Find the current ISO 8601 week number, look up the week for any date, or get the Monday-to-Sunday dates for any week of any year.
Week numbers are everyday currency in European business, manufacturing and logistics — 'shipping in W32', 'the sprint starts week 40'. If your calendar does not show them, you need to look them up, and the answer is less obvious than it sounds.
This page gives the current ISO week, the week for any date you enter, and the Monday-to-Sunday dates for any week of any year.
The standard has two rules. Weeks start on Monday and end on Sunday. Week 1 is the week containing the first Thursday of the January — equivalently, the week containing 4 January, or the first week with the majority of its days in the new year.
The consequence surprises people: the first days of January often belong to the last week of the previous ISO year, and the last days of December often belong to week 1 of the next. 1 January 2023 was a Sunday, so it fell in week 52 of ISO year 2022. 30 December 2019 was a Monday and belonged to week 1 of ISO year 2020. This is why an ISO week always comes with an ISO year, which is not always the calendar year of the date.
Most years have 52 ISO weeks; some have 53. A year has 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2015, 2020 and 2026 all have 53 weeks; 2024 and 2025 have 52.
This trips up year-on-year reporting. Comparing 'week 1 to week 52' between a 52-week year and a 53-week one silently drops a week of data, and a 53-week year makes quarterly week groupings uneven. Worth checking before building a comparison.
The ISO scheme is the norm in Europe and in most software, but it is not universal. The North American convention often starts weeks on Sunday and treats the week containing 1 January as week 1, which can differ from the ISO answer by one. Some retail and broadcast calendars use their own systems again, such as 4-4-5 accounting periods.
Everything here is ISO 8601. If a colleague's week number is one different from yours, a differing convention is the usual explanation — worth confirming which one their calendar uses before assuming an error.
The current ISO week number is shown at the top of the page, with its ISO year and the Monday-to-Sunday date range it covers.
Because ISO week 1 is the week containing the first Thursday of January. If 1 January falls late in the week — a Friday, Saturday or Sunday — that whole week has most of its days in the old year, so it stays with the previous ISO year.
52 in most years, 53 when the year starts on a Thursday or is a leap year starting on a Wednesday. The page shows the total for whichever ISO year you are looking at.
Monday, under ISO 8601, which is what this page uses. Some North American calendars start on Sunday and may report a different number for the same date.
The year that owns the week, which is not always the calendar year of the date. 30 December 2019 sits in week 1 of ISO year 2020. Always quote a week number together with its ISO year.