Date calculator
Week Number Calculator
Find the ISO 8601 week number for any date — plus the week start and end dates, the day of the week, and the ISO week-year. Results are shareable via URL.
How ISO weeks work
- ISO weeks always start on Monday and end on Sunday
- Week 1 is the week that contains the first Thursday of the calendar year
- Most years have 52 weeks; some have 53
- Late-December dates can fall in week 1 of the following year; early-January dates can fall in week 52 or 53 of the previous year
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Frequently Asked Questions
What is an ISO week number?
ISO 8601 week numbering divides the year into 52 or 53 weeks. Each week starts on Monday and ends on Sunday. Week 1 is the week that contains the first Thursday of the year — it always includes 4 January.
Why does the ISO week-year sometimes differ from the calendar year?
ISO weeks run Monday–Sunday and week 1 is anchored to the first Thursday of the calendar year. Late-December dates in a week whose Thursday falls in January will show the following calendar year. Conversely, early-January dates whose Thursday falls in December will show the previous year.
How many ISO weeks are in a year?
Most years have 52 ISO weeks. A year has 53 when 1 January falls on a Thursday, or when it is a leap year and 1 January falls on a Wednesday or Thursday.
Can I share my result?
Yes. Every result has its own URL. Copy it from the address bar or use the Copy URL button.