babies of the same age and sex. A weight at the 60th percentile means that your baby weighs more than about 60 of those 100 babies and less than the remaining 40.

Percentiles are not grades. There is no "passing grade" or target values. A baby in the 15th percentile can be just as healthy as a baby in the 85th percentile. What pediatricians look at is not only one measurement, but also the trend over time.

What percentiles tell us:

Growth charts are a compilation of growth data from thousands of healthy children. The distribution ranges for weight, length and head circumference are documented for each age and gender. A baby's percentile indicates its position within that distribution range.

The middle of the range is the 50th percentile, also called the median. Half of babies are above this value and half below it. The 3rd and 97th percentiles mark what's usually considered the normal range. Although measurements outside these limits are rare, they don't necessarily mean there's a problem and should be discussed with your doctor.

How to use this calculator:

Select whether it is a boy or girl. The reference data will differ depending on gender. Enter the age. You can change the unit to weeks or years if you prefer. Then either enter weight, height, or head circumference.

You do not have to enter all the measurements. A percentile chart will be generated for each measurement entered so you can check weight only or compare weight and height at the same time. When both weight and height are entered, the calculator also adds in the percentile of height. This allows you to compare your baby's weight with other babies of the same age but based on their height.

If you want a clinical representation, show the z-score. The z-score and percentile both give the same information but differ in that the z-score gives the deviation from the median in units of standard deviations.

Formula for calculating percentile:

The growth curves store for each age numerical values which are referred to as LMS-parameters: L, M and S. L indicates the degree of distortion of the distribution, M is the median and S the coefficient of variation. The baby's measurements are converted into a z-value with the following formula:

where X is the baby's measured value. If L is zero, then the formula becomes logarithmic. Z is equal to the natural log of X divided by M and then divided again by S. The z-score is then converted into a percentile using a standard normal distribution.

This becomes more intuitive if we look at specific examples. Suppose a boy who is 12 months old weighs 10.3 kg, which is close to the median weight for boys of that age. His z-score would be near zero and his percentile rank would be about 50. Another boy of the same age weighs 9.2 kg, which is roughly at the 16th percentile, one standard deviation below the median.

Typical measurements by age group:

The following table shows the median (50th percentile) values from the reference data used by this calculator. Babies are not expected to match these numbers exactly; they represent an average within a wide range of healthy values.

Length when lying down, length when standing up, head circumference:

In children under two years of age the length is measured while lying down. From the second year of life on, it switches to measuring the length standing up, with a value that comes out about half a centimeter lower. Regardless of the method, the calculation tool can be used by entering the measured values into age-related charts.

Head circumference is measured at the widest part of the head, along the eyebrows and above the ears. Head circumference grows fastest in the first year of life and is a useful indicator of brain development. For this reason doctors track it along with weight and length.

Pay attention to trends and not just numbers.

A single percentile is like a snapshot, while growth is more like a moving picture. Babies often grow along their own curve and a steady line that stays close to the same percentile for several months is reassuring. Regardless of starting points, doctors will look more closely at sudden increases or decreases that cross more than one major percentile line.

For premature babies special considerations are required. For the first two years doctors usually use corrected age - an age calculated from expected date of birth rather than actual date of birth. This ensures that percentiles reflect developmental stage more than the fact that the baby was born early.

This calculator is for general information and educational purposes only and does not constitute medical advice or a substitute for a pediatrician. Growth is just one part of your child's health. If you have concerns about your baby's growth, please consult with a qualified healthcare provider.