Age in weeks calculator
Babies' ages are often given in weeks; this tool gives the whole weeks and extra days for any birth date.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter date of birth and age on this date.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Days = calculation date − birth date. Whole weeks = floor(days ÷ 7); remaining days = days mod 7.
Example
Inputs: Date of birth = 2026-06-01, Age on this date = 2026-09-30. Result: Whole weeks: 17; Remaining days: 2; Age in weeks (decimal): 17.29.
Limits and caveats
- This is age from the date of birth, not gestational age, which is counted from the last menstrual period.
Questions about this tool
How many weeks are in a year?
A year is 52 weeks and 1 day, or 2 days in a leap year.
When is a baby 12 weeks old?
84 days after birth. Enter the birth date to see how many weeks have been completed so far.
What does the decimal age in weeks mean?
It is the total days divided by 7, so 17.29 means 17 weeks and 2 days.