What Week Is It? Week Number Calculator | ISO 8601 Calendar (All 52/53 Weeks)
Wondering "what week is it?" Just enter a date to instantly get its ISO 8601 week number (1-52/53) plus that week's start and end dates. You can also browse a full week number calendar for any year in a static table.
Weeks of 2027
2027 has 52 weeks in total.
| Week | Start (Mon) | End (Sun) |
|---|---|---|
| Week 1 | 2027/01/04 | 2027/01/10 |
| Week 2 | 2027/01/11 | 2027/01/17 |
| Week 3 | 2027/01/18 | 2027/01/24 |
| Week 4 | 2027/01/25 | 2027/01/31 |
| Week 5 | 2027/02/01 | 2027/02/07 |
| Week 6 | 2027/02/08 | 2027/02/14 |
| Week 7 | 2027/02/15 | 2027/02/21 |
| Week 8 | 2027/02/22 | 2027/02/28 |
| Week 9 | 2027/03/01 | 2027/03/07 |
| Week 10 | 2027/03/08 | 2027/03/14 |
| Week 11 | 2027/03/15 | 2027/03/21 |
| Week 12 | 2027/03/22 | 2027/03/28 |
| Week 13 | 2027/03/29 | 2027/04/04 |
| Week 14 | 2027/04/05 | 2027/04/11 |
| Week 15 | 2027/04/12 | 2027/04/18 |
| Week 16 | 2027/04/19 | 2027/04/25 |
| Week 17 | 2027/04/26 | 2027/05/02 |
| Week 18 | 2027/05/03 | 2027/05/09 |
| Week 19 | 2027/05/10 | 2027/05/16 |
| Week 20 | 2027/05/17 | 2027/05/23 |
| Week 21 | 2027/05/24 | 2027/05/30 |
| Week 22 | 2027/05/31 | 2027/06/06 |
| Week 23 | 2027/06/07 | 2027/06/13 |
| Week 24 | 2027/06/14 | 2027/06/20 |
| Week 25 | 2027/06/21 | 2027/06/27 |
| Week 26 | 2027/06/28 | 2027/07/04 |
| Week 27 | 2027/07/05 | 2027/07/11 |
| Week 28 | 2027/07/12 | 2027/07/18 |
| Week 29 | 2027/07/19 | 2027/07/25 |
| Week 30 | 2027/07/26 | 2027/08/01 |
| Week 31 | 2027/08/02 | 2027/08/08 |
| Week 32 | 2027/08/09 | 2027/08/15 |
| Week 33 | 2027/08/16 | 2027/08/22 |
| Week 34 | 2027/08/23 | 2027/08/29 |
| Week 35 | 2027/08/30 | 2027/09/05 |
| Week 36 | 2027/09/06 | 2027/09/12 |
| Week 37 | 2027/09/13 | 2027/09/19 |
| Week 38 | 2027/09/20 | 2027/09/26 |
| Week 39 | 2027/09/27 | 2027/10/03 |
| Week 40 | 2027/10/04 | 2027/10/10 |
| Week 41 | 2027/10/11 | 2027/10/17 |
| Week 42 | 2027/10/18 | 2027/10/24 |
| Week 43 | 2027/10/25 | 2027/10/31 |
| Week 44 | 2027/11/01 | 2027/11/07 |
| Week 45 | 2027/11/08 | 2027/11/14 |
| Week 46 | 2027/11/15 | 2027/11/21 |
| Week 47 | 2027/11/22 | 2027/11/28 |
| Week 48 | 2027/11/29 | 2027/12/05 |
| Week 49 | 2027/12/06 | 2027/12/12 |
| Week 50 | 2027/12/13 | 2027/12/19 |
| Week 51 | 2027/12/20 | 2027/12/26 |
| Week 52 | 2027/12/27 | 2028/01/02 |
What Is a Week Number Calendar?
A week number splits a year into Monday-to-Sunday weeks and numbers them sequentially, following the ISO 8601 international standard. It's the source of notations like "WK32" or "2026-W30" seen on manufacturing lot codes and shipping documents from overseas, and week 1 is officially defined as "the week containing January 4th."
This tool shows a static table of every week in a chosen year, complete with start and end dates, and also lets you type in a single date to instantly see which week it falls in and the exact range of that week. Because a year can have either 52 or 53 weeks depending on how the days fall, this is especially useful when you're dealing with dates near the turn of the year.
How to Look Up a Week Number
- Enter the date you want to check Pick a date from the calendar picker, or click "Today" to fill in the current date automatically.
- Read the week number and its range The tool shows which week the date belongs to, plus the start (Monday) and end (Sunday) dates of that week.
- Scan the yearly table for context Use the full-year table to see neighboring weeks and confirm whether the chosen year has 52 or 53 weeks in total.
Tips for getting more out of it
- 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.
When a Week Number Calendar Comes in Handy
Matching manufacturing lot codes
Trace a lot code printed on a product, such as "WK32", back to the exact date range it was manufactured.
Cross-checking sprints in project management tools
When Jira, Asana, or similar tools label a sprint "Week 32", confirm the actual dates using the same ISO 8601 standard.
Untangling dates around New Year's
Work out counterintuitive cases, like January 1st belonging to week 52 or 53 of the previous year, with an actual calculated result.
Labeling weekly reports and meeting notes
When a report needs to reference "week 32", pull the exact date range straight from the yearly table instead of counting by hand.
Glossary
- ISO 8601
- The international standard for representing dates and times. For week numbering, it defines the week as starting on Monday and uses a Thursday-based rule to determine which year week 1 belongs to.
- ISO week-year
- The year a given week is assigned to under this standard, which can differ from the calendar year near New Year's — for example, if January 1st falls on a Friday, Saturday, or Sunday, that week's ISO week-year is the previous year.
- 53-week year
- A year that ends up with one extra week beyond the usual 52, because of how the leftover days beyond a multiple of seven land relative to the Thursday rule. This happens roughly once every 5 to 6 years.
- Thursday rule
- The ISO 8601 convention that decides which year (and week number) a week belongs to based on the year containing that week's Thursday — the midpoint of the Monday-to-Sunday week.
- WK notation
- A shorthand like "WK32" commonly used on manufacturing lot codes, meaning "week 32 of the year". The exact convention (Monday-start vs. Sunday-start) can vary by country or industry.
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.