Free tool
Week Number Calculator: What Week of the Year Is It?
Find the week number of any date, ISO or US style, with the Monday to Sunday dates, weeks in the year and a 2026 and 2027 week table. Free and instant.
How to use the Week Number Calculator
- The date field starts on today. Pick any other date to see its week.
- Choose ISO (Monday start), US (Sunday start) or Excel system 2 (Monday start).
- Read the week number, the dates the week runs and how many weeks the year has.
- To go the other way, enter a year and a week number under Find the dates of a week.
How ISO week numbers work
ISO 8601 is the international standard for week numbers. Weeks run Monday to Sunday, and week 1 is the week that contains the year's first Thursday. That is the same as saying week 1 is the week with January 4 in it. A year has 52 or 53 weeks.
Because of this rule, the first days of January can belong to the last week of the old year, and the last days of December can belong to week 1 of the next year.
ISO, US and Excel week numbers compared
| ISO 8601 | US (Excel WEEKNUM, default) | Excel system 2 | |
|---|---|---|---|
| Week starts on | Monday | Sunday | Monday |
| Week 1 is | The week with the first Thursday | The week that contains January 1 | The week that contains January 1 |
| First and last weeks | Always full 7 day weeks | Can be partial | Can be partial |
| Weeks in a year | 52 or 53 | 53 or 54 | 53 or 54 |
If a report says "week 44", check which system it uses. In 2026, ISO week 1 started on Monday, December 29, 2025, while US week 1 started on Sunday, December 28, 2025 and holds only three days of the new year (January 1 to 3).
53-week years
An ISO year has 53 weeks when January 1 is a Thursday, or when it is a Wednesday in a leap year. The recent and upcoming ones are 2015, 2020, 2026 and 2032. In 2026 the last ISO week runs from Monday, December 28, 2026 to Sunday, January 3, 2027.
ISO week numbers for 2026
| Week | Monday | Sunday |
|---|---|---|
| 1 | Dec 29, 2025 | Jan 4, 2026 |
| 2 | Jan 5, 2026 | Jan 11, 2026 |
| 3 | Jan 12, 2026 | Jan 18, 2026 |
| 4 | Jan 19, 2026 | Jan 25, 2026 |
| 5 | Jan 26, 2026 | Feb 1, 2026 |
| 6 | Feb 2, 2026 | Feb 8, 2026 |
| 7 | Feb 9, 2026 | Feb 15, 2026 |
| 8 | Feb 16, 2026 | Feb 22, 2026 |
| 9 | Feb 23, 2026 | Mar 1, 2026 |
| 10 | Mar 2, 2026 | Mar 8, 2026 |
| 11 | Mar 9, 2026 | Mar 15, 2026 |
| 12 | Mar 16, 2026 | Mar 22, 2026 |
| 13 | Mar 23, 2026 | Mar 29, 2026 |
| 14 | Mar 30, 2026 | Apr 5, 2026 |
| 15 | Apr 6, 2026 | Apr 12, 2026 |
| 16 | Apr 13, 2026 | Apr 19, 2026 |
| 17 | Apr 20, 2026 | Apr 26, 2026 |
| 18 | Apr 27, 2026 | May 3, 2026 |
| 19 | May 4, 2026 | May 10, 2026 |
| 20 | May 11, 2026 | May 17, 2026 |
| 21 | May 18, 2026 | May 24, 2026 |
| 22 | May 25, 2026 | May 31, 2026 |
| 23 | Jun 1, 2026 | Jun 7, 2026 |
| 24 | Jun 8, 2026 | Jun 14, 2026 |
| 25 | Jun 15, 2026 | Jun 21, 2026 |
| 26 | Jun 22, 2026 | Jun 28, 2026 |
| 27 | Jun 29, 2026 | Jul 5, 2026 |
| 28 | Jul 6, 2026 | Jul 12, 2026 |
| 29 | Jul 13, 2026 | Jul 19, 2026 |
| 30 | Jul 20, 2026 | Jul 26, 2026 |
| 31 | Jul 27, 2026 | Aug 2, 2026 |
| 32 | Aug 3, 2026 | Aug 9, 2026 |
| 33 | Aug 10, 2026 | Aug 16, 2026 |
| 34 | Aug 17, 2026 | Aug 23, 2026 |
| 35 | Aug 24, 2026 | Aug 30, 2026 |
| 36 | Aug 31, 2026 | Sep 6, 2026 |
| 37 | Sep 7, 2026 | Sep 13, 2026 |
| 38 | Sep 14, 2026 | Sep 20, 2026 |
| 39 | Sep 21, 2026 | Sep 27, 2026 |
| 40 | Sep 28, 2026 | Oct 4, 2026 |
| 41 | Oct 5, 2026 | Oct 11, 2026 |
| 42 | Oct 12, 2026 | Oct 18, 2026 |
| 43 | Oct 19, 2026 | Oct 25, 2026 |
| 44 | Oct 26, 2026 | Nov 1, 2026 |
| 45 | Nov 2, 2026 | Nov 8, 2026 |
| 46 | Nov 9, 2026 | Nov 15, 2026 |
| 47 | Nov 16, 2026 | Nov 22, 2026 |
| 48 | Nov 23, 2026 | Nov 29, 2026 |
