How week numbers are counted
There are two common systems, and the tool shows both:
| ISO 8601 (Europe, much of Asia) | US system | |
|---|---|---|
| Week starts on | Monday | Sunday |
| Week 1 is | the week with the first Thursday of the year | the week that contains January 1 |
| Weeks per year | 52 or 53 | 52 or 53 (sometimes a partial 54th) |
Because of the ISO rule, the first days of January can still belong to week 52 or 53 of the previous year. January 1, 2027 is a Friday, so in ISO numbering it falls in week 53 of 2026.
Years with 53 ISO weeks
A year has 53 ISO weeks if it starts on a Thursday, or if it is a leap year starting on a Wednesday.
| Year | ISO weeks | Week 1 starts |
|---|---|---|
| 2025 | 52 | Mon, Dec 30, 2024 |
| 2026 | 53 | Mon, Dec 29, 2025 |
| 2027 | 52 | Mon, Jan 4, 2027 |
| 2028 | 52 | Mon, Jan 3, 2028 |
| 2032 | 53 | Mon, Dec 29, 2031 |
Week numbers in Excel and Google Sheets
| You want | Formula (date in A1) |
|---|---|
| ISO week number | =ISOWEEKNUM(A1) |
| US week number (Sunday start) | =WEEKNUM(A1) |
| Week starting Monday, week 1 = Jan 1 | =WEEKNUM(A1,2) |
Frequently asked questions
What week number is it today?
The current week number is shown at the top of this page and updates automatically based on your device’s date.
How many weeks are in 2026?
2026 has 53 ISO weeks because it starts on a Thursday. Week 53 runs from December 28, 2026 to January 3, 2027.
Why does my calendar show a different week number?
Many calendars in the US count weeks from Sunday with week 1 starting on January 1, while Europe uses ISO 8601. Switch the numbering above to compare.
Is week 1 always the first week of January?
In the US system, yes. In ISO 8601, week 1 can start as early as December 29 or as late as January 4.