ISO week number and day of year calculator
Find the ISO week, week-year, Monday–Sunday range and day of the year for any date. Check year-boundary weeks accurately.
Preparing your tool…
How it works
ISO weeks start on Monday. Week 1 is the week containing the year’s first Thursday. The ISO week-year can differ from the calendar year near January and December.
Example
1 January 2021 is ISO week 53 of 2020. Its week runs from 28 December 2020 to 3 January 2021.
Are all countries’ week numbers identical?
No. Local calendars may start on Sunday or use a different week-1 rule. This page deliberately uses the ISO convention and labels the week-year separately.