Week Number Calculator
Find the ISO-8601 week number for any date.
Free week number calculator. Enter a date to find its ISO-8601 week number and week-year, following the Monday-start standard used across business and Europe. Runs in your browser.
How to use the Week Number Calculator
- 01Pick a date.
- 02See its ISO-8601 week number and the corresponding week-year.
- 03The weekday is shown so you can confirm the Monday-start convention.
Frequently asked questions
How is the ISO week number defined?
Under ISO-8601, weeks start on Monday and week 1 is the week that contains the year's first Thursday. This means early January dates can belong to the previous year's last week.
Why does the week-year sometimes differ from the calendar year?
Because week 1 is tied to the first Thursday, a date like 1 January can fall in week 52 or 53 of the previous ISO week-year, which the tool reports separately.