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Week numbers

What week number is it?

ISO 8601 week numbers, with the exact Monday-to-Sunday range for every week of the year. Calculated in your browser — nothing is sent anywhere.

Week calendars by year

How ISO weeks work

Weeks run Monday to Sunday. Week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January. Because the rule anchors on a midweek day, every week sits wholly inside one year, and the first days of January often belong to the last week of the previous year.

That gives each year either 52 or 53 whole weeks. There is never a stray part-week at either end, which is exactly why planning and logistics systems favor the scheme.

The mismatch to watch for

ISO numbering is not the only convention. Much of North American payroll, retail, and broadcast scheduling starts weeks on Sunday and calls the week containing 1 January week 1. The two systems routinely disagree by one, and the disagreement is largest in early January. If a week number here does not match another system, check which convention that system uses before assuming an error.

Common questions

What is an ISO week number?
ISO 8601 numbers the weeks of a year from 1 upward. Weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of the year. Every week belongs entirely to one year, so no week is ever split across a year boundary.
Why do some years have 53 weeks?
A year has 53 ISO weeks when it begins on a Thursday, or begins on a Wednesday in a leap year. Otherwise it has 52. Over the long run roughly one year in seven has 53.
Why does my week number not match my payroll system?
Most likely because the two use different conventions. ISO weeks start on Monday and week 1 holds the first Thursday. Many North American payroll, retail, and broadcast calendars start weeks on Sunday and treat the week containing 1 January as week 1, which shifts every subsequent number.
Who uses ISO week numbers?
They are common in European business, logistics, manufacturing, and project planning, where a date is often given as a week rather than a day — "shipping in week 32". They are also the default in most software date libraries.

Related

Need working days rather than weeks? Use the business day calculator or the days between dates calculator. Every monthly calendar shows the ISO weeks it spans.