Week Number Calculator
Pick any date to see its ISO week number.
How it works
- Pick any date.
- See its ISO week number and the week's Monday-to-Sunday dates.
- Copy the result for planning or reporting.
Frequently asked questions
What is an ISO week number?
ISO 8601 numbers weeks from Monday to Sunday. Week 1 is the week containing the year's first Thursday, so it always has at least four days in the new year.
Why can early January be week 52 or 53?
If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous ISO year.
How many weeks are in a year?
Most ISO years have 52 weeks, but some have 53 — for example 2026, 2032 and 2037.
Do all countries number weeks this way?
ISO weeks are standard in Europe and much of Asia. Some systems, including many US calendars, start weeks on Sunday and number them differently.
Which week is it?
Week numbers are used for project planning, school timetables, production schedules and financial reporting. Pick any date to get its ISO 8601 week number and the dates that week covers.
ISO 8601 weeks
The international standard starts weeks on Monday and defines week 1 as the week with the year's first Thursday, so every week belongs to exactly one year.