Week Number Finder
Find the ISO week number and week-year for a date.
ISO calendar week
Week 44
Week-year: 2026
ISO weeks start on Monday. Week 1 contains the year's first Thursday.
Tool guide
About Week Number Finder
Find the week number for a selected date using the page's calendar convention. Week numbering systems differ, especially around New Year, so check the rule used when a result is for business or reporting.
How to use this tool
- 1Select the date you want to check.
- 2Read the week number shown by the tool.
- 3Confirm whether your organization uses ISO week dates or another convention.
How the tool works
A week-number system assigns dates to numbered seven-day weeks and decides which week counts as week 1. The ISO 8601 system starts weeks on Monday and defines week 1 as the week with the year's first Thursday; other calendars may use Sunday starts or different rules.
Worked real-world example
A date near January 1 can belong to the final week of the previous ISO week-year, even though its calendar year has begun.
Common scenarios and use cases
Prepare a week reference for scheduling, planning, or comparing dates in a spreadsheet.
Important details and limitations
The week number can differ between systems. Check the selected convention before using the result in payroll, logistics, or a formal report.
Check the output unit as well as the number. If the result will be reused in another calculation, retain full precision where possible and round only at the end. For measurements governed by a technical specification, use the standard and tolerance required by that document.
Frequently asked questions
Why can early January show week 52 or 53?
ISO week-years can begin in the previous calendar year.
Does every country use ISO week numbers?
No. Local business and calendar conventions vary.
Is the week number the same as the month number?
No. It counts weeks under a specified week-year rule.