Week-number guide
US-style week numbers
The US-style comparison on week-numbers.com uses Sunday-start weeks and counts the week containing January 1 as Week 1.
The week begins on Sunday.
It ends on Saturday.
Week 1 contains January 1.
The displayed week-year follows the calendar year.
Why can it differ from ISO?
ISO starts a new week on Monday, while the US-style system starts on Sunday. A Sunday can therefore already belong to the next US-style week while still being the last day of the current ISO week. The Week 1 rules also differ around New Year.
Example
Sunday, May 3, 2026 starts US-style Week 19, while the same date is still the final day of ISO Week 18.
Important terminology
There is no single universal “US week number” standard used in every American application. This site labels its comparison clearly as a simplified US-style Sunday-start system so the rule is transparent.
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