Reference hub
Week Number Calculator
Find ISO week ranges, week start dates, week end dates, and yearly week-number pages.
Date Time Encyclopedia answer, with calculation details and assumptions below.
Choose one of the focused calculators below. Each page gives a direct answer first, then shows the assumptions and related date or time lookups.
Snapshot
Week Number Calculator
Find ISO week ranges, week start dates, week end dates, and yearly week-number pages.
Choose one of the focused calculators below. Each page gives a direct answer first, then shows the assumptions and related date or time lookups.
- Week rule
- ISO weeks start Monday and end Sunday
- Week 1 rule
- Week 1 contains the first Thursday of the ISO year
- ISO week examples
- Years 2020 to 2035
- Calendar basis
- Gregorian calendar
Method
Hub pages group focused answer pages around a shared date, calendar, time zone, or timestamp intent.
Find Another Lookup
ISO week rule
ISO weeks start on Monday and end on Sunday. Week 1 is the week that contains the first Thursday of the ISO week-numbering year.
Week ranges by year
Open a year and week route to see the exact Monday-to-Sunday range, nearby ISO weeks, and the total number of ISO weeks in that year.
Related Lookups
FAQ
When does an ISO week start?
An ISO week starts on Monday and ends on Sunday.
Can a year have 53 ISO weeks?
Yes. Some ISO week-numbering years have 53 weeks depending on how the year starts and ends.