Week Number: What Week Is It?
What week is it? Today's ISO week number, the week of any date, the US week number and the list of all weeks of the year with their dates.
| Week | Monday | Sunday |
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What it does
Most of the world numbers the weeks of the year with the ISO 8601 standard: weeks start on Monday and week 1 is the week with the year's first Thursday, so a year has 52 or 53 weeks and the first days of January can belong to the last week of the year before. The United States and some other countries count weeks from Sunday, with week 1 being the week of January 1. This page shows today's week in both systems, the dates of the week and a list of all the weeks of the year.
How to use it
- Open the page to see today's week, or choose any date.
- See the ISO week, the dates from Monday to Sunday and the US week number.
- Change the year to list all of its weeks with their dates.
The arithmetic runs in your browser. Nothing you type is sent anywhere and nothing is stored, so you can use these with real numbers.
Frequently asked questions
Why do some years have 53 weeks?
A year has 52 weeks and one or two days. In the ISO system a year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year, as in 2020 and 2026.
Can January 1 be in week 52 or 53?
Yes. If January 1 falls on a Friday, Saturday or Sunday, it belongs to the last ISO week of the previous year, and week 1 starts on the following Monday. In the same way, December 29 to 31 can be in week 1 of the next year.
Which week numbering should I use?
In Europe, most of Asia and in business planning, the ISO week. In the United States, Canada and many calendars that start on Sunday, the US week. The two can differ by one.
How is an ISO week written?
As the year, the letter W and the week, such as 2026-W44, and with the day of the week as a number from 1 (Monday) to 7, such as 2026-W44-3 for a Wednesday.
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π Day of the Week Calculator
Wikipedia — ISO week date
ISO — ISO 8601 date and time format