What week is it?

What week of the year it is, twice

Ask “what week of the year is it” in London and in Chicago and you can get two answers a week apart. Both are right, which doesn’t help the meeting booked for week 26.

ISO weeks run Monday to Sunday, and week 1 is the one that holds the year’s first Thursday. US weeks run Sunday to Saturday, and week 1 is the one that holds January 1, even if only a day of it falls in the new year.

Why “what week is it” depends on where you are

The week starts on Sunday in the US, Canada, Japan, India, Brazil and Mexico, and on Monday in the UK. The UK also uses the ISO rule for week 1, so a British calendar and an American one can disagree by a week. Any plan that gives a week number needs a note saying whose.

Whatever the number, work due EOW is due on Friday, and how long until Friday counts down to it either way.

Week numbers in Excel

Excel’s WEEKNUM uses the US rule unless you tell it otherwise. WEEKNUM(date, 21) gives the ISO week, and so does ISOWEEKNUM in current versions of Excel. WEEKNUM(date, 2) starts weeks on Monday but still counts week 1 from January 1.

Years with 53 weeks

An ISO year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year. Between 2020 and 2040 that’s 2020, 2026, 2032 and 2037. Companies that count their year in weeks get a 53rd week on the same kind of schedule, which moves what quarter it is by a week for them. Somebody’s quarter gets an extra week of targets.

The US count runs to 53 weeks most years, and to 54 when a leap year starts on a Saturday. That happens in 2028, so December 31, 2028 lands in US week 54, a week one day long.

Week number calendar for 2026

ISO weeks, each from its Monday to its Sunday.

1. 1 Dec 29 to Jan 4
2. 2 Jan 5 to Jan 11
3. 3 Jan 12 to Jan 18
4. 4 Jan 19 to Jan 25
5. 5 Jan 26 to Feb 1
6. 6 Feb 2 to Feb 8
7. 7 Feb 9 to Feb 15
8. 8 Feb 16 to Feb 22
9. 9 Feb 23 to Mar 1
10. 10 Mar 2 to Mar 8
11. 11 Mar 9 to Mar 15
12. 12 Mar 16 to Mar 22
13. 13 Mar 23 to Mar 29
14. 14 Mar 30 to Apr 5
15. 15 Apr 6 to Apr 12
16. 16 Apr 13 to Apr 19
17. 17 Apr 20 to Apr 26
18. 18 Apr 27 to May 3
19. 19 May 4 to May 10
20. 20 May 11 to May 17
21. 21 May 18 to May 24
22. 22 May 25 to May 31
23. 23 Jun 1 to Jun 7
24. 24 Jun 8 to Jun 14
25. 25 Jun 15 to Jun 21
26. 26 Jun 22 to Jun 28
27. 27 Jun 29 to Jul 5
28. 28 Jul 6 to Jul 12
29. 29 Jul 13 to Jul 19
30. 30 Jul 20 to Jul 26
31. 31 Jul 27 to Aug 2
32. 32 Aug 3 to Aug 9
33. 33 Aug 10 to Aug 16
34. 34 Aug 17 to Aug 23
35. 35 Aug 24 to Aug 30
36. 36 Aug 31 to Sep 6
37. 37 Sep 7 to Sep 13
38. 38 Sep 14 to Sep 20
39. 39 Sep 21 to Sep 27
40. 40 Sep 28 to Oct 4
41. 41 Oct 5 to Oct 11
42. 42 Oct 12 to Oct 18
43. 43 Oct 19 to Oct 25
44. 44 Oct 26 to Nov 1
45. 45 Nov 2 to Nov 8
46. 46 Nov 9 to Nov 15
47. 47 Nov 16 to Nov 22
48. 48 Nov 23 to Nov 29
49. 49 Nov 30 to Dec 6
50. 50 Dec 7 to Dec 13
51. 51 Dec 14 to Dec 20
52. 52 Dec 21 to Dec 27
53. 53 Dec 28 to Jan 3