Time/Numbers
Week Number (ISO 8601)
Find the ISO 8601 week number for a date, or find the start/end dates of a specific week.
The ISO 8601 week number is a standardized way to number the weeks of a year (1-52 or 1-53), where a week starts on Monday and the year's first week is the one containing the first Thursday of January. This differs from a casual "since the start of the year" count.
How to use it
- Date → week number: enter a date to find its ISO 8601 week number and that week's ISO year.
- Week number → date: enter a year and week number to get that week's start (Monday) and end (Sunday) dates.
- For years with 53 weeks (rarer than 52), the tool handles that edge case correctly too.
Common uses
- Planning sprints or weekly reporting in project management, where ISO numbering is the convention.
- Cross-checking a week number against the ISO standard when it differs from what a calendar or another system shows.
- Finding the exact start and end dates of a specific work week for a schedule or invoice.
Things to keep in mind
A week's ISO year doesn't always match the calendar year: the first days of January can belong to the last week of the previous ISO year, and the last days of December to the first week of the next.
The number of weeks in an ISO year is 52 or 53, depending on what day of the week January 1 falls on and whether the year is a leap year.
Article about this tool: Week number: why different countries count weeks differently
Frequently asked questions
How is the week number determined according to ISO 8601?
Weeks start on Monday, and week 1 is the one containing the year's first Thursday (equivalently, the one that contains January 4).
Why can this week number differ from what another calendar shows?
Some systems (like in the US) start the week on Sunday and consider week 1 to be the one containing January 1 — this gives different numbering, especially at the start and end of the year.
Why can a date belong to a different "ISO year" than its calendar year?
The first or last days of January/December sometimes fall into a week where most days lie in the neighboring year — in that case, the ISO year of that week differs from the calendar year of the date itself.
Why do some years have 53 weeks instead of 52?
52 weeks is only 364 days, 1-2 days short of a calendar year, so those "extra" days periodically form an additional week. A 53rd week occurs in years starting on a Thursday, and in leap years starting on a Wednesday.
What happens if I enter a week number that doesn't exist for the chosen year?
The tool shows an error noting the maximum number of weeks for that specific ISO year (52 or 53), since that count differs from year to year.