Week Number Calculator
Look up the ISO week number (Monday-based) and corresponding week-year for planning and reports.
ISO week number
Find the ISO-8601 week number and week-year for any calendar date.
ISO week
40
Week year 2026
Need day counts? Use the date calculator.
What the ISO week number is
The page returns the ISO-8601 week number and the week-year for the date you pick. Weeks start on Monday. Week 1 is the week that contains the year’s first Thursday, which is the same rule as “the week that contains 4 January”.
The week-year can differ from the calendar year. 1 January can fall in week 52 or 53 of the previous week-year. Late December can fall in week 1 of the next week-year. Reports that say “week 1” should say which week-year they mean.
How to use it
- Pick a calendar date. The result updates from that date.
- Read the week number and the week-year together.
- If you are matching a payroll or school calendar, confirm they use ISO weeks and not a Sunday-start local week.
- For the number of days until that date, open Days Until.
The date stays in the browser. It is not uploaded.
Questions
Do weeks start on Sunday?
Why is the week-year not the same as the calendar year?
Is week 53 an error?
Frequently Asked Questions
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