ISO Week Calculator

An ISO week number is a standardized way of counting weeks within a year, defined by the ISO 8601 international standard. Every year contains 52 or 53 ISO weeks. ISO weeks always start on Monday and end on Sunday. ISO Week 1 is defined as the week containing the year's first Thursday — equivalently, it is the week containing January 4, or the first week that has the majority of its days in the new year.

Because of this definition, ISO week years can diverge from calendar years. The last few days of December may belong to ISO Week 1 of the next year, and the first few days of January may belong to ISO Week 52 or 53 of the previous year. This is why ISO 8601 defines both a calendar year and an ISO week year (ISOYEAR), which may differ from the Gregorian year.

ISO week numbers are widely used in European business, manufacturing, and logistics for weekly reporting. Rather than saying "the week of June 7," a European company would say "CW24" (calendar week 24) or "Week 24 of 2026." Many project management tools, ERP systems, and factory scheduling systems display ISO weeks natively.

The ISO week number is computed by first finding the Thursday of the current week, then counting how many weeks have elapsed since the first Thursday of the year.

The key property that makes ISO weeks special is the Thursday rule — it ensures that no week straddles two different year boundaries in a way that would split more than 3 days. Here are the edge cases:

Always pair the week number with the ISO year (not calendar year) to avoid ambiguity near year boundaries.

1. Date to ISO Week: Enter any date in the date picker. The calculator instantly shows the ISO week number, ISO year, day of the week (ISO 1–7), the week's Monday–Sunday date range, total ISO weeks in that year, and the ISO week date string.
2. ISO Week to Date: Enter an ISO year and week number to see the corresponding Monday–Sunday date range. This is useful when a report references "Week 24" and you need to know the calendar dates it covers.

ISO week numbers are the reporting standard for manufacturing, logistics, and supply chain in Europe and many international organizations. Production reports, delivery schedules, and inventory counts are all indexed by week number rather than month, because weeks have a fixed 7-day length (unlike months with 28–31 days) that makes period-over-period comparisons reliable.

Retailers and consumer goods companies — particularly those using the Nielsen or Kantar retail measurement services — receive data indexed by ISO week. Marketing agencies, brand managers, and analysts working with these data sources need to translate ISO week numbers to calendar dates to understand seasonal trends and plan promotions.

Software development teams using agile methodologies sometimes organize sprints by ISO week (e.g., Sprint W24–W25). Project management software like Microsoft Project and Jira can display timelines in ISO week view. Knowing which calendar dates correspond to a given sprint week is a common coordination need.