What Week Number Is It?
Today’s ISO week number is shown below. Pick any other date to find its week number and day of the year.
Your result appears here.
By Shah Rukh, software developer · Last updated:
How ISO week numbers work
The ISO 8601 standard, used in Europe, the UK and most business software, says:
- Weeks start on Monday.
- Week 1 is the week containing the year’s first Thursday (the week with 4 January in it).
- A year has 52 or 53 weeks.
So 1 January can belong to week 52 or 53 of the previous year, and 31 December can be in week 1 of the next year. That’s why the ISO week-year is shown too.
US week numbers
Some US calendars start weeks on Sunday and count the week of 1 January as week 1. Their week numbers can be one higher than ISO numbers for part of the year.
Frequently asked questions
How many weeks are in a year?
52 or 53 ISO weeks. A year has 53 weeks if it starts on a Thursday, or on a Wednesday in a leap year.
What week number is it today?
It’s shown at the top of this page as soon as it loads.
Does Excel use ISO weeks?
Use =ISOWEEKNUM(date) for ISO weeks. =WEEKNUM(date) uses the US system by default.
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