Week Number
Find a date’s ISO week number and see the full Monday-to-Sunday week.
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ISO week
Choose a date to see its week number.
How it works
Read the guide
ISO-8601 weeks start on Monday, and week 1 is the week containing the year’s first Thursday. A consequence catches many calculations out: 1 January is often not in week 1. It can belong to week 52 or 53 of the previous week-year, and the last days of December can belong to week 1 of the next one.
That is why the week-year is shown next to the week number — they are not always the same as the calendar year of the date. Years that begin on a Thursday, and leap years that begin on a Wednesday, have 53 weeks instead of 52.
The calendar uses the same numbering, so the week numbers shown there always agree with this tool.