How to use the week number calculator
- Choose Calendar date and pick today or another date to find its ISO week number.
- Or choose ISO week number and enter the week-year and a week from 1 to 52 or 53 to look up its dates.
- Read the week code, Monday start, Sunday end, and the number of weeks in that ISO year. Expand the year table to see every week.
Calculations run in your browser. Your selected date is not uploaded to a server. A shared link may contain the date in its URL fragment, so avoid sharing private dates.
What is an ISO week number?
ISO 8601 assigns every calendar date to a numbered Monday-to-Sunday week. The first week of an ISO year contains January 4, equivalently the first Thursday of January. The week-year follows that Thursday, not necessarily the ordinary calendar year of the date you selected. That distinction matters in weekly reports, project plans, timesheets, shipping schedules and spreadsheets.
For example, December 29, 2003 was a Monday, but its ISO week is 2004-W01. The same seven-day week ended Sunday, January 4, 2004. The year printed on the ISO week code is 2004, even though its Monday fell in 2003.
How the calculation works
First, find the Monday of the selected date’s week. Move three days forward to its Thursday: that Thursday determines the ISO week-year. Then locate the Monday of the week containing January 4 of that ISO year. Count complete seven-day intervals between the two Mondays, and add one. The result is an integer from 1 to 52 or 53.
Reverse lookup follows the same rule: find the Monday of ISO week 1 and add seven days for each preceding week. The calculator rejects week 53 for years that have only 52 ISO weeks rather than silently rolling into the next year.
Worked examples and year boundaries
- January 1, 2016: Friday, ISO week 2015-W53. The week began December 28, 2015 and ended January 3, 2016.
- January 1, 2021: Friday, ISO week 2020-W53, because the first Thursday of 2021 belonged to the following week.
- January 1, 2026: Thursday, ISO week 2026-W01, running from December 29, 2025 to January 4, 2026.
- December 31, 2026: Thursday, ISO week 2026-W53, ending January 3, 2027.
Why some years have 53 ISO weeks
Most ISO week-years have 52 full weeks. A year has 53 when January 1 is a Thursday, or when it is a leap year beginning on Wednesday. That is why 2020 and 2026 have 53 ISO weeks, but 2021 and 2027 have 52. A date in early January can therefore have the previous year’s week label; a date in late December can already belong to week 1 of the next ISO year.
ISO weeks versus US calendar weeks
ISO weeks always start on Monday. Many US calendars start their rows on Sunday and may number partial weeks differently. A US Sunday-start week number is not automatically an ISO week number, even when the numeric value happens to match. This tool intentionally reports ISO 8601 only; for payroll, contracts or fiscal schedules, confirm which convention your organization requires.
The calculation uses UTC calendar dates so daylight saving time does not shift a date into the wrong week. It does not count holidays, business days, fiscal 4-4-5 weeks or the number of weeks elapsed since a project began. For elapsed days use the date difference calculator; for the weekday of a date use the weekday calculator.
Sources and verification
Week-year and first-Thursday rules follow ISO 8601 and the Python datetime ISO calendar reference. The independent reference examples include 2003-12-29 = 2004-W01 and 2016-01-01 = 2015-W53.
Frequently asked questions
What week number is it today?
Leave Calendar date selected and keep today’s date. The calculator shows the ISO week number and its Monday–Sunday range.
Can January 1 be in the previous ISO year?
Yes. January 1, 2016 was in ISO week 2015-W53. ISO years are assigned using the Thursday in each week.
What is week 53?
An extra ISO week found in some years. For example, 2026 has 53 ISO weeks; 2027 has 52. The reverse lookup checks this.
Do ISO weeks start on Sunday?
No. ISO weeks run Monday through Sunday. Some US calendars use Sunday-first numbering instead.
How do I convert week 1 to a date?
Choose ISO week number, enter the ISO year and week 1. The result shows the Monday and Sunday of that week.
Last updated: October 17, 2026