Week Number Finder

Find the ISO 8601 week number for any date free online. Real ISO week-numbering algorithm, the standard used in business, project planning, and many calendars.

Browser only -- real ISO 8601 algorithm.

How to Use This Tool

1
Pick a dateAny date you want the ISO week number for
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See the resultReal ISO 8601 algorithm, not an approximation
3
Use for business or project planningStandard week numbering for scheduling

Real ISO 8601 Logic, Not a Simple Division

ISO week numbering isn't simply "day of year divided by 7" -- it specifically anchors week 1 to the week containing the year's first Thursday, with weeks running Monday through Sunday. This means dates near year boundaries can belong to a week number from the adjacent year, a deliberate and standardized behavior this calculator implements correctly.

Why Some Years Have 53 Weeks

52 weeks cover exactly 364 days, but calendar years have 365 or 366 days, so the extra day or two occasionally pushes a year into having 53 ISO weeks rather than 52, depending on which weekday January 1st falls on for that particular year.

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Frequently Asked Questions

ISO 8601 defines week 1 as the week containing the year's first Thursday, with weeks running Monday to Sunday -- this means the first few days of January sometimes belong to the last week of the previous year, and the last few days of December sometimes belong to week 1 of the next year.
This rule ensures week 1 always contains at least 4 days of the new year, since the Thursday-based calculation guarantees the majority of that week falls within January -- a deliberate design choice for consistency across years.
Since 52 weeks only covers 364 days but most years have 365 (or 366 in leap years), the extra day(s) occasionally accumulate into a 53rd week under the ISO counting rules, which happens in certain years depending on which day of the week January 1st falls on.
Business reporting, project planning, payroll systems, and many European calendars and contexts where consistent week-based scheduling matters more than calendar month boundaries.
Many calendar apps support ISO week numbering as an option, though some default to a different week-numbering convention -- checking your specific app's settings confirms whether it matches the ISO standard this tool calculates.
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