Not every “week number” is calculated the same way, and the ISO-8601 standard used here has two rules that trip people up if they’re used to a more casual system. First, ISO weeks start on Monday, not Sunday — so a week runs Monday through Sunday, unlike the Sunday-through-Saturday layout common on many US calendars and some spreadsheet defaults. Second, week 1 of a year isn’t simply “the week containing January 1st.” It’s the week that contains the year’s first Thursday.
Why the first-Thursday rule matters
That rule has a genuinely surprising consequence at the edges of the year. Because week 1 is anchored to the first Thursday rather than to January 1st itself, the first few days of January can belong to week 52 or 53 of the previous year — if January 1st falls on a Friday, Saturday, or Sunday, the new year starts partway through the old year’s last week. The same logic works in reverse: the last days of December can already belong to week 1 of the next year if December 31st falls early in its week. So it’s entirely possible to look at a date in late December and have it reported as week 1 of the following year.
52 or 53 weeks
Most years contain 52 ISO weeks. A year gets a 53rd week when it starts on a Thursday, or when a leap year starts on a Wednesday — those are the arrangements where a full extra Monday-to-Sunday block fits before the next year’s week 1 begins.
Where ISO week numbers are used
This standard is deliberately unambiguous, which is why it’s the default for international business systems, ISO-compliant software, manufacturing schedules (“we ship in week 34”), and academic timetables. The number shown here is calculated fresh from your device’s local date each time the page loads.
ISO week numbers for 2026
2026 has 53 ISO weeks because it starts on a Thursday. Each week runs Monday to Sunday:
| Week | Monday | Sunday |
|---|---|---|
| 1 | December 29, 2025 | January 4 |
| 2 | January 5 | January 11 |
| 3 | January 12 | January 18 |
| 4 | January 19 | January 25 |
| 5 | January 26 | February 1 |
| 6 | February 2 | February 8 |
| 7 | February 9 | February 15 |
| 8 | February 16 | February 22 |
| 9 | February 23 | March 1 |
| 10 | March 2 | March 8 |
| 11 | March 9 | March 15 |
| 12 | March 16 | March 22 |
| 13 | March 23 | March 29 |
| 14 | March 30 | April 5 |
| 15 | April 6 | April 12 |
| 16 | April 13 | April 19 |
| 17 | April 20 | April 26 |
| 18 | April 27 | May 3 |
| 19 | May 4 | May 10 |
| 20 | May 11 | May 17 |
| 21 | May 18 | May 24 |
| 22 | May 25 | May 31 |
| 23 | June 1 | June 7 |
| 24 | June 8 | June 14 |
| 25 | June 15 | June 21 |
| 26 | June 22 | June 28 |
| 27 | June 29 | July 5 |
| 28 | July 6 | July 12 |
| 29 | July 13 | July 19 |
| 30 | July 20 | July 26 |
| 31 | July 27 | August 2 |
| 32 | August 3 | August 9 |
| 33 | August 10 | August 16 |
| 34 | August 17 | August 23 |
| 35 | August 24 | August 30 |
| 36 | August 31 | September 6 |
| 37 | September 7 | September 13 |
| 38 | September 14 | September 20 |
| 39 | September 21 | September 27 |
| 40 | September 28 | October 4 |
| 41 | October 5 | October 11 |
| 42 | October 12 | October 18 |
| 43 | October 19 | October 25 |
| 44 | October 26 | November 1 |
| 45 | November 2 | November 8 |
| 46 | November 9 | November 15 |
| 47 | November 16 | November 22 |
| 48 | November 23 | November 29 |
| 49 | November 30 | December 6 |
| 50 | December 7 | December 13 |
| 51 | December 14 | December 20 |
| 52 | December 21 | December 27 |
| 53 | December 28 | January 3, 2027 |
2027 has 52 weeks; its week 1 starts on Monday, January 4, 2027.
Related counts
For the countdown version, see how many weeks are left in the year. The day-based equivalents are what day of the year it is and how many days are left. To confirm the calendar date, use today’s date.