Week number
Date & timeWhat week number is it?
Today's ISO week number, the week of any date, and the dates of any week.
What you enter stays on your device while you use this tool. Opening a shared link sends the settings in its address to HowToolbox to prepare the page.
Leave it empty for the current week.
Dates of a week
This week
Week …
The week
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Today's ISO week number, the week of any date, and the dates of any week.
How to use
- Read this week's number at the top.
- Pick a date to see its week number and the Monday and Sunday of that week.
- To find the dates of a week, enter its number and the year under Dates of a week.
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How it works
Week numbers here follow the international standard ISO 8601, used in Finland and most of Europe. A week runs from Monday to Sunday, and week 1 is the week that contains the year's first Thursday. Put another way, week 1 is always the week with 4 January in it.
A week belongs to the year its Thursday falls in. So a few days around New Year can belong to the neighbouring year's weeks: Saturday 1 January 2005 is in week 53 of 2004, and Monday 2 January 2006 starts week 1 of 2006. The full ISO week date shows this, for example 2026-W42-3 for Wednesday 14 October 2026.
Most years have 52 weeks. A year has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks, and its week 53 runs from 28 December 2026 to 3 January 2027.
Calendars in the United States usually number weeks differently: weeks start on Sunday and week 1 is the week with 1 January, so their numbers can differ. Today is the date on your own device.
Frequently asked questions
Why is 1 January sometimes in week 52 or 53?
Because the week belongs to the year of its Thursday. When 1 January falls on a Friday, Saturday or Sunday, that week's Thursday is still in December, so the days belong to the last week of the previous year.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. 2020 and 2026 have 53 weeks; 2021 and 2027 have 52.
Why does my US calendar show a different week number?
US calendars often start the week on Sunday and count week 1 from the week with 1 January. This page uses the ISO 8601 rule, as Finland and most of Europe do.
Can I look up the dates of a week number?
Yes. Enter the week-year and week number to see Monday through Sunday.
Can ISO week 1 begin in December?
Yes. For example, week 1 of 2026 starts on 29 December 2025.
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