Baby Growth Percentile Calculator (0–36 Months)

About the Baby Growth Percentile Calculator (0–36 Months)

Enter your baby’s date of birth and a measurement — weight, length and head circumference — to see each one as a percentile and z-score for babies of the same sex and age, plus weight-for-length, on the CDC growth charts for birth to 36 months. For babies born early (before 37 weeks) the tool uses the corrected age, which the American Academy of Pediatrics advises using during the first two years.

Add earlier measurements from your baby’s health record to see the trend on the growth curves: one reading is a snapshot, and doctors look at how a child grows over time. For babies under 2 years, CDC recommends the WHO growth standards; this tool uses the CDC charts (see below), so ask your doctor for the WHO reading too.

Common uses

• Checking where a new weight or length from a check-up falls, and plotting it with earlier visits from the baby’s health record.
• Following a premature baby’s growth on corrected age.
• Converting a percentile your doctor mentioned into a z-score, or the other way round.

How the percentiles are worked out

CDC publishes three numbers for every month of age, separately for boys and girls: L (a Box-Cox power for skew), M (the median) and S (a coefficient of variation). The calculator interpolates them to your baby’s exact age, as CDC’s own programs do, then:

• z-score = ((measurement ÷ M)^L − 1) ÷ (L × S)
• percentile = Φ(z) × 100, where Φ is the standard normal distribution

Age is counted in days since birth ÷ 30.4375. Weight-for-length uses the length instead of the age (45 to 103.5 cm). A measurement CDC would flag as biologically implausible (a modified z-score below −5 or above +8 for weight, below −5 or above +4 for length, beyond ±5 for head circumference, below −4 or above +8 for weight-for-length) is shown with a warning to check it.

Corrected age for babies born early

A baby born early is compared with babies of the same age since their due date: corrected age = age since birth − (40 weeks − weeks of pregnancy at birth). A baby born at 32 weeks is 8 weeks early, so at 16 weeks old the corrected age is 8 weeks. The tool corrects for babies born before 37 weeks (the definition of preterm birth) during the first 2 years, the period for which the AAP advises using corrected age; you can switch it off. Before the due date the corrected age is negative and these charts do not apply — preterm growth charts are used then.

Lying-down length or standing height

The CDC charts for birth to 36 months use recumbent length, measured lying down. Standing height is about 0.8 cm (about 0.3 inch) shorter, so if your child was measured standing the tool adds 0.8 cm before comparing, as CDC’s own programs do for children under 2. From age 2, doctors usually switch to the CDC charts for 2 to 20 years, which use standing height — for those years see the child BMI percentile calculator.

CDC or WHO charts?

For babies and children under 2 years, CDC and the American Academy of Pediatrics recommend the WHO growth standards, which describe how healthy children should grow in good conditions, from repeated measurements of the same children; CDC’s infant charts were built from cross-sectional data, with no weights between birth and 2 months and small samples before 6 months. From 2 years, CDC recommends its own charts. This calculator uses the CDC birth-to-36-months charts; WHO offers its standards in the free WHO Anthro software. The two can place the same baby at different percentiles, so use the chart your doctor uses when comparing.

Sources

• CDC, growth chart data files — LMS tables for weight-for-age, length-for-age, head circumference-for-age and weight-for-length, birth to 36 months
• Kuczmarski RJ et al. CDC growth charts for the United States: methods and development. Vital Health Stat 11(246)
• Flegal KM, Cole TJ. Construction of LMS parameters for the CDC growth charts. National Health Statistics Reports No. 63
• CDC, SAS program for the CDC growth charts — age in days, interpolation, implausible-value flags, length versus height
• CDC, WHO growth chart recommendations and rationale, based on Grummer-Strawn LM et al., MMWR Recommendations and Reports 59(RR-9)
• American Academy of Pediatrics, Corrected age for preemies

Limitations

• A percentile is not a diagnosis. Healthy babies grow along many different percentiles; what matters is the pattern over time, together with feeding, health and family size. Very low or very high values, or a change across several percentile lines between visits, are reasons to talk to your child’s doctor.
• CDC recommends the WHO standards under 2 years; this tool shows the CDC charts only.
• The CDC charts describe US children measured decades ago. Babies born very early or with medical conditions may need specialised charts.
• Accepted values: weight 0.3–35 kg, length 35–125 cm, head circumference 20–60 cm; ages from birth (or the due date) to 36 months.

Privacy

Everything is calculated in your browser. Measurements stay in this page unless you tick “Keep on this device”, which stores them in this browser only (untick it to remove them); a CSV you download stays on your device. Nothing is uploaded.