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What week number is it?

Reading your clock…

Week number for any date

Why week 1 hinges on a Thursday

ISO weeks start on Monday, so the week containing 1 January usually straddles two years. Whichever year owns the majority of those seven days (4 or more) gets the week. Pick the weekday of 1 January:

That week has 4 January days, so it is week 1 of the new year. Notice the Thursday: whenever it lands in January, January has the majority. That's why the rule is usually stated as "the week with the first Thursday", or equivalently "the week containing 4 January".

ISO or US week numbers?

ISO 8601 (Monday start, first-Thursday rule) is the standard across Europe and much of Asia, and in tools like ISOWEEKNUM in Excel or %V in programming languages. US-style numbering starts week 1 on 1 January and weeks on Sunday, so it can differ by one.

If a colleague in Germany says "KW …" (Kalenderwoche), they mean the ISO week. When in doubt, quote the dates as well as the number.

ISO years have 52 or 53 weeks. 2026 has 53.

All weeks of 2026

W01Dec 29 – Jan 4
W02Jan 5 – Jan 11
W03Jan 12 – Jan 18
W04Jan 19 – Jan 25
W05Jan 26 – Feb 1
W06Feb 2 – Feb 8
W07Feb 9 – Feb 15
W08Feb 16 – Feb 22
W09Feb 23 – Mar 1
W10Mar 2 – Mar 8
W11Mar 9 – Mar 15
W12Mar 16 – Mar 22
W13Mar 23 – Mar 29
W14Mar 30 – Apr 5
W15Apr 6 – Apr 12
W16Apr 13 – Apr 19
W17Apr 20 – Apr 26
W18Apr 27 – May 3
W19May 4 – May 10
W20May 11 – May 17
W21May 18 – May 24
W22May 25 – May 31
W23Jun 1 – Jun 7
W24Jun 8 – Jun 14
W25Jun 15 – Jun 21
W26Jun 22 – Jun 28
W27Jun 29 – Jul 5
W28Jul 6 – Jul 12
W29Jul 13 – Jul 19
W30Jul 20 – Jul 26
W31Jul 27 – Aug 2
W32Aug 3 – Aug 9
W33Aug 10 – Aug 16
W34Aug 17 – Aug 23
W35Aug 24 – Aug 30
W36Aug 31 – Sep 6
W37Sep 7 – Sep 13
W38Sep 14 – Sep 20
W39Sep 21 – Sep 27
W40Sep 28 – Oct 4
W41Oct 5 – Oct 11
W42Oct 12 – Oct 18
W43Oct 19 – Oct 25
W44Oct 26 – Nov 1
W45Nov 2 – Nov 8
W46Nov 9 – Nov 15
W47Nov 16 – Nov 22
W48Nov 23 – Nov 29
W49Nov 30 – Dec 6
W50Dec 7 – Dec 13
W51Dec 14 – Dec 20
W52Dec 21 – Dec 27
W53Dec 28 – Jan 3

Questions people ask

What week number is it?
The big number at the top is the current ISO 8601 week, calculated from your device clock. ISO weeks run Monday to Sunday.
How is the ISO week number calculated?
Week 1 is the week (Monday to Sunday) that contains the year's first Thursday, which is the same as the week containing 4 January. Every other week follows on from there. The Thursday explainer on this page lets you see why.
Why does my US calendar show a different week number?
Many US calendars and spreadsheets start week 1 on 1 January and begin weeks on Sunday. That count can run one ahead of the ISO number for much of the year. Excel's WEEKNUM uses the US system by default; ISOWEEKNUM gives the ISO one.
Does every year have 52 weeks?
No. ISO years have 52 or 53 weeks. A year has 53 weeks when it starts on a Thursday, or is a leap year starting on a Wednesday. 2026 is one (it starts on a Thursday); the next is 2032.
Can the first days of January belong to last year?
Yes. If 1 January is a Friday, Saturday or Sunday, those days belong to the final week (52 or 53) of the previous ISO year. Likewise 29–31 December can be week 1 of the next year.