2026-03-20 · 5 min read
What Is ISO Week Numbering?
A clear explanation of ISO-8601 week rules, why week 1 can start in the prior year, and how to avoid reporting errors.
ISO week numbering is the global standard used in logistics, operations, and finance reporting. It is defined by ISO-8601 and keeps week labels consistent across countries.
Under ISO rules, weeks start on Monday. Week 1 is the week that contains the first Thursday of the year, which is equivalent to the week containing January 4.
That definition means late December dates can belong to week 1 of the next ISO year, and early January dates can still belong to the last ISO week of the previous year.
If your dashboards mix calendar-year logic with ISO-week logic, trends can look broken at year boundaries. Always store both the calendar date and ISO year-week label.
A week-number calculator is a fast way to verify unusual boundary dates and align analytics, planning, and payroll reports.