📅 Week Number Calculator
Find the ISO week number for any date.
How to Find the Week Number of a Date
Instantly determine the ISO week number (1-52 or 53) for any date. This tool is essential for project planning, payroll processing, and international business scheduling. It follows the ISO 8601 standard, where weeks start on Monday and Week 1 contains the first Thursday of the year.
Example: How to Calculate ISO Week Numbers
Suppose you want to find the ISO week number for December 31, 2025.
Although this date falls in December 2025, it actually belongs to Week 1 of 2026, according to the ISO 8601 standard.
This happens because ISO weeks always start on Monday, and Week 1 is the week containing the first Thursday of the new year.
For example:
- December 29, 2025 (Monday): Week 1 of 2026.
- December 31, 2025 (Wednesday): Week 1 of 2026.
- January 1, 2026 (Thursday): Week 1 of 2026.
Our free week number calculator automatically handles these situations, helping you identify the correct ISO week number for any date.
This is especially useful for business schedules, project deadlines, payroll planning, and international calendars.
Frequently Asked Questions
What is the ISO week number?
It’s an international standard (ISO 8601) for identifying weeks. Each week starts on Monday, and Week 1 is the week containing the first Thursday of January.
Why does the year sometimes differ from the calendar year?
If a date falls in early January but belongs to the last week of the previous year (or late December belonging to Week 1 of the next year), the ISO week year will adjust accordingly.
Is this calculator free and private?
Yes. It runs entirely in your browser with no data collection or server requests.