WEEK NUMBERS
Week number today
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Calendar with week numbers
How week numbers work
Europe and many other countries follow ISO 8601, the international standard for week numbers. Its rules are simple:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week with the year’s first Thursday in it. Another way to say it: January 4 is always in week 1.
- A year has 52 or 53 weeks. It gets 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.
Because of rule 2, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next. ISO week codes such as 2026-W43 carry the week’s own year to avoid that confusion.
US week numbers
Week numbers are less common in the US, and when they are used they usually follow a different rule: weeks start on Sunday, and week 1 is the week containing January 1, however short. This is the default in Excel’s and Google Sheets’ WEEKNUM function. Weeks are cut at the year boundary, so a year has 53 US weeks, and occasionally 54: in 2028 the last one is just Sunday, December 31. Because the two systems start weeks on different days, US and ISO numbers differ by one on every Sunday, and around New Year they can be completely different: January 1, 2027 is ISO week 53 but US week 1. In years that start on a Friday or Saturday, such as 2027 and 2028, they differ on every day of the year; after the first few days of January the US number is one higher, or two higher on Sundays. When a team spans countries, say which system you mean, or quote dates.
Week 53 in 2026
2026 starts on a Thursday, so it is a 53-week ISO year. Week 53 runs from December 28, 2026 to January 3, 2027. Weekly reports, payroll calendars and sprint plans that assume 52 weeks can be off by a week in early 2027. The next 53-week ISO years are 2032 and 2037.
Week numbers in Excel and Google Sheets
=ISOWEEKNUM(date) gives the ISO week number in both. =WEEKNUM(date) uses the US rule (Sunday start, week 1 contains January 1); =WEEKNUM(date, 21) switches it to ISO. To get the Monday that starts an ISO week, look the week up above, or use the tables below.
All weeks in 2026 and 2027
ISO week numbers with their Monday and Sunday dates.
2026: 53 weeks
| Week | Starts (Monday) | Ends (Sunday) |
|---|---|---|
| 1 | Mon, Dec 29, 2025 | Sun, Jan 4 |
| 2 | Mon, Jan 5 | Sun, Jan 11 |
| 3 | Mon, Jan 12 | Sun, Jan 18 |
| 4 | Mon, Jan 19 | Sun, Jan 25 |
| 5 | Mon, Jan 26 | Sun, Feb 1 |
| 6 | Mon, Feb 2 | Sun, Feb 8 |
| 7 | Mon, Feb 9 | Sun, Feb 15 |
| 8 | Mon, Feb 16 | Sun, Feb 22 |
| 9 | Mon, Feb 23 | Sun, Mar 1 |
| 10 | Mon, Mar 2 | Sun, Mar 8 |
| 11 | Mon, Mar 9 | Sun, Mar 15 |
| 12 | Mon, Mar 16 | Sun, Mar 22 |
| 13 | Mon, Mar 23 | Sun, Mar 29 |
| 14 | Mon, Mar 30 | Sun, Apr 5 |
| 15 | Mon, Apr 6 | Sun, Apr 12 |
| 16 | Mon, Apr 13 | Sun, Apr 19 |
| 17 | Mon, Apr 20 | Sun, Apr 26 |
| 18 | Mon, Apr 27 | Sun, May 3 |
| 19 | Mon, May 4 | Sun, May 10 |
| 20 | Mon, May 11 | Sun, May 17 |
| 21 | Mon, May 18 | Sun, May 24 |
| 22 | Mon, May 25 | Sun, May 31 |
| 23 | Mon, Jun 1 | Sun, Jun 7 |
| 24 | Mon, Jun 8 | Sun, Jun 14 |
| 25 | Mon, Jun 15 | Sun, Jun 21 |
| 26 | Mon, Jun 22 | Sun, Jun 28 |
| 27 | Mon, Jun 29 | Sun, Jul 5 |
| 28 | Mon, Jul 6 | Sun, Jul 12 |
| 29 | Mon, Jul 13 | Sun, Jul 19 |
| 30 | Mon, Jul 20 | Sun, Jul 26 |
| 31 | Mon, Jul 27 | Sun, Aug 2 |
| 32 | Mon, Aug 3 | Sun, Aug 9 |
| 33 | Mon, Aug 10 | Sun, Aug 16 |
| 34 | Mon, Aug 17 | Sun, Aug 23 |
| 35 | Mon, Aug 24 | Sun, Aug 30 |
| 36 | Mon, Aug 31 | Sun, Sep 6 |
| 37 | Mon, Sep 7 | Sun, Sep 13 |
| 38 | Mon, Sep 14 | Sun, Sep 20 |
| 39 | Mon, Sep 21 | Sun, Sep 27 |
| 40 | Mon, Sep 28 | Sun, Oct 4 |
| 41 | Mon, Oct 5 | Sun, Oct 11 |
| 42 | Mon, Oct 12 | Sun, Oct 18 |
| 43 | Mon, Oct 19 | Sun, Oct 25 |
| 44 | Mon, Oct 26 | Sun, Nov 1 |
| 45 | Mon, Nov 2 | Sun, Nov 8 |
| 46 | Mon, Nov 9 | Sun, Nov 15 |
| 47 | Mon, Nov 16 | Sun, Nov 22 |
| 48 | Mon, Nov 23 | Sun, Nov 29 |
| 49 | Mon, Nov 30 | Sun, Dec 6 |
