Find the ISO week number for any date — free and instant.
The ISO 8601 week-numbering system divides the year into 52 or 53 numbered weeks, each running Monday through Sunday. Week 1 is defined as the week containing the year's first Thursday — equivalently, the week containing January 4th — which means the last few days of December can belong to week 1 of the *following* year, and the first few days of January can belong to the *last* week of the *previous* year.
This matters for anything scheduled or reported "by week number" — payroll cycles, project sprints, manufacturing batch codes, and many business calendars all use ISO week numbers rather than calendar months.
Because ISO weeks are defined by which year contains that week’s Thursday, not by the calendar month — if December 29-31 fall in a week whose Thursday is already in January, that week belongs to the new year.
No — most years have 52 weeks, but years where January 1st falls on a Thursday (or, in a leap year, a Wednesday) have 53.
No — that naive approach doesn’t account for which day of the week January 1st falls on, and gives wrong results for most years. The ISO definition specifically anchors week 1 to the first Thursday.