Baby Growth Percentile Calculator (Weight, Length, Head)

Find your baby's weight, length and head circumference percentiles from birth to 24 months, using the WHO growth standards that the CDC recommends for children under 2.

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How baby growth percentiles are calculated

A growth percentile tells you where one measurement sits among healthy babies of the same age and sex. A 6-month-old girl at the 40th percentile for weight weighs more than about 40 of every 100 girls in the reference group and less than about 60.

The reference. For children under 2, the CDC recommends the WHO Child Growth Standards. They come from the WHO Multicentre Growth Reference Study, which followed healthy, mostly breastfed babies in six countries raised in conditions that support healthy growth. The CDC publishes the WHO parameters as public-domain data files, and this calculator uses those files for weight-for-age, length-for-age and head circumference-for-age, separately for girls and boys, at every month from birth to 24 months.

The LMS method. Each month has three numbers: L (skewness), M (the median) and S (spread). A measurement X becomes a z-score with z = ((X/M)^L - 1) / (L x S), and the z-score becomes a percentile through the standard normal distribution. For an age between whole months, the calculator interpolates L, M and S linearly between the two nearest months. A value exactly at the median is the 50th percentile, and the calculator reproduces the CDC files' own 5th and 95th percentile columns.

What the numbers do and don't mean

There is no "good" percentile. A baby at the 10th percentile and a baby at the 90th percentile can both be growing perfectly. What pediatricians watch is the pattern over time: a baby who follows roughly the same curve from visit to visit is usually growing as expected for their own build.

  • Below the 2.3rd or above the 97.7th percentile. The CDC's 2010 recommendations use these cut-offs (2 standard deviations) on the WHO charts to flag measurements that deserve a closer look. Many of these babies are healthy, but it is worth a conversation.
  • Crossing two major percentile lines (for example from above the 50th to below the 10th) over a few months is more meaningful than any single reading.
  • Head circumference tracks brain growth and is checked at every well-baby visit in the first two years.
  • Premature babies are usually plotted using corrected age (age from the due date) until about 2 years. Enter corrected age in months for a closer comparison, and follow your pediatrician's advice.

Measuring at home: common mistakes

Home measurements vary more than clinic measurements, so treat a surprising result as a reason to re-measure, not to worry.

  • Weighing with clothes or a full diaper. A wet diaper and a onesie can add 5 to 10 ounces, which moves a young baby's percentile noticeably. Weigh undressed, or weigh yourself holding the baby and subtract your own weight on the same scale.
  • Measuring length standing up. Under 2, length is measured lying flat, from the top of the head to the bottom of the heel with the leg straightened. Standing height is shorter by about 0.7 cm and belongs to the 2-to-20-year charts.
  • Tape too high or too low on the head. Place it just above the eyebrows and ears and around the widest part at the back, and take the largest of three readings.
  • Rounding the age. In the first months babies grow fast, so a 2-week difference matters. Use decimals: 3 and a half months is 3.5.
  • Using the wrong chart. These WHO standards stop at 24 months; from age 2 the CDC 2000 growth charts are used instead.

Frequently Asked Questions

Common questions about the Baby Growth Percentile Calculator (Weight, Length, Head)

Any percentile can be normal. Healthy babies are spread across the whole chart. Doctors look for measurements below the 2.3rd or above the 97.7th percentile and for a baby who crosses major percentile lines over time, not for a particular number.

Sources & References

WHO Growth Chart Data Files (LMS parameters, birth to 24 months)

Source of the L, M and S parameters and percentile columns for weight-for-age, length-for-age and head circumference-for-age used in the calculation (US government, public domain).
→Source: Centers for Disease Control and Prevention

Use of World Health Organization and CDC Growth Charts for Children Aged 0-59 Months in the United States (MMWR Recomm Rep 2010;59(RR-9))

CDC recommendation to use the WHO growth standards under age 2 and to use the 2.3rd and 97.7th percentiles as screening cut-offs.
→Source: CDC Morbidity and Mortality Weekly Report

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