The differences between the calculators are due to different approaches to the data. Please choose the correct calculator that best suits your needs.
Different summaries of the data in the background. The calculator chosen should be based on the form of the anthropometric data you have (is the data completed weeks or actual age?). The use of completed weeks is historic, when the data was only collected with weekly precision. When whole weeks are entered into the two calculators, the calculators give different values (e.g. 32 weeks 0 days) - this is not an error.
Completed weeks is how most research data is collected, that is when only the whole weeks gestational age data is available (24 0/7 to 24 6/7 is summarized as 24 weeks). The Completed Weeks Calculator uses data that has summarized all the size of all the babies that were born that week.
The Actual Age Calculator data has been adjusted to allow for daily charting in electronic growth charts, and the database has an individual data values for each day. The data was adjusted as documented in our 2013 paper to produce the actual age version, and the daily values were obtained from the splined data curves. The Actual Age Calculator should only be used if you have weeks and days of gestational age, and if you are confident that you need this precision.
Assign gestational age size at birth
The 6 country meta-analysis sex-specific percentiles, which were used to develop the Fenton Preterm Growth Charts, can be used to assign size for gestational age (small, large and appropriate) at birth. This data is made freely available under an open Creative Commons License for non-commercial use.
Actual Age Calculator
Use only if age is defined in both weeks and days
Research Bulk Calculator
Uses Completed weeks
for weight, Head circumference, and Length - can calculate up to 500 babies' completed weeks z-scores and exact percentiles at one time.
Many thanks to Dr. Tim Stevens for his assistance in preparation of these spreadsheets.
These results differ from the Actual Age Calculator since they are based on Completed weeks (24 weeks and 0 to 6 days are categorized as 24 weeks), which is usually of utility for research purposes.
Do not enter days of age into this calculator as it causes this calculator to incorrectly round up or down
Size-for-Gestational Age and Gestational-Age Calculator
A Web-based calculator is available here: