The ISO 8601 system defines weeks to prevent ambiguity. Weeks start on Monday, and week 1 of a year is the one containing the first Thursday of the year. This means some years can have 53 weeks. You can use our Week Number Calculator to find the ISO week for any date.
Why ISO week numbers matter
In international business, finance, and logistics, referencing a week by number eliminates confusion caused by different calendar systems and date formats. A project manager in Germany saying "deliver by Week 23" is unambiguous — whereas "deliver by the week of June 1st" could be interpreted differently depending on whether you count weeks starting Sunday or Monday.
How ISO week numbering works
Under ISO 8601, Monday is always the first day of the week. Week 1 of a year is the week that contains the first Thursday of January (equivalently, the week containing January 4th). This means that December 29-31 can sometimes fall in Week 1 of the next year, and January 1-3 can sometimes fall in Week 52 or 53 of the previous year. For example, January 1, 2026 is a Thursday — so it falls in Week 1 of 2026. But January 1, 2027 is a Friday — so it falls in Week 53 of 2026.
Which countries use ISO week numbers?
Most of continental Europe, Scandinavia, and much of Asia uses ISO week numbers in business and government contexts. The US, Canada, and UK commonly use the "Sunday-start" week numbering system for business calendars, though ISO weeks are standard in international contexts. Many project management tools (Jira, Asana, Monday.com) support both systems.