Week Number Calculator
Under ISO 8601, weeks start on Monday and week 1 is the week with the year's first Thursday. Week 1 of 2026 runs from Mon, Dec 29, 2025 to Sun, Jan 4, 2026, and 2026 has 53 weeks.
How to use
- To find a week number, choose Date to week and pick a date. It starts on today's date.
- To find the dates of a week, choose Week to dates, then enter the year and the week number.
- Read the ISO week number and year, the weekday, and the Monday to Sunday range of that week.
How it works
ISO 8601 numbers weeks from 1 to 52 or 53. A week starts on Monday, and week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January.
To find the week of a date, move to the Thursday of that week. The year of that Thursday is the ISO year, and the week number is the number of whole weeks since 1 January of that year, plus one. For example 2 January 2021 is a Saturday, its Thursday is 31 December 2020, so it belongs to week 53 of 2020.
Because of this, the first days of January can fall in week 52 or 53 of the previous year, and the last days of December can fall in week 1 of the next year. A year has 53 weeks if 1 January is a Thursday, or if it is a leap year and 1 January is a Wednesday. Otherwise it has 52. For example 2020 and 2026 have 53.
Other conventions exist. In the US and some other countries, weeks often start on Sunday and week 1 contains 1 January, which gives different numbers from ISO. This tool uses ISO 8601 only.
Examples of weeks that cross the new year
| Date | ISO week |
|---|---|
| 1 Jan 2005 (Saturday) | 2004-W53-6 |
| 31 Dec 2007 (Monday) | 2008-W01-1 |
| 29 Dec 2008 (Monday) | 2009-W01-1 |
| 31 Dec 2020 (Thursday) | 2020-W53-4 |
| 3 Jan 2021 (Sunday) | 2020-W53-7 |
| 30 Dec 2024 (Monday) | 2025-W01-1 |
Source
ISO 8601-1:2019, "Date and time: Representations for information interchange", clause on week dates. The notation YYYY-Www-D gives the ISO year, week and weekday (Monday = 1).
FAQ
Why is 1 January sometimes in week 52 or 53?
ISO weeks belong to the year that contains their Thursday. If 1 January falls on a Friday, Saturday or Sunday, its week has its Thursday in the previous year, so it counts as week 52 or 53 of that year.
Which years have 53 weeks?
Years where 1 January is a Thursday, or leap years where 1 January is a Wednesday. Recent examples are 2015, 2020 and 2026, and the next is 2032.
Is the ISO week number the same as the US week number?
Not always. US calendars often count Sunday to Saturday weeks, with week 1 as the one containing 1 January. Their numbers can differ by one from ISO.
Is my data sent anywhere?
No. The calculation runs in your browser and nothing you enter is sent to a server.