Week Number Finder

ISO 8601 is the international standard for week numbering: weeks start on Monday and week 1 is the week containing January 4 of the calendar year. This tool implements an established algorithm that converts the input date to a Julian Day Number (JDN), derives weekday and day-of-year, then applies the ISO formula to compute the week number.

The US-style method treats Sunday as the first day of the week and designates the week containing January 1 as week 1. The simple Jan 1 method divides the year into consecutive 7-day blocks beginning on January 1. The custom method shifts the start-of-week and first-week alignment per user selection.

All calculations use proleptic Gregorian rules (Gregorian leap-year rules applied for all years) and integer arithmetic to avoid rounding issues. Boundary cases (for example, ISO week 1 that begins in the previous calendar year) are handled according to the chosen method; see the FAQ for specifics.

Select the method that matches your reporting or scheduling needs: ISO 8601 for international consistency, US-style for Sunday-based calendars, simple Jan 1 for the simplest partitioning, or Custom to match an organizational convention.

All computations use standard Gregorian calendar arithmetic and JDN conversions. Review boundary-case behavior before using results in compliance-sensitive contexts.