ISO Week Number Calculator
Determine the ISO-8601 week number and ISO week year of a calendar date.
Description
Find the ISO week number and ISO week-numbering year for a calendar date.
ISO Week Number Calculator find the ISO week number and ISO week-numbering year for a calendar date.
Use this for weekly reporting, filenames, schedules, and cross-year grouping where ordinary month and calendar-year labels are insufficient.
When to use ISO Week Number Calculator
Use this for weekly reporting, filenames, schedules, and cross-year grouping where ordinary month and calendar-year labels are insufficient.
How it works
ISO weeks start Monday, and week 1 is the week containing the year's first Thursday. Dates near New Year can therefore belong to the preceding or following ISO year.
Interpreting the result
Keep the input units, direction conventions, encoding, and ordering rules visible when reusing the result. Preserve the full returned value for downstream calculations and round only for presentation.
Assumptions and limitations
The result is a week label, not an elapsed-week duration. Use the returned ISO year together with the week number to avoid ambiguity.