| 50 | Mon, Dec 7 | Sun, Dec 13 |
| 51 | Mon, Dec 14 | Sun, Dec 20 |
| 52 | Mon, Dec 21 | Sun, Dec 27 |
| 53 | Mon, Dec 28 | Sun, Jan 3, 2027 |
2027: 52 weeks
| Week | Starts (Monday) | Ends (Sunday) |
|---|---|---|
| 1 | Mon, Jan 4 | Sun, Jan 10 |
| 2 | Mon, Jan 11 | Sun, Jan 17 |
| 3 | Mon, Jan 18 | Sun, Jan 24 |
| 4 | Mon, Jan 25 | Sun, Jan 31 |
| 5 | Mon, Feb 1 | Sun, Feb 7 |
| 6 | Mon, Feb 8 | Sun, Feb 14 |
| 7 | Mon, Feb 15 | Sun, Feb 21 |
| 8 | Mon, Feb 22 | Sun, Feb 28 |
| 9 | Mon, Mar 1 | Sun, Mar 7 |
| 10 | Mon, Mar 8 | Sun, Mar 14 |
| 11 | Mon, Mar 15 | Sun, Mar 21 |
| 12 | Mon, Mar 22 | Sun, Mar 28 |
| 13 | Mon, Mar 29 | Sun, Apr 4 |
| 14 | Mon, Apr 5 | Sun, Apr 11 |
| 15 | Mon, Apr 12 | Sun, Apr 18 |
| 16 | Mon, Apr 19 | Sun, Apr 25 |
| 17 | Mon, Apr 26 | Sun, May 2 |
| 18 | Mon, May 3 | Sun, May 9 |
| 19 | Mon, May 10 | Sun, May 16 |
| 20 | Mon, May 17 | Sun, May 23 |
| 21 | Mon, May 24 | Sun, May 30 |
| 22 | Mon, May 31 | Sun, Jun 6 |
| 23 | Mon, Jun 7 | Sun, Jun 13 |
| 24 | Mon, Jun 14 | Sun, Jun 20 |
| 25 | Mon, Jun 21 | Sun, Jun 27 |
| 26 | Mon, Jun 28 | Sun, Jul 4 |
| 27 | Mon, Jul 5 | Sun, Jul 11 |
| 28 | Mon, Jul 12 | Sun, Jul 18 |
| 29 | Mon, Jul 19 | Sun, Jul 25 |
| 30 | Mon, Jul 26 | Sun, Aug 1 |
| 31 | Mon, Aug 2 | Sun, Aug 8 |
| 32 | Mon, Aug 9 | Sun, Aug 15 |
| 33 | Mon, Aug 16 | Sun, Aug 22 |
| 34 | Mon, Aug 23 | Sun, Aug 29 |
| 35 | Mon, Aug 30 | Sun, Sep 5 |
| 36 | Mon, Sep 6 | Sun, Sep 12 |
| 37 | Mon, Sep 13 | Sun, Sep 19 |
| 38 | Mon, Sep 20 | Sun, Sep 26 |
| 39 | Mon, Sep 27 | Sun, Oct 3 |
| 40 | Mon, Oct 4 | Sun, Oct 10 |
| 41 | Mon, Oct 11 | Sun, Oct 17 |
| 42 | Mon, Oct 18 | Sun, Oct 24 |
| 43 | Mon, Oct 25 | Sun, Oct 31 |
| 44 | Mon, Nov 1 | Sun, Nov 7 |
| 45 | Mon, Nov 8 | Sun, Nov 14 |
| 46 | Mon, Nov 15 | Sun, Nov 21 |
| 47 | Mon, Nov 22 | Sun, Nov 28 |
| 48 | Mon, Nov 29 | Sun, Dec 5 |
| 49 | Mon, Dec 6 | Sun, Dec 12 |
| 50 | Mon, Dec 13 | Sun, Dec 19 |
| 51 | Mon, Dec 20 | Sun, Dec 26 |
| 52 | Mon, Dec 27 | Sun, Jan 2, 2028 |
What week is it now?
The current week number is shown at the top of this page, for the date on your device, and it updates at midnight. Under the ISO system a new week starts on Monday; under the US system it starts on Sunday.
How many weeks are in 2026?
2026 has 53 ISO weeks, because it starts on a Thursday. Week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027. Most years, including 2027, have 52 ISO weeks. The next 53-week ISO years are 2032 and 2037.
When does week 1 of 2027 start?
Under ISO numbering, week 1 of 2027 starts on Monday, January 4, 2027; January 1 to 3 still belong to week 53 of 2026. Under the US system, week 1 is the short week of Friday, January 1 and Saturday, January 2, 2027.
Why do two calendars show different week numbers?
They are probably using different rules. ISO weeks start on Monday and week 1 is the week with the year’s first Thursday. Many US calendars and Excel’s and Google Sheets’ WEEKNUM start weeks on Sunday and count the week containing January 1 as week 1. They differ by one on every Sunday, and around New Year they can be completely different (January 1, 2027 is ISO week 53 but US week 1). In years that start on a Friday or Saturday, such as 2027 and 2028, they differ on every day: by one, or by two on Sundays, after the first few days of January.
How do I add week numbers to my calendar?
Choose a year and a numbering system, then use Add week numbers to your calendar to download a calendar file with a small label for each week. Open the file in Outlook or Apple Calendar, or import it in Google Calendar on the web (Settings › Import & export). In Google Calendar, first create a separate calendar (Settings › Add calendar › Create new calendar) and choose it when importing, so the labels are easy to hide or remove later.
Counting days rather than weeks? Count working days between two dates or find a date a number of days away. Planning across time zones? See our guides for remote teams.