Why can January 1 be week 52 or 53?

ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday. Early January days before that can belong to the previous year's final week.

Is the US week number different?

The US convention often starts weeks on Sunday and numbers from January 1, so it can differ by one from ISO. This tool uses ISO 8601, the international standard.

Week Number Calculator

ISO week for any date.

DateDefaults to today

ISO week

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Pick a date.

Much of Europe plans by week number — “delivery in week 34”. This shows the ISO 8601 week for any date, plus the Monday-to-Sunday range it covers, so week talk and date talk stop getting lost in translation.

How to use it

1.Pick a date (it defaults to today).
2.Read the ISO week number and year.
3.Check the Monday–Sunday range the week spans.

Questions people ask

Why can January 1 be week 52 or 53?: ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday. Early January days before that can belong to the previous year's final week.
Is the US week number different?: The US convention often starts weeks on Sunday and numbers from January 1, so it can differ by one from ISO. This tool uses ISO 8601, the international standard.

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