Example 1
- Date: January 1, 2026
Week 1 of 2026
What week number is January 1, 2026? Week 1, from December 29, 2025 to January 4, 2026.
Week Number
The calculator reports the ISO week number for the date you enter. January 1, 2026 is week 1. December 31, 2024 is week 1 of 2025, because that week's Thursday already falls in 2025.
This is not a today's-date page. Civil days between two dates are on the date difference page. Clock hours are on the hours calculator.
Result
Enter a date. The ISO week number will appear here.
How it works
The calculator uses ISO 8601 week numbering. A week starts on Monday. The week belongs to the year that contains its Thursday. Week 1 is the week that contains January 4.
January 1, 2026 is a Thursday, so it is week 1 of 2026. That week runs from December 29, 2025 to January 4, 2026.
December 31, 2024 is a Tuesday. The Thursday of that week is January 2, 2025, so the number is week 1 of 2025, not 2024.
Some years have 53 ISO weeks. December 31, 2020 is a Thursday and week 53 of 2020. That week runs from December 28, 2020 to January 3, 2021.
The today button inserts the device calendar day. The page still reports the week number for that date. It is not a separate "what is today's date" tool.
The U.S. layout counts the week from Sunday and starts week 1 on January 1. That number can differ. The calculator shows it in the note under the result, and the main result follows ISO.
Formula
An ISO week belongs to the year of that week's Thursday. Week 1 contains January 4.
Monday = date minus (weekday number − 1), where Monday = 1 and Sunday = 7. Sunday closes the same week.
How to use
- Enter a date, for example January 1, 2026, or use the today button.
- Choose Calculate. The result is the ISO week number and the ISO year.
- Below that you see the Monday and Sunday of that week and the weekday of the chosen date.
- When the ISO year differs from the calendar year, the note explains it. December 31, 2024 is that case.
Week 1 from January 1, 2026
January 1, 2026 is a Thursday, so it is week 1 of 2026. The ISO week runs from December 29, 2025 to January 4, 2026.
- ISO 8601
- Week numbering from Monday. Week 1 contains January 4 or the first Thursday of the year.
- ISO year
- The year that contains the Thursday of that week. December 31, 2024 has ISO year 2025.
- Week 53
- An extra ISO week. December 31, 2020 is week 53 of 2020.
Examples
Example 2
- Date: December 31, 2024
Week 1 of 2025
What week number is December 31, 2024? Week 1 of 2025, because that week's Thursday is already in 2025.
Example 3
- Date: December 31, 2020
Week 53 of 2020
Is December 31, 2020 week 53? Yes. It is a Thursday and the last ISO week of that year.
Related calculators
Frequently asked questions
What week number is January 1, 2026?
Week 1 of 2026. That day is a Thursday, so the ISO week runs from December 29, 2025 to January 4, 2026.
What week number is December 31, 2024?
Week 1 of 2025. The Thursday of that week falls in 2025, so the ISO year is 2025, not 2024.
Is December 31, 2020 week 53?
Yes. It is a Thursday, ISO week 53 of 2020. The week runs from December 28, 2020 to January 3, 2021.
Which day starts an ISO week?
Monday. Sunday closes the same week. That is ISO 8601, not a Sunday-start week.
How is the ISO year different from the calendar year?
The ISO year belongs to the Thursday of that week. December 31 can therefore belong to week 1 of the next year.
Is this a today's-date page?
No. The calculator reports the week number for the date you enter. The today button only inserts the device calendar day.
How is this different from the date difference?
The date difference counts civil days between two dates. Here you look up one day and its ISO week number.
How many weeks are in a year?
Usually 52. An ISO year has 53 weeks when January 1 or December 31 is a Thursday, and in a leap year also when January 1 is a Wednesday.
Is the U.S. week the same?
Not always. In a Sunday-start layout with week 1 from January 1, the number can differ. The calculator shows that number in the note under the result.
Is December 29, 2025 already week 2026?
Yes. December 29, 2025 is the Monday of week 1 of 2026, because that week's Thursday is January 1, 2026.
Sources
The week number on this page is the ISO 8601 layout: week from Monday, year by Thursday.
Page updated in 2026.