Week number calculator
Find the ISO 8601 week number (the standard used in France and most European countries) for any date. The calculation runs locally, with no data sent.
How it works
The ISO 8601 week number is widely used in business (schedules, reporting, weekly billing) because it removes the ambiguity of systems where weeks start on Sunday. Under this standard, the week starts on Monday and week number 1 of the year is the one that contains the first Thursday of January.
Why isn't January 1 always in week 1?
Because week 1 is defined relative to the first Thursday of the year, not to January 1 itself. If January 1 falls on a Friday, Saturday or Sunday, the days that precede it in the same calendar week actually belong to the last week of the previous year (week 52 or 53).
Can a year have 53 weeks?
Yes, this happens for years where January 1 falls on a Thursday, or for leap years where January 1 falls on a Wednesday: in these cases, the year has 53 full ISO weeks instead of the usual 52.
No date you enter is sent to a server: the calculation stays entirely local to your browser.