Week Number Calculator - ISO 8601 & US Weeks
ISO 8601 standard
It's currently Week 37 of 2026
ISO 8601 Week
Week 37, 2026
International standard (Monday-based). Week 1 is the first week with a Thursday.
US Week Number
Week 37
US standard (Sunday-based). Week 1 starts with January 1st.
UK/EU Week Number
Week 37
UK/EU standard (Monday-based, equivalent to ISO 8601).
How Week Numbering Works
Different countries and standards define week numbers differently. The three main systems are:
ISO 8601 (International Standard)
Used by most of Europe, Asia, and the international business community. Weeks start on Monday. Week 1 is the first week of the year that contains a Thursday (or equivalently, the week containing January 4th). This means December 29-31 can belong to Week 1 of the next year, and January 1-3 can belong to Week 52 or 53 of the previous year.
US / North American System
Used in the United States and Canada. Weeks start on Sunday. Week 1 is the week containing January 1st, regardless of what day it falls on. This system is simpler but can result in a partial first week with only 1-2 days.
Middle Eastern System
Used in some Middle Eastern countries. Weeks start on Saturday. Similar to the US system but shifted by one day.
Frequently Asked Questions
What week is it now?
The current ISO 8601 week number is shown at the top of this page. It updates automatically based on today's date. Most European countries, international businesses, and software systems use this standard.
Why do different countries have different week numbers?
The difference comes down to two factors: which day starts the week (Monday, Sunday, or Saturday) and how Week 1 is defined. ISO 8601 says Week 1 must contain a Thursday, while the US system says Week 1 simply contains January 1st. These rules can produce different week numbers, especially in early January and late December.
Which week numbering system does my country use?
Most of Europe, Asia, and Oceania use ISO 8601 (Monday start). The US, Canada, and Japan commonly use Sunday-start weeks. Some Middle Eastern countries use Saturday-start weeks. When in doubt, ISO 8601 is the safest choice for international communication.
When does Week 1 start?
In ISO 8601, Week 1 starts on the Monday of the first week containing a Thursday. In practice this means Week 1 always includes January 4th. In the US system, Week 1 starts on the Sunday on or before January 1st. The difference can shift week numbers by up to a week between the two systems.
Full Year Week Numbers
| Week | Start | End |
|---|---|---|
| 1 | Dec 29 | Jan 4 |
| 2 | Jan 5 | Jan 11 |
| 3 | Jan 12 | Jan 18 |
| 4 | Jan 19 | Jan 25 |
| 5 | Jan 26 | Feb 1 |
| 6 | Feb 2 | Feb 8 |
| 7 | Feb 9 | Feb 15 |
| 8 | Feb 16 | Feb 22 |
| 9 | Feb 23 | Mar 1 |
| 10 | Mar 2 | Mar 8 |
| 11 | Mar 9 | Mar 15 |
| 12 | Mar 16 | Mar 22 |
| 13 | Mar 23 | Mar 29 |
| 14 | Mar 30 | Apr 5 |
| 15 | Apr 6 | Apr 12 |
| 16 | Apr 13 | Apr 19 |
| 17 | Apr 20 | Apr 26 |
| 18 | Apr 27 | May 3 |
| 19 | May 4 | May 10 |
| 20 | May 11 | May 17 |
| 21 | May 18 | May 24 |
| 22 | May 25 | May 31 |
| 23 | Jun 1 | Jun 7 |
| 24 | Jun 8 | Jun 14 |
| 25 | Jun 15 | Jun 21 |
| 26 | Jun 22 | Jun 28 |
| 27 | Jun 29 | Jul 5 |
| 28 | Jul 6 | Jul 12 |
| 29 | Jul 13 | Jul 19 |
| 30 | Jul 20 | Jul 26 |
| 31 | Jul 27 | Aug 2 |
| 32 | Aug 3 | Aug 9 |
| 33 | Aug 10 | Aug 16 |
| 34 | Aug 17 | Aug 23 |
| 35 | Aug 24 | Aug 30 |
| 36 | Aug 31 | Sep 6 |
| 37 | Sep 7 | Sep 13 |
| 38 | Sep 14 | Sep 20 |
| 39 | Sep 21 | Sep 27 |
| 40 | Sep 28 | Oct 4 |
| 41 | Oct 5 | Oct 11 |
| 42 | Oct 12 | Oct 18 |
| 43 | Oct 19 | Oct 25 |
| 44 | Oct 26 | Nov 1 |
| 45 | Nov 2 | Nov 8 |
| 46 | Nov 9 | Nov 15 |
| 47 | Nov 16 | Nov 22 |
| 48 | Nov 23 | Nov 29 |
| 49 | Nov 30 | Dec 6 |
| 50 | Dec 7 | Dec 13 |
| 51 | Dec 14 | Dec 20 |
| 52 | Dec 21 | Dec 27 |
| 53 | Dec 28 | Jan 3 |
How the Week Number Tool Works
- 01Select a date or use the current date as default
- 02The tool calculates the ISO 8601 week number for that date
- 03View the start and end dates of the selected week
- 04Navigate between weeks to plan ahead or review past weeks
Week Number Guide
Week numbers follow the ISO 8601 standard, which is widely used in business, project management, and international commerce. Under this standard, weeks start on Monday and the first week of the year is the one that contains the first Thursday of January. This means week 1 always includes January 4th. Week numbers are particularly useful for planning sprints, scheduling deliveries, and coordinating across teams that reference weeks by number rather than date ranges.
Common Use Cases
- Identify the current week number for sprint planning and agile project management
- Convert dates to week numbers for reporting and data analysis
- Coordinate delivery schedules and logistics that reference ISO week numbers
- Plan marketing campaigns, content calendars, and editorial schedules by week
- Align international team schedules when different countries reference week numbers
Expert Tips
- Remember that ISO weeks start on Monday, so a week number change always happens between Sunday and Monday
- Be aware that the first and last weeks of a year can span two calendar years under ISO 8601
- Use week numbers in file names and reports (e.g., 'report-2026-W14') for consistent chronological sorting
- When communicating week numbers internationally, specify the year too, since 'W1' can mean different dates in different standards
Frequently Asked Questions
What standard does the week numbering follow?→
Why does the US use a different week numbering system?→
Can a year have 53 weeks?→
How is the week number useful for project planning?→
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<a href="https://toolforte.com/tools/week-number">Week Number Calculator</a> - MD
[Week Number Calculator](https://toolforte.com/tools/week-number)