Week Number Calculator
Find the ISO 8601 week number for any date.
That date is in week 3 of 2026 (ISO 8601).
About this tool
This tool calculates the ISO 8601 week number of a date — the international standard used across Europe and by most global businesses for scheduling, reporting and planning.
Under ISO 8601, weeks run Monday to Sunday, and week 1 of any year is defined as the week that contains that year's first Thursday. This rule (rather than simply starting week 1 on January 1) ensures every ISO week is fully contained within a single year in the vast majority of cases, and that week numbers stay consistent for comparing the same period year over year.
Week numbers are especially common in manufacturing, logistics, payroll and project management, where referring to "week 34" is more consistent for planning than a date range that shifts every year.
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Frequently asked questions
- How is the week number calculated?
- This tool uses the ISO 8601 standard, where week 1 of a year is defined as the week containing that year's first Thursday, and weeks always start on Monday and end on Sunday.
- Why does the ISO week year sometimes differ from the calendar year?
- Dates near the very start or end of a calendar year can belong to a week that ISO 8601 officially counts as part of the previous or next year — for example, December 31 sometimes falls in "week 1" of the following year.
- Why use week numbers instead of just a date?
- Week numbers are widely used in business and project planning (payroll cycles, manufacturing schedules, sprint planning) because they make it easy to talk about "week 12" as a single unit, rather than a range of dates that shifts every year.
- Do all countries use the same week-numbering system?
- No. ISO 8601 (Monday-start weeks, first week contains the year's first Thursday) is the international and European standard, but the United States commonly uses a different convention where week 1 simply starts on January 1, which can produce a different week number for the same date.