This week of year calculator finds the week number for any date you choose. It shows both the ISO week number used across Europe and most of the world, and the US week number used in many American calendars and spreadsheets.
For example, September 30, 2026 falls in ISO week 40 of 2026, a week that runs from September 28, 2026 to October 4, 2026.
ISO Week Numbers
The ISO 8601 standard counts weeks starting on Monday. Week 1 is the week that contains the first Thursday of the year, which is the same as the week holding January 4. Because of this rule, the first few days of January can belong to the last week of the previous year, and late December days can belong to week 1 of the next year.
US Week Numbers
The common US method counts week 1 as the week containing January 1, with weeks beginning on Sunday. This is simpler but it does not match the ISO standard, so the two numbers can differ by one near the start and end of the year.
What People Use It For
- Planning around fiscal and payroll week numbers
- Scheduling production and delivery by week
- Matching week numbers used in spreadsheets and project tools
- Coordinating teams that use ISO weeks
Common Questions
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