Week Number Calculator
Find the ISO week number, week-year and week dates for any date.
Select a date to find its ISO week number.
About the Week Number Calculator
The Week Number Calculator finds the ISO-8601 week number for any selected Gregorian calendar date. It also shows the ISO week-year and the Monday-to-Sunday date range for that week.
ISO weeks begin on Monday, and week 1 is the week containing January 4. This definition keeps complete weeks consistent but means some dates near New Year belong to the previous or following ISO week-year.
For example, January 1 can be part of week 52 or 53 of the previous ISO year, while late December can belong to week 1 of the next ISO year. The calculator calls out this boundary whenever it occurs.
All calculations use date-only UTC arithmetic and run locally in the browser, so timezone and daylight-saving changes cannot shift the selected date.
Key features
- ISO-8601 week number
- ISO week-year identification
- Monday week start and Sunday week end
- Correct New Year boundary handling
How to use
- 1Select the calendar date you want to check.
- 2Read its ISO week number and ISO week-year.
- 3Review the Monday start and Sunday end dates for the complete week.
Frequently asked questions
- What is an ISO week number?
- ISO-8601 numbers weeks from Monday through Sunday. Week 1 is the week that contains January 4.
- Why can the ISO week-year differ from the calendar year?
- The first and last few calendar days of a year can belong to a week whose Thursday falls in the adjacent year. ISO assigns the whole week to that Thursday's year.
- Which day starts an ISO week?
- Monday is the first day and Sunday is the final day of every ISO week.
- Can an ISO year have 53 weeks?
- Yes. Depending on how weekdays and leap years align, an ISO week-year can contain either 52 or 53 weeks.
- Does timezone affect the week number?
- No. The input is handled as a calendar date without local timestamp conversion, so timezone changes do not alter the result.