Time & Productivity Utility
Week Number Calculator
Find the ISO 8601 week number, week-year, and weekday for a calendar date.
Find ISO week number
Method
ISO 8601 weeks begin on Monday. Week 1 is the week containing the year’s first Thursday. The calculator uses UTC date-only arithmetic, converts Sunday from browser day 0 to ISO day 7, shifts the chosen date to the Thursday in its ISO week, and then computes the week index within that ISO week-year.
Worked example
January 1, 2026 is a Thursday. It belongs to ISO week 1, ISO week-year 2026, and ISO weekday 4.
Why week-year can differ from calendar year
Dates near New Year’s Day can belong to the previous or next ISO week-year. That is expected behavior under ISO week numbering. A late-December Monday, for example, can be part of week 1 of the following ISO week-year.
Privacy
The selected date is processed locally in the browser.
Limitations
This tool uses ISO 8601 week numbering only. Some calendars, accounting systems, and regional conventions begin weeks on Sunday or use different rules for week 1, so their week numbers can differ.
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