How week numbers work
Many businesses, factories and project teams — especially in Europe — refer to weeks by number instead of by date ("we ship in week 32"). This calculator returns the ISO 8601 week number for any date you pick, so you always know which calendar week you are in and exactly which days it covers.
The ISO 8601 rule
Under ISO 8601, weeks start on Monday and end on Sunday. Week 1 is the week containing the year's first Thursday — equivalently, the week that contains January 4. Because of this rule, the first days of January can belong to the last week of the previous year, and late-December days can fall into week 1 of the next year. A year has either 52 or 53 weeks.
Common uses
- Coordinating production and delivery schedules by week
- Planning sprints and reporting periods
- Reading European calendars that show week numbers
- Tracking how far through the year you are
Frequently asked questions
What week number is it right now?
With today's date in the field, the big number is the current ISO week. Click "Use today" any time to jump back to it.
Why can a year have 53 weeks?
A year has 53 ISO weeks when January 1 or December 31 falls on a Thursday (or on a Wednesday in a leap year). Otherwise it has 52.
Does the week start on Sunday or Monday?
ISO 8601 weeks always start on Monday. That is the standard used across most of Europe and in international business.