How to use the iso week number calculator
Choose a date to see its Monday-based ISO week and the full Monday-to-Sunday span.
How the calculation works
ISO weeks start on Monday. Week 1 is the week containing 4 January, so the ISO week-year can differ from the date’s calendar year around New Year.
Check your inputs carefully: an incorrect date can change a result. Dates and times are entered using your device settings, and no account is needed. For formal, legal, medical or financial purposes, verify the applicable local rules and official records.
Frequently asked questions
Can early January belong to last year’s week?
Yes. ISO week-years can differ from calendar years around 1 January.
Which time zone is used?
Dates use your device’s local calendar day. Date-only calculations avoid daylight-saving day-length errors.
Are the dates I enter uploaded?
No. AgeGlide calculates your results in your browser. Entered dates are not sent to our server for processing.