Week Number Calculator (ISO 8601)
Find the ISO 8601 week number of a date, the Monday to Sunday range of that week, and how many weeks the year has.
ISO week number
Type a date as YYYY-MM-DD.
Week
Week 53
ISO week date
2026-W53
Monday to Sunday
2026-12-28 to 2027-01-03
2026 has 53 ISO weeks
Weeks in that ISO year
Same as the calendar year
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On this page
Week number examples
| Date | ISO week | Why |
|---|---|---|
| 2026-01-01 | 2026-W01 | a Thursday: week 1 |
| 2021-01-01 | 2020-W53 | belongs to the last week of 2020 |
| 2024-12-30 | 2025-W01 | belongs to week 1 of 2025 |
| 2020-12-31 | 2020-W53 | 2020 has 53 weeks |
| 2016-01-03 | 2015-W53 | belongs to week 53 of 2015 |
Formula
week 1 is the week with the first Thursday of the year; weeks run Monday to Sunday
How the iso week number works
- Step oneType the date as YYYY-MM-DD.
- Step twoRead the week number and the ISO week date, such as 2026-W53.
- Step threeThe range line shows the Monday and the Sunday of that week.
What this tool does not do
- It follows ISO 8601 only; it does not give US-style or other week numbering.
- It uses the Gregorian calendar and the years 1000 to 9999.
- It does not know fiscal or school calendars, which can number weeks differently.
Frequently asked questions
Why can January 1 be in week 52 or 53 of the old year?
ISO weeks belong to the year that holds their Thursday. If January 1 falls on a Friday, Saturday or Sunday, it is still part of the last week of the year before.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. In 2026, which starts on a Thursday, there are 53.
Is this the same as the week number on a US calendar?
Not always. The US often counts weeks from Sunday and calls the week with January 1 week 1. This tool follows ISO 8601 only.