Week Number Calculator
Find the ISO 8601 week number, ISO year, and day of week for any date.
ISO Week Number
Week 33
2026iso year•
Wednesdayday of week•
Wednesday, Aug 12, 2026selected date
Find the ISO 8601 week number and ISO year for any date on the calendar.
[!NOTE] This calculator strictly implements the international ISO 8601 week numbering standard.
Worked Example
- January 1, 2026 (Thursday) = ISO Week 1 of 2026
- December 30, 2024 (Monday) = ISO Week 1 of 2025
Frequently Asked Questions
Which week numbering standard does this tool use?+
This tool explicitly uses the international ISO 8601 week numbering standard, where weeks begin on Monday and Week 1 of any year is the week containing the first Thursday of that year.
Why can a date in late December or early January belong to a different ISO year?+
Under ISO 8601, if December 30 or 31 falls on a Monday, Tuesday, or Wednesday, it belongs to Week 1 of the upcoming ISO year.
Which day does the ISO week start on?+
ISO 8601 specifies Monday as the first day of the week.
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