Week Number Calculator
Find the ISO week number of any date, a numbering that many businesses and schools use for planning. You can also go the other way and find the dates of a numbered week.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
ISO weeks start on Monday. A week belongs to the year that holds its Thursday, so the week number is found from the Thursday of the same week.
To find the dates of a week, start from the Monday of week one, which is the week that holds January 4, and add seven days for each week after the first.
Worked example
2026-11-05 falls in ISO week 45. In 2,027, week 10 runs from 2027-03-08 to 2027-03-14.
Questions
Why can week one start in December?
Week one is the week that contains January 4, so it can begin in late December of the year before.
Do all years have the same number of weeks?
No. Some years have 53 ISO weeks. Type 53 for the week to check whether a year has one.
Is this the same as US week numbering?
No. Many countries use ISO weeks, but some systems start the week on Sunday and number it differently.
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