Week Number Calculator
Find the ISO week number and ISO week-year for a date.
Results are provided for general informational use. Verify important financial, health or legal figures before relying on them.
About this Week Number Calculator
The Week Number Calculator reports the ISO 8601 week number for a selected date. ISO weeks begin on Monday, and week 1 is the week containing the year’s first Thursday.
For related tasks, try Date Difference Calculator, Day of Year Calculator or Add Days to Date.
How the calculation works
The calculation shifts the selected date to the Thursday of its ISO week and then measures the week position from the start of the ISO year.
How to use it
- Choose a calendar date.
- Select Calculate week number.
- Review the ISO week and ISO week-year.
Practical example
A date near the start or end of a year can belong to the adjacent ISO week-year depending on the weekday layout.
Frequently asked questions
Does ISO week 1 always start on January 1?
No. ISO week boundaries depend on Mondays and the first Thursday rule.
Can December dates belong to next year’s week 1?
Yes. Some late-December dates do.
Can early January belong to the prior ISO year?
Yes.
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