Week Number Calculator (ISO)
Find the ISO week number for any date — what week of the year is it?
Uses the ISO 8601 standard: weeks start on Monday and week 1 is the week containing the year's first Thursday. Near New Year the ISO week-year can differ from the calendar year. Runs in your browser.
Find out which week of the year a date falls in, using the ISO 8601 standard (weeks start on Monday and week 1 contains the year's first Thursday). Enter any date or use today. The tool also shows the ISO week-year, which can differ from the calendar year around New Year. Useful for planning, payroll periods and project schedules. Runs in your browser.
How to use the Week Number Calculator
- Pick a date (or tap This week).
- Read the ISO week number.
- Note the ISO week-year if it's near New Year.
Frequently asked questions
How is the week number calculated?
By the ISO 8601 standard: weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing January 4th).
Why does the week-year sometimes differ from the calendar year?
Early January can belong to the last ISO week of the previous year, and late December can belong to week 1 of the next — so the ISO week-year occasionally differs from the calendar year.