Useful for project plans, payroll, ISO 8601 dates and academic calendars. Pair it with the Countdown Date Calculator to plan a specific week.
Calculator
How to use the Week Number Finder
- Pick a date from the calendar.
- Choose the week-numbering rule that matches your context.
- Read the week number, day of year and week start.
- Use the table to plan a specific working week.
About this tool
The Week Number Finder is built for speed and accuracy: every calculation runs in your browser, so your data never leaves your device and the result updates as you type. It works equally well on a phone, a tablet or a desktop, and it is free to use as many times as you like.
Common use cases
- Aligning project milestones to ISO weeks.
- Comparing week numbers across the Atlantic.
- Calculating days left in the year for budgeting.
- Building academic week lists.
Tips for the best result
- Use the latest figures you have - even a small change in rate or term can shift the outcome by hundreds.
- For annual figures, divide by 12 to get a monthly number you can compare with the result.
- Bookmark this page and revisit the calculation as the inputs change.
Frequently asked questions
What is an ISO week?
ISO weeks start on Monday, and week 1 of the year is the one containing the first Thursday. The first ISO week of 2026 is the week of 29 December 2025.
Why does January 1 sometimes show as week 53?
If Jan 1 falls on a Friday-Sunday in the US system, or in some leap-year edge cases in the ISO system, the date can belong to the last week of the previous year.
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