US week numbering uses Sunday-start weeks, with week 1 always containing 1 January. It is the default in Microsoft Excel's WEEKNUM function and the basis for US payroll, retail, and many corporate calendars.
What is US week numbering?
US week numbering is the Sunday-start convention used across United States business systems. Each week runs from Sunday through Saturday. Week 1 is the week that contains 1 January, regardless of how many days from the new year fall inside it.
This convention is older than the ISO 8601 standard. It is built into Microsoft Excel as the default WEEKNUM mode and into most US payroll, retail, and broadcast systems. It dominates US commercial software the way ISO 8601 dominates international software.
Because the rule is "week 1 contains 1 January," every US year has at least 53 numbered weeks. Leap years where 1 January is a Saturday extend to 54 weeks.
How US week numbers are calculated
The rule is simple. Find the Sunday on or before 1 January of the target year. That Sunday starts week 1. Every following Sunday starts the next numbered week. The week number of any date is the count of Sunday-starts from that first week up to and including the week containing the date.
This is mathematically equivalent to Microsoft Excel's WEEKNUM(date, 1) formula. Excel documents this behaviour at the official WEEKNUM function reference.
For the step-by-step algorithm and code, see How US Week Numbers Are Calculated.
Examples
Three concrete dates show the rule at work.
- 1 January 2027 is a Friday, so it falls in US week 1 of 2027.
- 28 May 2026 is a Thursday, which falls in US week 22 of 2026.
- 31 December 2026 is a Thursday, which falls in US week 53 of 2026.
The same date can sit in a different week under ISO 8601, because ISO requires four days of the new year to be in week 1.
US vs ISO 8601
The two standards disagree on two basic questions: which day starts the week, and how to assign week 1.
| Feature | US | ISO 8601 |
|---|---|---|
| First day of week | Sunday | Monday |
| Week 1 rule | Contains 1 January | Contains first Thursday |
| Minimum days in week 1 | 1 | 4 |
| Excel function | WEEKNUM(date, 1) | ISOWEEKNUM(date) |
| Primary users | US payroll, retail, Excel | International software, EU business |
For 1 January 2022, a Saturday, US returns week 1 of 2022, while ISO 8601 returns week 52 of 2021.
Where US weeks are used
The Sunday-start convention is the default in several major US systems.
- US payroll. Biweekly cycles in providers such as ADP and Paychex default to Sunday–Saturday weeks.
- Microsoft Excel.
WEEKNUM(date)andWEEKNUM(date, 1)return Sunday-start week numbers. - NRF 4-5-4 retail calendar. The National Retail Federation calendar uses Sunday-start weeks, with its own fiscal-year rule.
- US broadcast calendar. Broadcast accounting uses week numbers tied to Monday-starting weeks, which is a distinct convention from this one.
- US corporate reporting. Many US head-office calendars assume Sunday-start weeks for internal reports.
Note that the US broadcast calendar starts its weeks on Monday, not Sunday. It shares the country but not the convention.
Related terms
Compare with other standards
ISO 8601 Week Numbering uses Monday-start weeks and the first-Thursday rule. It is the international default and the right choice for cross-border data exchange.
Australian Financial Year Week Numbering numbers weeks against the Australian financial year, which runs from 1 July to 30 June. It is used by the Australian Taxation Office and Australian government reporting.
FAQ
How is US week numbering different from ISO 8601? US weeks start on Sunday and put 1 January in week 1. ISO 8601 weeks start on Monday and put the first Thursday of the year in week 1. The two often disagree near year boundaries.
What does Excel's WEEKNUM function return by default?
WEEKNUM(date) and WEEKNUM(date, 1) return the US Sunday-start week number. To get the ISO 8601 week number in Excel, use =ISOWEEKNUM(date) or =WEEKNUM(date, 21) instead.
Do all US businesses use the same week numbering? No. US payroll, Excel, and most office tools default to Sunday-start weeks. The US broadcast calendar uses Monday-start weeks. The NRF retail calendar uses Sunday-start weeks but a fiscal-year offset.
When does week 53 occur in the US system? Every US year has at least 53 weeks, because 365 days span 53 Sunday-start weeks once the partial first week is counted. Leap years where 1 January is a Saturday extend to 54 weeks.
For the step-by-step algorithm and code, see How US Week Numbers Are Calculated.
Articles in this section
- How US Week Numbers Are Calculated — How to calculate US week numbers with examples for Excel WEEKNUM type 1 and 2, and code snippets in JavaScript and Python.