Week Number Calculator
Enter a date to find its ISO 8601 week number and the Monday-to-Sunday range of that week.
How ISO week numbers work
The ISO 8601 standard defines weeks as starting on Monday and ending on Sunday, with week 1 of a year being the week that contains that year's first Thursday (equivalently, the week containing January 4th).
Because of this definition, the last few days of December can belong to week 1 of the following year, and the first few days of January can belong to the last week (52 or 53) of the previous year — this calculator handles those edge cases automatically and shows the correct ISO year alongside the week number.
Frequently asked questions
Why does the ISO week start on Monday instead of Sunday?
The ISO 8601 standard specifically defines Monday as the first day of the week, which differs from the Sunday-first convention common in everyday calendars in some countries.
Can a year have 53 weeks?
Yes — most years have 52 weeks, but years where January 1st falls on a Thursday (or it's a leap year starting on Wednesday) have 53 ISO weeks.
Why might a date in late December show as week 1?
If December 31st falls early in the week (Monday through Wednesday), it can belong to week 1 of the following ISO year, since that week's Thursday falls in January.