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ISO Week Number Today

Free ISO week number tool: see the current ISO 8601 week, look up the ISO week for any date, and read the exact rule weeks follow.

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What ISO week is it?

Pick any date to see its ISO 8601 week. The default is today, so you get the current ISO week straight away.

ISO 8601 weekWeek 31 of 2026
Week number (weeks start Sunday)31
Day of the year210
ISO weeks left this year22

ISO 8601 weeks start on Monday, and week 1 is the week that contains the year's first Thursday. Because of that rule, the first days of January can belong to the last week of the previous year, and late December can belong to week 1 of the next.

See the US week numberCount working daysPlan across time zones
2026

All weeks in 2026

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

How the ISO week number works

ISO 8601 is the international standard for dates, and it defines weeks precisely: every week starts on Monday, and week 1 of a year is the week that holds that year's first Thursday. An equivalent rule is that week 1 is the week containing January 4. Years have either 52 or 53 ISO weeks.

Because week 1 is anchored to a Thursday rather than to January 1, a date in early January can fall in the final week of the previous ISO year, and a date in late December can fall in week 1 of the next. Sprints, shift rotations, and European business calendars rely on this count, which is why it often differs from a simple Sunday-start week number.

A year has 53 ISO weeks when January 1 falls on a Thursday, or on a Wednesday in a leap year; otherwise it has 52. Because ISO weeks never split across the Monday-to-Sunday boundary, every ISO week has exactly seven days, which makes the count reliable for scheduling and reporting.

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Get the ISO week number in Excel or Google Sheets

Both Excel and Google Sheets have an ISOWEEKNUM function that returns the ISO 8601 week number. Give it a date and it returns the ISO week of the year.

=ISOWEEKNUM(TODAY())=ISOWEEKNUM(A2)

In Excel you can also use WEEKNUM with a return type of 21 to get the ISO week. For the Sunday-start week number instead, use WEEKNUM with its default settings.

ISO week number questions

What is the current ISO week?

The current ISO week is shown at the top of this page and updates automatically every Monday. ISO weeks start on Monday, and week 1 is the week that contains the year's first Thursday.

Does this year have 52 or 53 ISO weeks?

Most years have 52 ISO weeks. A year has 53 when January 1 is a Thursday, or a Wednesday in a leap year. You can see the full count in the week list above by browsing to a year.

Why is the ISO week different from the US week number?

ISO weeks start on Monday and anchor week 1 to the first Thursday of the year. The common US count starts weeks on Sunday and puts January 1 in week 1. Around New Year the two often show different weeks, and even different years.

Can a date in January be in week 52 or 53?

Yes. Under ISO 8601, the first days of January belong to the last week of the previous year when they fall before that year's first full week. For example, a Friday, January 1 sits in the final ISO week of the year before.