Week Number Calculator
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ISO 8601 week number for any date — week 1 is the week with the year's first Thursday.
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ISO week
Week 31, 2026
ISO 8601 week number for 2026-07-31
- Week number
- 31
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ISO week numbers
ISO 8601 defines week numbers so that week 1 is the week containing the first Thursday of the year — which means early January dates can belong to the previous year's week 52 or 53, and late December dates can be week 1 of the next year.
ISO weeks start on Monday. This is the standard used in Europe for business reporting, manufacturing schedules, and payroll periods. US calendars often use Sunday-start weeks without ISO numbering — do not mix conventions in the same report.
Example: July 31, 2026 falls in ISO week 31 of 2026. January 1, 2026 is in week 1 of 2026 because that week contains the first Thursday (January 1, 2026 is a Thursday).
Common questions
Why can January dates show the previous year's week?
If January 1–3 fall before the first Thursday, ISO assigns them to the last week of the previous year — keeping every week aligned to Monday–Sunday boundaries.
Is this the same as US week numbers?
US informal "week of the year" counts often start on Sunday and use January 1 as week 1 regardless of weekday. ISO week numbers differ near year boundaries.