Under the ISO 8601 standard, weeks are numbered 1 to 52 (or 53), each starting on Monday — and you can find the number for any date on the [week numbers](/week-numbers) tool.
How ISO week numbering works
Three simple rules define it:
- Weeks start on Monday.
- Week 1 is the week containing the year's first Thursday (equivalently, the week with 4 January in it).
- A year has 52 or 53 numbered weeks.
This is why the first days of January sometimes belong to the last week of the previous year, and late-December days can fall in week 1 of the next year.
Why 52 or 53 weeks?
A year is 52 weeks plus 1 or 2 days, so the leftover days accumulate. A year has 53 ISO weeks when it starts on a Thursday, or a Wednesday in a leap year.
Find the current week number
The live week numbers page shows this week's number and a full grid for the year. You can also see week numbers down the side of every monthly calendar.
FAQ
What week number is it today? Open the week numbers tool for the current ISO week.
Does the week start on Monday or Sunday? ISO 8601 (used in business and Europe) starts the week on Monday; the US convention often starts on Sunday.