Free Date Tool
Week Number Calculator
Find the ISO week number for any date. See which week of the year it is.
ISO Week Number
Week 0
2026
ISO Year
Monday
Day of Week
Week Number Calculator: Find the ISO Week Number
Our week number calculator tells you the ISO week number for any date. This is useful for business planning, project scheduling, and understanding date formats that use week numbers.
The ISO week number calculator follows the international standard where weeks start on Monday and Week 1 contains the first Thursday of the year. This differs from some systems that start weeks on Sunday.
ISO Week Number: Weeks start on Monday. Week 1 contains the first Thursday of the year.
Week Numbers for 2026
- January 1, 2026 (Thursday) → Week 1
- January 12, 2026 (Monday) → Week 3
- March 1, 2026 (Sunday) → Week 9
- July 1, 2026 (Wednesday) → Week 27
- December 31, 2026 (Thursday) → Week 53
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Frequently Asked Questions
What is ISO week number?
ISO week numbering starts with Week 1 containing the first Thursday of January. Weeks start on Monday. Week 1 is the week with the year's first Thursday. Dec 28-31 and Jan 1-3 may belong to the same ISO week.
How many weeks in a year?
Most years have 52 weeks. Some have 53 weeks if the year starts on a Thursday (or is a leap year starting on Wednesday). 2026 has 52 weeks.
What week number is today?
Use our calculator to find the current week number. Simply click the date picker and select today's date, or enter any date you want to check.
Does the week start on Sunday or Monday?
ISO standard uses Monday as the first day of the week. Some countries (US, Canada) start weeks on Sunday. Our calculator uses the ISO standard (Monday start).
What is week 1 of 2026?
Week 1 of 2026 starts on Monday, December 29, 2025 and ends on Sunday, January 4, 2026. This is because January 1, 2026 (Thursday) falls in Week 1 per ISO rules.