Week Number Calculator
Find the ISO 8601 week number, ISO year, and day of week for any date.
ISO Week Number
Week 34
2026iso year•
Aug 17 – Aug 23, 2026week range•
Wednesdayday of week
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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