Date tool · ISO 8601
Week calculator
Convert any date to its ISO week number, date range, US week and holidays. Works for any year from 1900 to 2100.
Convert any date to its ISO week number.
Pick a date below and you'll see its ISO 8601 week number right away, plus the full Monday to Sunday range, the US week number, the quarter and any holidays that week. Every calculation runs the moment you change the date, no page reload.
Want today instead? Check this week's number. Need the whole year? Browse the full 2026 calendar, or read how ISO week numbering works.
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How the week calculator works
Every date belongs to exactly one ISO week. ISO 8601 starts each week on Monday, and week 1 is the one holding the year's first Thursday. So a date in early January can land in week 52 or 53 of the year before. The calculator handles all of that for you.
It also shows the US week number alongside the ISO one. The US system starts weeks on Sunday and counts January 1 as week 1, so the two can differ by a week near year boundaries. If you work in spreadsheets, the homepage has ready-made Excel and Google Sheets formulas for both.
Curious how many weeks a year has? The weeks in a year guide explains 52 versus 53, and the ISO week number page covers the standard end to end.
Week calculator: questions and answers
How do I find the week number for any date?
Enter any date in the calculator above. It instantly shows the ISO week number, the full Monday–Sunday range, the US week number, and any holidays in that week.
What is the difference between ISO week and US week number?
ISO 8601 weeks start on Monday. The US system starts weeks on Sunday. Near year boundaries, the two systems can differ by one. Most international business uses ISO week numbering.
What is week 1 of a year?
Under ISO 8601, Week 1 is always the week containing the first Thursday of the year. This means Week 1 can start as early as December 29 or as late as January 4.
Can a year have 53 weeks?
Yes. A year has 53 ISO weeks when January 1 falls on a Thursday. 2026 is a 53-week year.
How do I convert a week number back to a date?
Each ISO week starts on Monday. Week 1 of a year starts on the Monday of the week containing January 4. From there, add 7 days per week. The results show the full date range for any week you look up.