Week Number Calculator
Find the ISO week number for any date and look up all dates in any week of the year. Free, instant, runs entirely in your browser.
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What is the ISO Week Number?
ISO 8601 defines a standard week numbering system used widely in Europe and business contexts. Weeks start on Monday, and the first week of the year (Week 1) is the week that contains the year's first Thursday. This means some years have 53 weeks, and early January dates can sometimes fall in Week 52 or 53 of the previous year.
Week Numbering Rules
| Rule | Detail |
|---|---|
| Week starts | Monday |
| Week 1 | Contains the year's first Thursday |
| Weeks per year | 52 or 53 |
| Standard | ISO 8601 |
| 53-week years | When Jan 1 falls on Thu, or Wed in a leap year |
Common Uses
Week numbers are used in business reporting, project management, payroll cycles, manufacturing schedules and delivery tracking. Many European countries print week numbers on calendars by default.