Category: Date & time
Week Number Calculator
Find the ISO week number, week-year and weekday for any date
Find the ISO-8601 week number and week-year for any date, along with its weekday, day of the year, and the Monday-to-Sunday range that week covers. Calculated locally as you pick a date.
Everything on this page is processed in your browser. Nothing is uploaded.
What this tool does
The ISO-8601 standard numbers weeks from 1 to 52 or 53, starting each week on Monday and assigning week 1 to the week containing the year's first Thursday. That rule means a date can belong to a week numbered in the previous or next calendar year, which is why this tool reports the week-year separately from the calendar year. It also shows the weekday, the ordinal day of the year, and the exact Monday-to-Sunday range for the week your date falls in.
How to use it
- Pick a date with the date field.
- Read its ISO week number, week-year and weekday.
- Check the Monday-to-Sunday range for that week.
Privacy
This tool runs entirely in your browser. Your input is never uploaded, stored or shared — closing the tab removes it.
Frequently asked questions
- Why does the week-year sometimes differ from the calendar year?
- ISO weeks always start on Monday and week 1 is defined as the week holding the first Thursday of January, so the last days of December can fall in week 1 of the next year, and the first days of January can fall in week 52 or 53 of the previous year.
- Does this use the US convention where weeks start on Sunday?
- No. This follows ISO-8601, the international standard used across most of Europe and by most software systems, where weeks start on Monday.
- Is my date sent anywhere?
- No. Everything is calculated locally in your browser.