Week number
The page shows today’s ISO week, or the week of any date you pick, with that week’s Monday and Sunday. You can also turn a week number back into dates.
- ISO week number
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How to use it
- Leave the date on today, or pick another date.
- Read the ISO week, the week-year and the Monday to Sunday range.
- Open a week number to see its dates, and download the year’s week table.
ISO weeks and the US count
ISO 8601 weeks start on Monday. Week 1 is the week that contains the first Thursday, which is also the week that contains 4 January. A date in late December or early January can belong to the neighbouring year: 31 December 2020 and 3 January 2021 are both in 2020-W53.
The US option counts weeks from Sunday, and week 1 is the week that contains 1 January. Looking up a week number lists that week’s first and last day, and the table under the result lists every ISO week of the year as a CSV.
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Questions
What is an ISO week number?
A number from 1 to 52 or 53. The week starts on Monday, and week 1 contains the first Thursday of the calendar year.
Why is 1 January sometimes last year’s week?
If 1 January falls on Friday, Saturday or Sunday, that day is still in the previous ISO week-year. 1 January 2021 is in 2020-W53.
What is the US week number?
Tick the US line. Weeks start on Sunday, and the week that contains 1 January is week 1. It often differs from the ISO number.
Which dates are in 2026 week 1?
ISO 2026-W01 runs from Monday 29 December 2025 to Sunday 4 January 2026. 1 January 2026 is a Thursday in that week.
How do I list a whole year?
The table under the result is every ISO week of the week-year, with Monday and Sunday. Download CSV saves it.