Week of year is the ordinal position of a week within a year. The numeric value depends on which week-numbering standard is in use.
Ranges by standard
The 3 standards used on this site produce different ranges:
- ISO 8601 — weeks 1 to 52 or 53. Week 1 contains the first Thursday of the calendar year.
- US Week Numbering — weeks 1 to 54. Week 1 contains 1 January regardless of weekday.
- Australian Financial Year — weeks 1 to 52 or 53. Week 1 starts 1 July.
The same calendar date typically produces 3 different week-of-year values, one per standard. For example, 1 January 2026 is ISO week 1 of 2026, US week 1 of 2026, and AU FY week 27 of FY2026.
Choosing a standard
The correct standard depends on jurisdiction and use case. International software and European business use ISO. US retail and broadcast use the US convention, often with NRF 4-5-4 variants. Australian tax and statutory reporting use the financial year.