ISO Week Number Calculator
Enter a date to find its ISO 8601 week number along with the start and end dates of that week. Also see a static table listing all 52 or 53 weeks of a given year.
Weeks of 2023
2023 has 52 weeks in total.
| Week | Start (Mon) | End (Sun) |
|---|---|---|
| Week 1 | 2023/01/02 | 2023/01/08 |
| Week 2 | 2023/01/09 | 2023/01/15 |
| Week 3 | 2023/01/16 | 2023/01/22 |
| Week 4 | 2023/01/23 | 2023/01/29 |
| Week 5 | 2023/01/30 | 2023/02/05 |
| Week 6 | 2023/02/06 | 2023/02/12 |
| Week 7 | 2023/02/13 | 2023/02/19 |
| Week 8 | 2023/02/20 | 2023/02/26 |
| Week 9 | 2023/02/27 | 2023/03/05 |
| Week 10 | 2023/03/06 | 2023/03/12 |
| Week 11 | 2023/03/13 | 2023/03/19 |
| Week 12 | 2023/03/20 | 2023/03/26 |
| Week 13 | 2023/03/27 | 2023/04/02 |
| Week 14 | 2023/04/03 | 2023/04/09 |
| Week 15 | 2023/04/10 | 2023/04/16 |
| Week 16 | 2023/04/17 | 2023/04/23 |
| Week 17 | 2023/04/24 | 2023/04/30 |
| Week 18 | 2023/05/01 | 2023/05/07 |
| Week 19 | 2023/05/08 | 2023/05/14 |
| Week 20 | 2023/05/15 | 2023/05/21 |
| Week 21 | 2023/05/22 | 2023/05/28 |
| Week 22 | 2023/05/29 | 2023/06/04 |
| Week 23 | 2023/06/05 | 2023/06/11 |
| Week 24 | 2023/06/12 | 2023/06/18 |
| Week 25 | 2023/06/19 | 2023/06/25 |
| Week 26 | 2023/06/26 | 2023/07/02 |
| Week 27 | 2023/07/03 | 2023/07/09 |
| Week 28 | 2023/07/10 | 2023/07/16 |
| Week 29 | 2023/07/17 | 2023/07/23 |
| Week 30 | 2023/07/24 | 2023/07/30 |
| Week 31 | 2023/07/31 | 2023/08/06 |
| Week 32 | 2023/08/07 | 2023/08/13 |
| Week 33 | 2023/08/14 | 2023/08/20 |
| Week 34 | 2023/08/21 | 2023/08/27 |
| Week 35 | 2023/08/28 | 2023/09/03 |
| Week 36 | 2023/09/04 | 2023/09/10 |
| Week 37 | 2023/09/11 | 2023/09/17 |
| Week 38 | 2023/09/18 | 2023/09/24 |
| Week 39 | 2023/09/25 | 2023/10/01 |
| Week 40 | 2023/10/02 | 2023/10/08 |
| Week 41 | 2023/10/09 | 2023/10/15 |
| Week 42 | 2023/10/16 | 2023/10/22 |
| Week 43 | 2023/10/23 | 2023/10/29 |
| Week 44 | 2023/10/30 | 2023/11/05 |
| Week 45 | 2023/11/06 | 2023/11/12 |
| Week 46 | 2023/11/13 | 2023/11/19 |
| Week 47 | 2023/11/20 | 2023/11/26 |
| Week 48 | 2023/11/27 | 2023/12/03 |
| Week 49 | 2023/12/04 | 2023/12/10 |
| Week 50 | 2023/12/11 | 2023/12/17 |
| Week 51 | 2023/12/18 | 2023/12/24 |
| Week 52 | 2023/12/25 | 2023/12/31 |
Tips
- Notations like "WK32" or "2026-W30" seen on manufacturing lot codes or shipping documents follow the same ISO 8601 week numbering shown by this tool.
- If a project management tool (Jira, Asana, etc.) labels a sprint "Week 32", look up the corresponding date here to cross-check against the same ISO standard.
- Dates around New Year's deserve extra care: if January 1st falls on a Sunday, it can actually belong to week 52 of the previous year.
- Click "Today" to instantly fill in the current date and see which week number you're in right now.
FAQ
Side Note — Why Thursday Decides the Week Number
The ISO 8601 rule for week numbering can feel counterintuitive at first: which year (and which week number) a week belongs to is decided not by its start (Monday) but by its Thursday. The logic is that a week belongs to whichever year contains the majority of its seven days. Since Thursday is the fourth day of a Monday-starting week — exactly the midpoint — checking which year Thursday falls in is a simple shortcut for determining which year holds the majority of that week's days.
A side effect of this rule is that the week containing January 1st isn't always "week 1." If January 1st falls on a Friday, Saturday, or Sunday, that week's Thursday actually falls in the previous year, so January 1st gets counted as part of the previous year's final week (week 52 or 53). Conversely, if December 31st falls on a Monday, Tuesday, or Wednesday, the week containing it becomes week 1 of the following year.
Whether a year has 52 or 53 weeks is also a consequence of this Thursday rule. A common year (365 days) is 52 weeks plus one extra day, and a leap year (366 days) is 52 weeks plus two extra days — and whichever year that leftover day's Thursday falls into determines whether the year needs a 53rd week. This happens roughly 71 times every 400 years, or about once every 5 to 6 years.
Because ISO week numbers (Monday-based) differ from the US-style calendar week numbering (Sunday-based) used in some spreadsheets and calendars, the same date can be reported as a different week number depending on the convention. When week numbers matter for cross-border business — such as manufacturing lot codes or international logistics — it's worth explicitly stating that ISO 8601 is the convention being used.