Who is this calculator for?
This version is for babies who were born at 37 weeks or later and were younger than 2 on the measurement date. You can use a measurement from an earlier checkup, as long as it fits that age range.
If your baby was born before 37 weeks, or you're not sure, this version will not calculate a result. Ask your pediatrician which chart and age to use. We do not make a corrected-age adjustment.
For U.S. children, the CDC recommends WHO growth standards before age 2 and CDC charts from age 2. Our age limit follows that U.S. guidance; WHO also publishes standards for older children.
Read the CDC's chart guidanceWhat can you calculate?
You can enter any one of these measurements, or all three. Each gets its own result using the selected boys' or girls' chart.
- Weight for age
- How your baby's weight compares with the weight reference for the same age and sex.
- Length for age
- How your baby's length compares. Use a measurement taken while your baby is lying down.
- Head circumference for age
- How the measurement around your baby's head compares with the reference.
If you enter several measurements, use ones taken on the same date. This version does not calculate weight-for-length, BMI, or a combined growth score.
From your entry to a percentile
Find the age on that day
We count calendar days between the date of birth and the measurement date. We do not round the age to whole months. Dates before birth or in the future cannot be used.
Use the matching WHO reference
We select the daily data for that age, chart, and measurement. U.S. measurements are converted to kilograms or centimeters for the calculation.
Explain the result
We calculate the percentile and show a point on the matching reference chart. The point represents this measurement. It does not show your baby's growth over time.
Switching between U.S. and metric units preserves the measurement used in the calculation. The values shown in the fields may be rounded for readability.
What does the result mean?
An example, not your baby's result
If a weight result is at the 40th percentile, about 40 out of 100 babies of the same age and sex on that chart weigh less.
A higher percentile is not a better score. One number cannot tell you whether your baby is healthy. Your pediatrician considers measurements over time together with your baby's overall health. See how the CDC describes growth assessment.
Results usually show one decimal place. At the very ends of the chart, we show “Below 0.1” or “Above 99.9.” These describe chart position, not a diagnosis.
If an entry is outside this calculator's supported reporting range, we ask you to check the measurement and unit. That message is not a judgment about your baby's health.
Why might another result look different?
Start by checking that both results use the same information:
- The date of birth and measurement date.
- The boys' or girls' chart and the same growth standard.
- The same measurement, units, and measurement method.
- The same type of comparison, such as weight-for-age rather than weight-for-length.
Rounding can also change the number shown. If a result differs from one given at a checkup, ask your pediatrician to help you compare them.
Your entries stay in your browser
The calculation happens on your device. This website does not save or send your baby's dates, chart selection, answer about birth before 37 weeks, measurements, or results to a server.
There is no account or saved growth history. Clear form removes the current entries and results. Printing uses your browser's print feature.
Ordinary hosting access logs may record page requests and IP addresses. Your browser controls its own autofill and page history. These are separate from the calculator's handling of your entries.
The sources behind the calculator
We use published WHO daily reference data. We check our calculations against independently generated outputs from the official WHO R functions. These checks verify the software's calculations; they do not replace medical assessment or clinical review.
- WHO Child Growth Standards
Growth standards and supporting information from the World Health Organization.
- CDC: Using WHO growth standard charts
U.S. guidance on chart selection, measurements, and interpreting growth over time.
- WHO software and calculation tools
Official tools, including the anthro R package used for our reference checks.
More about the calculation method
Data and calculation
The data comes from a fixed WHO anthro source snapshot. We use its daily weight, length, and head circumference tables for both chart categories.
The LMS method uses three values from each daily record to calculate a statistical position called a z-score. We then convert that position to a percentile. Weight calculations include the WHO adjustment for values beyond ±3 z-scores. Length and head circumference use the ordinary LMS method.
WHO's source data-quality flags are checked using its two-decimal rounding behavior. Flagged inputs produce a recheck message in this calculator. These flags are not healthy or unhealthy percentile boundaries.
Dates and unit conversion
Eligibility ends at the second calendar birthday. For a February 29 birth, this website uses February 28 when that anniversary year has no February 29. This is our eligibility convention; elapsed days still determine the calculation age.
Conversions use 1 pound = 0.45359237 kilograms, 16 ounces = 1 pound, and 1 inch = 2.54 centimeters. The calculation retains unrounded values.
You can inspect the calculation source, reference files, and verification method in the calculator source download. The included WHO material and calculator adaptation carry the GPL-3.0 license.
This is an independent website. Using WHO standards does not mean WHO or the CDC endorses this calculator.