Week Number Calculator
Calculate the week number for any date and find the corresponding week, year, and date range using standard calendar week rules.
Understanding Week Numbers
A week number (or week of the year) is a standardized calendar designation that identifies a specific 7-day period within a year. Because years contain approximately 52.14 weeks, managing recurring weekly schedules, payroll, or business reporting requires a rigid mathematical standard to avoid misalignment.
By using standardized week numbers, teams across different countries and timezones can accurately coordinate milestones without needing to cross-reference exact dates or deal with shifting month boundaries.
The ISO 8601 Week Standard
Our calculator uses the globally recognized ISO 8601 standard to determine week numbers. This standard defines strict rules for handling edge cases at the beginning and end of the year:
The First Week
ISO Week 1 is defined as the week that contains the first Thursday of the year. This ensures that Week 1 always has at least 4 days (a majority) falling in the new year.
Start of the Week
Under ISO 8601, every week officially begins on a Monday and concludes on a Sunday, providing a consistent structure for business operations.
Year Boundary Surprises
Because weeks always start on a Monday, the ISO Week Year can sometimes drift away from the standard calendar year. It is highly common for dates around New Year's Eve and New Year's Day to exhibit unexpected week numbers:
- Late December: If December 31st falls on a Monday, Tuesday, or Wednesday, it actually belongs to Week 1 of the next ISO year.
- Early January: If January 1st falls on a Friday, Saturday, or Sunday, it still belongs to Week 52 or 53 of the previous ISO year.
- 53-Week Years: Any calendar year that starts on a Thursday (or a leap year starting on a Wednesday) will contain 53 full weeks.
Frequently Asked Questions
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