Week Number Calculator | What Week of the Year is it?
Find out the current ISO week number, or determine the week number of any given date.
2026
Apr
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29 | 30 | 31 | 01 | 02 | 03 | 04 |
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19 | 20 | 21 | 22 | 23 | 24 | 25 |
26 | 27 | 28 | 29 | 30 | 01 | 02 |
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About ISO Week Numbers
What is an ISO Week?
The ISO 8601 data elements standard specifies a week-numbering system. Weeks start on Monday. The first week of a year is the week that contains the first Thursday of the year.
Week 1
Week 1 of any year is the week containing January 4th. This means that sometimes, the first few days of January might belong to the last week (Week 52 or 53) of the previous year.
52 or 53 Weeks?
Most years have 52 weeks. A year has 53 weeks if January 1st falls on a Thursday, or if it's a leap year and January 1st falls on a Wednesday.