ISO Week Number Finder
Find the ISO week number for any date, with optional timezone support.
Enter Date & Timezone
Result
Enter a date above to find its ISO week number
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What is an ISO Week Number?
The ISO Week Number is a standardized way to identify a specific week within a year, defined by the international ISO 8601 standard. Instead of messy, irregular month lengths, the ISO calendar divides the year into exactly 52 (or sometimes 53) 7-day weeks.
By definition, an ISO week always starts on a Monday, and the very first week of the year (Week 1) is defined as the week that contains the first Thursday of the new year.
Who Uses ISO Week Numbers?
- Project Managers: Agile development sprints and global project timelines are often scheduled by week numbers (e.g., "Feature launch in CW42") rather than specific dates.
- Finance & Accounting: Many companies align their fiscal reporting, payroll, and quarterly tax planning with standardized ISO weeks.
- Logistics & Supply Chain: Global shipping and manufacturing schedules heavily rely on week numbers to coordinate international freight.
How to Find Your Week Number
- Click the Date input box to select the day you are inquiring about.
- Ensure the correct Timezone is selected (as a week can shift if your timezone is across the midnight boundary).
- Click "Find Week Number".
- The result will show the exact integer (1-53) representing that week in the ISO calendar.
Frequently Asked Questions (FAQ)
Can a year have 53 weeks?
Yes. While most years have 52 ISO weeks, a year will have 53 weeks if January 1st falls on a Thursday (or on a Wednesday in a leap year). The next 53-week years are 2026 and 2032.
Can early January dates belong to the previous year's week?
Yes. According to ISO 8601 rules, if January 1st, 2nd, or 3rd falls on a Friday, Saturday, or Sunday, those days belong to the final week (Week 52 or 53) of the previous year.
What does "CW" stand for?
CW stands for "Calendar Week." You will often see ISO week numbers written as CW42, W42, or 2024-W42 to differentiate them from standard monthly dates.