Date & time · Converter
ISO week number calculator.
Find the ISO 8601 week number of any date (2026-10-01 is 2026-W40-4), turn a week date back into a calendar date, and see how it differs from the US week number. Runs in your browser; nothing is sent anywhere.
Input
Weeks of a year
Result
Enter a date or a week date and press Convert.
How ISO 8601 week numbers work
An ISO week starts on Monday and ends on Sunday. Week 1 is the first week that contains a Thursday, which is the same as the week containing 4 January. A week date is written YYYY-Www-D, where ww is 01 to 53 and D is 1 (Monday) to 7 (Sunday). Example: 2026-W40-4 is Thursday 1 October 2026.
52 or 53 weeks. A year has 53 ISO weeks when 1 January is a Thursday, or a Wednesday in a leap year. Otherwise it has 52. 2026 has 53 weeks; 2025 has 52.
The week-year can differ from the calendar year. Near New Year the week belongs to the year that holds its Thursday. So 31 December 2020 is 2020-W53-4 and 3 January 2021 is still 2020-W53-7, while 29 December 2003 is 2004-W01-1.
ISO versus US numbering. The common US convention (Excel WEEKNUM system 1) starts weeks on Sunday and makes the week containing 1 January week 1, even if that week has only one day in the new year. ISO makes week 1 the week with at least four days in the new year. The two often differ by one, and in some years the US count reaches week 53 or restarts earlier. This tool shows both for every date. Other systems (Sunday-start with the first Sunday, Saturday-start in parts of the Middle East) exist and are not covered.
Sources: RFC 3339, Appendix A (week date grammar: date-wday 1 is Monday, date-week 01 to 53), the Python datetime documentation (week 1 is the first week containing a Thursday; 52 or 53 weeks) and Microsoft WEEKNUM (the two numbering systems). ISO 8601-1 itself is paywalled. Sources read 2026-09-30.
Frequently asked questions
What week number is it today?
Press Today to fill in your device date and see its ISO week.
Why is 1 January sometimes in week 52 or 53?
If 1 January falls on Friday, Saturday or Sunday, its week has fewer than four days in the new year, so ISO counts it as the last week of the previous year. For example 1 January 2021 is 2020-W53-5.
Which years have 53 weeks?
Years where 1 January is a Thursday, or a Wednesday in a leap year. Recent examples: 2015, 2020, 2026, 2032. The next after 2026 is 2032.
Is the US week number the same as ISO?
Not always. US numbering starts weeks on Sunday and puts 1 January in week 1. ISO starts on Monday and puts the first Thursday in week 1. The result panel shows both.
Calendar arithmetic only
This tool uses the proleptic Gregorian calendar for years 1 to 9999 and does not account for historical calendar changes. Your payroll, reporting or business calendar may define weeks differently, so check its own rule before relying on a week number. Nothing you type is stored or sent anywhere.