US week numbering is a Sunday-start convention where Week 1 always contains 1 January. It is the default used by US payroll, US retail, and Microsoft Excel's WEEKNUM function.
How weeks are numbered
The US convention applies 2 fixed rules:
- Sunday is day 1 — each week runs Sunday through Saturday.
- Week 1 contains 1 January — the week that holds New Year's Day is numbered 1, even if it starts in the previous December.
Week 1 can therefore be a short week. If 1 January falls on a Wednesday, Week 1 covers Sunday 29 December through Saturday 4 January.
Difference from ISO 8601
US week numbering does not apply the first-Thursday rule. ISO 8601 requires Week 1 to be the week containing the year's first Thursday and starts weeks on Monday. The two systems can disagree on the week number for the same date by up to 1 week near January.
Where it appears
US payroll software, US retail reporting, and Excel's default WEEKNUM(date) and WEEKNUM(date, 1) calls all use this convention. Excel exposes ISO numbering separately as WEEKNUM(date, 21) or ISOWEEKNUM(date).