How to use the week number calculator
- To find the week of a date, keep The week number of a date selected and pick the date.
- Read the ISO 8601 week and week-year, then the US week, along with the first and last day of each week.
- To find the dates in a week, choose The dates in a week number, enter the year and week number and pick ISO or US numbering.
- Use the CSV button if you want the result in a spreadsheet.
Formula
ISO 8601: weeks run Monday to Sunday and week 1 is the week that contains the first Thursday of the year (the week containing January 4)
ISO week = whole weeks from week 1 to the Thursday of the date's week, plus 1
US: weeks run Sunday to Saturday and week 1 is the week that contains January 1
US week = (day of the year - 1 + weekday of January 1, with Sunday = 0) / 7, rounded down, plus 1
The ISO week-year can differ from the calendar year for a few days at the start and end of a year, because a week belongs to the year that holds its Thursday.
Worked examples at the year boundary
January 1, 2026 (Thursday): its week contains the first Thursday of 2026, so it is ISO week 1 of 2026. That week runs Monday, December 29, 2025 to Sunday, January 4, 2026. In US numbering it is week 1 as well, running Sunday, December 28 to Saturday, January 3.
January 1, 2027 (Friday): its week began on Monday, December 28, 2026, and the Thursday of that week is December 31, 2026. So it is ISO week 53 of 2026. 2026 has 53 ISO weeks because it starts on a Thursday.
December 30, 2024 (Monday): the Thursday of that week is January 2, 2025, so it is ISO week 1 of 2025, even though the calendar year is 2024.
Dates in a week: ISO week 41 of 2026 runs from Monday, October 5 to Sunday, October 11, 2026.
Which numbering should you use
ISO 8601 numbering is standard in much of Europe, in many software systems and in supply chain and project planning. The United States commonly counts weeks from Sunday with week 1 containing January 1, which is why a US year can reach week 53 or even week 54.
Other systems exist. Some retail and payroll calendars use fiscal weeks that start on other days or other months. If a schedule has to match an organization's calendar, confirm which definition it uses before relying on a week number.
Years with 53 weeks
An ISO year has 53 weeks when January 1 is a Thursday, or when it is a Wednesday in a leap year. Years 2015, 2020 and 2026 are examples. The tool rejects a week number that does not exist, such as ISO week 53 of 2027, and tells you how many weeks that year has.
Weeks that straddle two years
When a week begins in one year and ends in the next, the result tells you. For example, ISO week 53 of 2026 ends on Sunday, January 3, 2027, and ISO week 1 of 2026 begins on December 29, 2025.
Assumptions and limits
- ISO weeks start on Monday. US weeks start on Sunday.
- Dates use the Gregorian calendar between the years 1000 and 9999.
- Fiscal, retail and school calendars may number weeks differently and are not modeled.
- No time zone is involved. The answer is for the calendar date you enter.
Frequently asked questions
What week number is it today?
Leave the date on today's date, which fills in automatically in your browser. The ISO week is shown first and the US week below it.
Why does December 30 sometimes belong to week 1?
ISO weeks belong to the year that contains their Thursday. If a week starts on December 29, 30 or 31, its Thursday falls in January, so it counts as week 1 of the new year.
Why can a year have 53 weeks?
A year has 52 weeks and one or two extra days. When those extra days push a Thursday into the 53rd week, ISO counts a 53rd week. In the 400-year Gregorian cycle, 71 years have one.
Is the US week the same as the ISO week?
Often they match, but not always. ISO weeks start on Monday and US weeks start on Sunday, and week 1 is defined differently, so numbers can differ by one near the start and end of the year.