Week number
Today's week number, the week of any date, and every week of the year with its dates.
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All weeks
ISO week numbers vs US week numbers
ISO 8601, used across Europe and in most business software, starts weeks on Monday. Week 1 is the week that contains the first Thursday of the year, so January 1 can belong to the last week of the previous year, and some years have 53 weeks (2026 is one of them).
The US system starts weeks on Sunday, and week 1 is simply the week that contains January 1. The two systems often differ by one around the start of the year.
Examples around the new year
The two systems agree for most of the year and differ at the edges:
| Date | ISO week | US week |
|---|---|---|
| Mon, Dec 29, 2025 | Week 1 of 2026 | Week 53 of 2025 |
| Thu, Jan 1, 2026 | Week 1 of 2026 | Week 1 |
| Wed, Jul 1, 2026 | Week 27 | Week 27 |
| Wed, Nov 4, 2026 | Week 45 | Week 45 |
| Thu, Dec 31, 2026 | Week 53 of 2026 | Week 53 |
| Fri, Jan 1, 2027 | Week 53 of 2026 | Week 1 of 2027 |
| Mon, Jan 4, 2027 | Week 1 of 2027 | Week 2 |
Which years have 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 happens in 2020, 2026, 2032 and 2037. Payroll and retail calendars that run on ISO weeks get an extra week in those years, which can shift monthly totals.
Week numbers in spreadsheets
In Excel and Google Sheets, =ISOWEEKNUM(A1) gives the ISO number shown here. =WEEKNUM(A1) uses the US system by default (weeks start on Sunday and week 1 holds January 1). =WEEKNUM(A1, 2) starts weeks on Monday but still counts week 1 as the week of January 1, so it is not the same as ISO.
Quarters and key dates in 2026
Planners often need the week number of a quarter's first and last day. In 2026 the two systems happen to give the same numbers for every date below, because January 1, 2026 is a Thursday: ISO week 1 starts on Monday, December 29, 2025, and US week 1 on Sunday, December 28.
| Date | Week | ISO week runs |
|---|---|---|
| Thu, Jan 1 (Q1 starts) | 1 | Mon, Dec 29, 2025 to Sun, Jan 4 |
| Tue, Mar 31 (Q1 ends) | 14 | Mon, Mar 30 to Sun, Apr 5 |
| Tue, Jun 30 (Q2 ends) | 27 | Mon, Jun 29 to Sun, Jul 5 |
| Wed, Sep 30 (Q3 ends) | 40 | Mon, Sep 28 to Sun, Oct 4 |
| Thu, Nov 26 (US Thanksgiving) | 48 | Mon, Nov 23 to Sun, Nov 29 |
| Fri, Dec 25 (Christmas) | 52 | Mon, Dec 21 to Sun, Dec 27 |
| Thu, Dec 31 (Q4 ends) | 53 | Mon, Dec 28 to Sun, Jan 3, 2027 |
Notice that each quarter ends in the middle of a week, so weeks 14, 27 and 40 are shared between two quarters. If a report runs "by week", decide up front which quarter those weeks belong to. 2027 has only 52 ISO weeks, and its week 1 starts on Monday, January 4, 2027, so the first three days of January 2027 still belong to week 53 of 2026. The All weeks list near the top shows every week with its dates, and its arrows step to the next or previous year.
Where each system is used
ISO week numbers are standard in much of Europe and in Asia for planning, shipping and school calendars, and they're common in software and manufacturing everywhere. US-style numbering shows up on many calendars printed in North America. If a colleague says "week 1", it's worth checking which one they mean in the first days of January.
Questions
Which week number should I use?
Use ISO if you work with European colleagues, Excel's ISOWEEKNUM, or most planning tools. Use US numbering for calendars printed in the United States or Excel's WEEKNUM with default settings.
Why does December 31 sometimes show week 1?
In ISO numbering a week belongs to the year that holds its Thursday. If December 31 is a Monday, Tuesday or Wednesday, that week's Thursday falls in January, so it is week 1 of the next year.
Why did the page open on US weeks for me?
The page starts in US numbering for visitors from the United States and Canada and in ISO for everyone else, based on your browser's language and time zone. Pick the other system once and it stays chosen on this device next time. A link with ?sys=iso or ?sys=us always opens in that system, which is handy when you share a week number with someone abroad.