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What Week Is It

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The big number is this week’s ISO week number, used in Europe and most of the world, where weeks start on Monday and week 1 is the week with the year’s first Thursday. The US-style number (weeks starting on Sunday, week 1 contains 1 January) is shown too, with this week’s dates and a list of every week in the year.

This week

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Two ways to number weeks

  • ISO 8601 (Europe, Asia, international business): weeks start on Monday. Week 1 is the week that contains the year’s first Thursday, which is the same as the week containing 4 January. A year has 52 or 53 weeks, and the first days of January can belong to the last week of the previous year.
  • US system: weeks start on Sunday, and week 1 is the week that contains 1 January. The last days of December can fall in week 1 of the next year.

Most of the time the two numbers are the same or differ by one. When someone says “week 12”, check which system their calendar uses.

Which years have 53 weeks?

An ISO year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year. Recent and upcoming 53-week years are 2015, 2020, 2026, 2032 and 2037. Payroll and retail calendars sometimes need an extra pay period or a “53rd week” in these years.

Where week numbers are used

  • Project planning and sprints (“deliver in week 42”)
  • Payroll, shift rotas and school timetables
  • Shipping, manufacturing and retail reporting
  • Diaries and planners in Europe, which print the week number on each page

Find the week of any date

Pick a date to see its week number, or change the year to list every week with its start and end dates. In Excel or Google Sheets, =ISOWEEKNUM(A1) gives the ISO week and =WEEKNUM(A1) the US week.

Frequently asked questions

What week number is it today?

Today’s ISO week number is shown in large type at the top of this page, along with the US-style week number and this week’s dates.

When does week 1 start?

In the ISO system, week 1 is the week (Monday to Sunday) that contains the year’s first Thursday. In the US system, it’s the week (Sunday to Saturday) that contains 1 January.

How many weeks are in a year?

52 weeks and 1 day (2 in a leap year). ISO years therefore have 52 or 53 numbered weeks.

Why do the ISO and US week numbers differ?

They start weeks on different days and define week 1 differently, so the numbers can be one apart, especially at the start and end of the year.

How do I get the week number in Excel?

Use =ISOWEEKNUM(date) for the ISO week or =WEEKNUM(date) for the US-style week.

Last updated October 2026 · More time & dates