What does this tool do?
Week numbering is not universal: ISO weeks start on Monday and are tied to the first Thursday, while US weeks start on Sunday with week 1 beginning on 1 January.
The Formula
US week = floor((day of year − 1 + weekday of 1 January) ÷ 7) + 1Frequently Asked Questions
Why do the ISO and US week numbers differ?
They use different first days and a different rule for week 1. ISO uses Monday and the week containing 4 January, while US numbering starts on Sunday and treats 1 January as part of week 1.
Which week number is used for payroll or contracts?
It depends on the jurisdiction and the agreement. ISO weeks are common in Europe and in scheduling standards, while US calendars often use Sunday-based weeks.