Week number — ISO 8601 and US conventions
Look up a date’s ISO week and its Monday-to-Sunday range, or compare it with the Sunday-start week number shown alongside.
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ISO weeks start on Monday; week 1 contains the first Thursday. Early January can belong to the previous ISO year.
The US-style count here starts weeks on Sunday and puts 1 January in week 1. Check which system your calendar or workplace uses.
How to use
- Choose the date. The initial value is today’s UTC date; change it if you need a different local or reporting date.
- Read the week and its year together. 1 January 2021 is ISO week 53 of 2020, covering 28 December 2020 through 3 January 2021.
- Match the reporting convention. The same date is week 1 under the Sunday-start convention shown as US week number.
Questions
How does ISO week numbering work?
Weeks run Monday to Sunday. Week 1 contains 4 January, equivalently the first Thursday of the year. The whole week belongs to the ISO year containing that Thursday, which can differ from the date’s calendar year.
What does the US week number mean here?
This calculator starts weeks on Sunday and puts 1 January in week 1, even when that first week is short. Organisations and software may use other conventions, so check which one your report expects.
Can an ISO year have 53 weeks?
Yes. It has 53 weeks when 1 January or 31 December falls on a Thursday. The tool shows the number of weeks in the selected date’s ISO year.
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