Week Number Calculator
A week number calculator returns the official calendar week of the year for any date according to the ISO 8601 international scheduling standard. It identifies work weeks from Week 1 to Week 52 (or 53), showing corporate planners the standardized fiscal week containing any given day.
By Daniel Okoro (Mathematics Editor) · Reviewed by Maya ChenUpdated · Published
Presets:
CALENDAR DAYS ELAPSED
Equal to 4 weeks and 2 days (720 hours).
Full Weeks:4 Weeks
Remaining Days:2 Days
Total Hours:720 hrs
How Does the Week Number Calculator Work?
- Pick any calendar date.
- The calculator computes the ISO week number and starting/ending dates of that week.
What Is the Week Number Formula?
ISO Week = Week containing the first Thursday of the calendar year
ISO-8601 standard defines week 1 as the week starting on Monday that contains January 4th.
Variables
ISO Week:Official calendar week integer (1–53)
How Do You Calculate Week Number Step by Step?
Checking week number for October 5, 2026Date: October 5, 2026
Find the ISO week containing October 5
Determine start (Monday) and end (Sunday)
Final Solution:Week 41 of 2026
Week Number Questions and Answers
A regular year has 365 days, which is 52 weeks and 1 day (or 2 days in a leap year). When the year starts or ends on a Thursday, an official 53rd week occurs in ISO 8601.
Where Do These Week Number Standards Come From?
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