What Is an ISO Week Number?
An ISO week number identifies a week of the year under the ISO 8601 international standard. Weeks start on Monday, and week 1 is the week that contains the year's first Thursday. As a result, a year has 52 or 53 ISO weeks, and the first days of January can belong to the last week of the previous year.
ISO vs. US Week Numbers
The common US convention starts weeks on Sunday and counts the week containing January 1 as week 1, so the same date can have different week numbers in the two systems.
Notation
ISO weeks are written as year-W-week, for example 2026-W39.
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🔗 Related Terms
More Time & Dates Terms
- 24-Hour Clock (Military Time)
- Business Days
- Calendar Quarter
- Chronological Age
- Day of the Year
- Daylight Saving Time (DST)
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Last updated: September 27, 2026. This definition is for general educational purposes only. It is not financial, investment, tax, legal, or medical advice, and it does not recommend any product or course of action. For decisions about your own situation, consult a qualified professional.