Week Number
Find the ISO 8601 week number for any date and see which dates fall in that week. Useful for project planning, payroll periods, and international reporting standards.
How to use the week number
Pick a date — ISO week number, ISO year, and day-of-year are computed.
Formula & explanation
ISO 8601 weeks start Monday; week 1 contains January 4th.
Examples
January 1 might be week 52 or 53 of the previous ISO year if it falls on a Friday/Saturday/Sunday.
Frequently asked questions
- Why does January 1 sometimes belong to week 52 of the previous year?
- ISO 8601 defines week 1 as the week containing the first Thursday of the year. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous ISO year.
- What is the difference between ISO week year and calendar year?
- The ISO week year can differ from the calendar year by ±1 day for dates at the start or end of January. The ISO week year is always the year that contains Thursday of that week.
- How is the day-of-year counted?
- Day 1 is January 1. Day 365 (or 366 in a leap year) is December 31. The count resets to 1 on January 1 each calendar year.
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