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Week Number Calculator

Find the ISO 8601 week number of any date, the dates of any week and a table of all the weeks of a year, with US week numbering as an option.

Week numbering
Week number
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Week-numbering year
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Week code
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Week runs from
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Day of the year
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Weeks in that year
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Find the dates of a week

All the weeks of a year

WeekFromTo

How to use the week number calculator

  1. Pick a Date. Today’s date is filled in when the page opens, and the Today button brings it back.
  2. Under Week numbering, keep ISO 8601 (used in Europe, Canada and most of the world) or switch to the United States system.
  3. Read the Week number, the Week-numbering year, the Week code (such as 2026-W40-3, meaning the third day of week 40), the Monday-to-Sunday range under Week runs from, the Day of the year and the number of Weeks in that year.
  4. To go the other way, use Find the dates of a week: enter a Year and a Week to get its first and last day.
  5. All the weeks of a year lists every week with its start and end dates. The week of your chosen date is highlighted, and Copy copies the table as tab-separated text you can paste into a spreadsheet.

How ISO 8601 week numbers work

The ISO 8601 standard, used in Europe and by most business software, defines weeks this way:

  • weeks start on Monday and end on Sunday;
  • week 1 is the week that contains the year’s first Thursday, which is also the week that contains January 4;
  • a year therefore has 52 or 53 weeks.

Because of the Thursday rule, the days around New Year don’t always belong to the calendar year you would expect. January 1, 2027 is a Friday: its week has more days in 2026 than in 2027, so it is week 53 of 2026. In the other direction, Monday, December 30, 2024 was already in week 1 of 2025. This is why the tool shows a separate week-numbering year and explains when it differs from the calendar year.

A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2020 and 2026 have 53 weeks; 2021 to 2025 have 52.

US week numbers

In the United States (and in many calendars printed there), weeks start on Sunday and week 1 is the week that contains January 1, even if it only has one day in the new year. With this system, week numbers always stay within the calendar year: they go from 1 to 53, or even 54 in a leap year that starts on a Saturday, such as 2000 or 2028. This matches the WEEKNUM function of spreadsheet software with its default settings.

The two systems often disagree by one week, so always check which one a colleague or a client means.

Useful examples

  • Tuesday, December 31, 2024 is ISO week 1 of 2025, but US week 53 of 2024.
  • January 1, 2021 (a Friday) is ISO week 53 of 2020.
  • September 30, 2026 is ISO week 40, from Monday, September 28 to Sunday, October 4.

Frequently asked questions

What week is it today?

When the page opens, the Date field holds today’s date, so the Week number shown is the current week in the selected numbering.

Why is January 1 in week 52 or 53?

Under ISO 8601, a week belongs to the year that contains its Thursday. When January 1 falls on a Friday, Saturday or Sunday, its week belongs to the previous year.

Which countries use ISO week numbers?

Most of Europe, including France, Germany and the Nordic countries, as well as many businesses in Canada and elsewhere. The United States, and to some extent the Middle East, use Sunday-based weeks.

Can a year have 54 weeks?

Not in ISO 8601, which has at most 53. With US numbering, a leap year that starts on a Saturday touches 54 weeks, the last one containing only December 31.

How do I find the dates of week 1?

Type the year and 1 in Find the dates of a week. For ISO week 1 of 2026, it is Monday, December 29, 2025 to Sunday, January 4, 2026.

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