Guide
ISO 8601 week numbers explained
The rules behind Monday-first week numbers, ISO years, New Year boundaries and week 53.
The four facts that determine an ISO week
- Weeks start on Monday
- Monday is day 1 and Sunday is day 7.
- Week 1 contains the first Thursday
- This is the same as saying week 1 contains January 4.
- The ISO year follows the Thursday
- Near New Year, the ISO year can differ from the calendar year printed in the date.
- An ISO year has 52 or 53 weeks
- The last ISO week is the week containing December 28.
Why the ISO year can differ
Consider Friday, January 1, 2021. Its week began Monday, December 28, 2020, and its Thursday was December 31. The date therefore belongs to ISO year 2020: 2020-W53.
At the other side of a boundary, Monday, December 31, 2018 belonged to 2019-W01, because that week's Thursday fell in 2019.
When does week 53 happen?
A year has ISO week 53 when it begins on a Thursday, or when a leap year begins on a Wednesday. A practical validation rule is simpler: December 28 is always in the final ISO week of its year.
ISO compared with common US numbering
ISO fixes both the start day and week-1 rule. In US software, weeks often start Sunday and week 1 may be the week containing January 1. Microsoft Excel makes the distinction visible: ISOWEEKNUM(date) returns the ISO week, while WEEKNUM(date, return_type) can use other systems.
When sharing a week number internationally, include the full code, such as 2026-W36, and state that it follows ISO 8601.