ISO Week Number Calculator

ISO 8601 week numbering exists to give every date a position in an unbroken sequence of complete seven-day blocks. Weeks run Monday (day 1) through Sunday (day 7), and week 01 is defined as the week containing the first Thursday of January. Four equivalent statements of the same rule are in common use: the week containing January 4, the week containing the year's first Thursday, the first week with at least four days in the new year, and the week whose Monday falls between December 29 and January 4. Anchoring on Thursday, the midpoint of the week, is what guarantees the majority-of-days property. The written form is YYYY-Www-D, so 2025-W11-6 denotes the Saturday of week 11 in ISO year 2025. The practical trap is the week-numbering year, which is not the calendar year. Up to three days at either end of January and December belong to the neighbouring ISO year: January 1, 2023 falls in 2022-W52, and December 31, 2019 falls in 2020-W01. Pairing an ISO week number with a calendar year is therefore a common source of off-by-one and off-by-52 errors in reporting systems. Year length is 52 weeks (364 days) or 53 weeks (371 days); a long year occurs when January 1 is a Thursday, or a Wednesday in a leap year, which happens 71 times in each 400-year cycle, about 17.75 percent of years. Note also that ISO weeks never align with month or quarter boundaries, so week-based and month-based reporting periods cannot be reconciled exactly.

Week numbering became necessary when industry needed a scheduling unit finer than a month but insensitive to the irregular lengths of months. Continental European practice adopted it well before international standardisation: Germany codified week numbering in DIN 1355, and week numbers have long been printed by default on Scandinavian, German and Swiss calendars, where a delivery quoted for KW 37 or vecka 37 is entirely routine. The international rule was first published by the International Organization for Standardization as ISO 2015 in 1976, which defined the Monday start and the first-Thursday rule. That standard was absorbed into the consolidated date and time standard ISO 8601 in 1988, which was revised in 2000 and 2004 and then split in 2019 into ISO 8601-1, covering basic rules, and ISO 8601-2, covering extensions. The specification has since been mirrored into ordinary computing infrastructure, appearing in the strftime %V and %G codes of the C library and POSIX, in the ISOWEEKNUM function added to Microsoft Excel in 2013, and in the ISO week functions of PostgreSQL, Java, Python and JavaScript date libraries. Adoption has been slower in the United States, where broadcast and retail calendars often use their own week conventions with weeks starting on Sunday or Monday and the year divided into 4-4-5 blocks, which is why cross-border reporting still requires care about which week definition is in force.