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Week Number Calculator

Enter a date to find its ISO 8601 week number, week-year and Monday-to-Sunday dates. You can also compare a calendar-week rule that starts on Sunday, Monday or Saturday.

Custom mode counts the week containing January 1 as week 1 and labels it with the calendar year. ISO 8601 can assign dates near New Year to a different week-year. Date-only calculations use UTC; no time-zone conversion or fiscal-calendar rules are applied.

How do I find the week number for a date?

Choose ISO 8601 or a calendar-week rule, enter the date and calculate. ISO weeks start Monday, and week 1 is the week containing January 4. The week-year can differ from the calendar year near New Year.

Frequently asked questions

How do I find the week number for a date?

Choose ISO 8601 or a calendar-week rule, enter the date and calculate. ISO weeks start Monday, and week 1 is the week containing January 4. The week-year can differ from the calendar year near New Year.

Is my date sent to a server?

No. This date calculation runs in your browser. The date is not uploaded or saved by this tool.

Why can the ISO week-year differ from the calendar year?

An ISO week belongs to the year containing its Thursday. A date in early January can therefore belong to the last ISO week of the previous year, and a date in late December can belong to week 1 of the next ISO year.

What does the custom calendar-week option mean?

It numbers the seven-day period that contains January 1 as week 1, using the selected first day. The label stays with the calendar year. This is a transparent comparison rule, not a claim about your country or employer’s official convention.

Does this calculate fiscal week numbers?

No. Fiscal calendars can use organization-specific start dates and 4-4-5 or other rules. This tool only calculates ISO weeks or the stated January 1 calendar-week rule.