Week Number Calculator
Find the ISO week number and week-year for any date, including the Monday start and Sunday end of that week.
Find an ISO week number
See the ISO week and the Monday-to-Sunday week range.
What is an ISO week?
ISO 8601 numbers weeks from Monday through Sunday. Week 1 is the week containing the year's first Thursday. Because of that rule, the first few days of January can belong to the previous ISO week-year, and the final days of December can belong to the next one.
DateTimeWise shows both the week number and week-year so that boundary cases are clear.
Examples at the year boundary
| Date | ISO week | Monday–Sunday range |
|---|---|---|
| January 1, 2021 | Week 53 of 2020 | December 28, 2020–January 3, 2021 |
| December 29, 2025 | Week 1 of 2026 | December 29, 2025–January 4, 2026 |
| September 22, 2026 | Week 39 of 2026 | September 21–27, 2026 |
The ISO week-year is part of the answer: writing only “week 1” can be ambiguous. For schedules and reports, record both the week number and its week-year.
Why another calendar may show a different week
Some calendars begin weeks on Sunday or treat the week containing January 1 as week 1. This calculator consistently uses Monday-start ISO weeks; it does not implement fiscal, retail or organization-specific week calendars. An ISO year contains either 52 or 53 weeks.
For the weekday alone, use the Weekday Calculator. For the number of weeks in an interval, use Days Between Dates. These answer different questions from the numbered week of a year.