Find the ISO 8601 week number for any date — the standard used in business and software.
ISO 8601 numbers the weeks of the year 1–52 (sometimes 53). Weeks start on Monday, and week 1 is the week containing the year's first Thursday.
Because of the first-Thursday rule, the first days of January can belong to the last week of the previous year, and late December can fall in week 1 of the next year.
In business, manufacturing, payroll and software across Europe and in standards worldwide — e.g. ‘2026-W25’.