| 49 | Nov 30, 2026 | Dec 6, 2026 |
| 50 | Dec 7, 2026 | Dec 13, 2026 |
| 51 | Dec 14, 2026 | Dec 20, 2026 |
| 52 | Dec 21, 2026 | Dec 27, 2026 |
| 53 | Dec 28, 2026 | Jan 3, 2027 |
ISO week numbers for 2027
| Week | Monday | Sunday |
|---|---|---|
| 1 | Jan 4, 2027 | Jan 10, 2027 |
| 2 | Jan 11, 2027 | Jan 17, 2027 |
| 3 | Jan 18, 2027 | Jan 24, 2027 |
| 4 | Jan 25, 2027 | Jan 31, 2027 |
| 5 | Feb 1, 2027 | Feb 7, 2027 |
| 6 | Feb 8, 2027 | Feb 14, 2027 |
| 7 | Feb 15, 2027 | Feb 21, 2027 |
| 8 | Feb 22, 2027 | Feb 28, 2027 |
| 9 | Mar 1, 2027 | Mar 7, 2027 |
| 10 | Mar 8, 2027 | Mar 14, 2027 |
| 11 | Mar 15, 2027 | Mar 21, 2027 |
| 12 | Mar 22, 2027 | Mar 28, 2027 |
| 13 | Mar 29, 2027 | Apr 4, 2027 |
| 14 | Apr 5, 2027 | Apr 11, 2027 |
| 15 | Apr 12, 2027 | Apr 18, 2027 |
| 16 | Apr 19, 2027 | Apr 25, 2027 |
| 17 | Apr 26, 2027 | May 2, 2027 |
| 18 | May 3, 2027 | May 9, 2027 |
| 19 | May 10, 2027 | May 16, 2027 |
| 20 | May 17, 2027 | May 23, 2027 |
| 21 | May 24, 2027 | May 30, 2027 |
| 22 | May 31, 2027 | Jun 6, 2027 |
| 23 | Jun 7, 2027 | Jun 13, 2027 |
| 24 | Jun 14, 2027 | Jun 20, 2027 |
| 25 | Jun 21, 2027 | Jun 27, 2027 |
| 26 | Jun 28, 2027 | Jul 4, 2027 |
| 27 | Jul 5, 2027 | Jul 11, 2027 |
| 28 | Jul 12, 2027 | Jul 18, 2027 |
| 29 | Jul 19, 2027 | Jul 25, 2027 |
| 30 | Jul 26, 2027 | Aug 1, 2027 |
| 31 | Aug 2, 2027 | Aug 8, 2027 |
| 32 | Aug 9, 2027 | Aug 15, 2027 |
| 33 | Aug 16, 2027 | Aug 22, 2027 |
| 34 | Aug 23, 2027 | Aug 29, 2027 |
| 35 | Aug 30, 2027 | Sep 5, 2027 |
| 36 | Sep 6, 2027 | Sep 12, 2027 |
| 37 | Sep 13, 2027 | Sep 19, 2027 |
| 38 | Sep 20, 2027 | Sep 26, 2027 |
| 39 | Sep 27, 2027 | Oct 3, 2027 |
| 40 | Oct 4, 2027 | Oct 10, 2027 |
| 41 | Oct 11, 2027 | Oct 17, 2027 |
| 42 | Oct 18, 2027 | Oct 24, 2027 |
| 43 | Oct 25, 2027 | Oct 31, 2027 |
| 44 | Nov 1, 2027 | Nov 7, 2027 |
| 45 | Nov 8, 2027 | Nov 14, 2027 |
| 46 | Nov 15, 2027 | Nov 21, 2027 |
| 47 | Nov 22, 2027 | Nov 28, 2027 |
| 48 | Nov 29, 2027 | Dec 5, 2027 |
| 49 | Dec 6, 2027 | Dec 12, 2027 |
| 50 | Dec 13, 2027 | Dec 19, 2027 |
| 51 | Dec 20, 2027 | Dec 26, 2027 |
| 52 | Dec 27, 2027 | Jan 2, 2028 |
These tables are fixed for 2026 and 2027. The calculator above works for any date from 1900 to 2200.
Other date tools
Counting working days instead? Use the business days calculator. To work out how old someone is, use the age calculator. Waiting on a date? See the days until Christmas countdown or the days until Thanksgiving countdown. For time instead of dates, try the hours and minutes calculator.
Frequently asked questions
How do I find the week number of a date?
Pick the date in the calculator. With ISO numbering, weeks start on Monday and week 1 is the week that holds the year's first Thursday, so January 4 is always in week 1.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday and week 1 is the first week with four or more days in the new year. The common US system (and Excel's WEEKNUM) starts weeks on Sunday and makes the week that holds January 1 week 1, even if it has only one day in the new year. The two can differ by one week or more.
Does a year ever have 53 weeks?
Yes. An ISO year has 53 weeks when January 1 is a Thursday, or a Wednesday in a leap year. 2020 and 2026 are 53-week years, and 2032 is the next after 2026.
Why is January 1 sometimes in week 52 or 53?
In ISO numbering a week belongs to the year that holds its Thursday. January 1, 2027 is a Friday, so it sits in week 53 of 2026. Week 1 of 2027 starts on Monday, January 4.
Can I get the dates of a week from its number?
Yes. Use Find the dates of a week under the calculator. Enter the year and the ISO week number, and it shows the Monday and Sunday of that week.