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Reference

What Is an ISO Week Number?

ISO 8601 defines a consistent way to number weeks across calendars. It's used in business, logistics, and software wherever week-based scheduling matters.

Step-by-step

  1. Week 1 is the week containing the first Thursday of the year.
  2. Weeks start on Monday and end on Sunday.
  3. Years usually have 52 ISO weeks. They have 53 when January 1 falls on a Thursday, or when it falls on a Wednesday in a leap year.
  4. Late December dates can belong to the ISO week of the following year, and early January dates can belong to the previous year — the year is reported alongside the week (e.g., 2026-W01).

Examples

  • 2026: 53 ISO weeks. January 1, 2026 falls on Thursday — Thursday → that week is week 1.
  • 2027: 52 ISO weeks. January 1 is a Friday, so it still belongs to week 53 of 2026 (since the first Thursday of 2027 is January 7).

Use our week number tool to look up any date.

Where ISO weeks are used

Germany, the Nordic countries, Switzerland, and Austria use ISO weeks extensively in everyday life. In the US, week numbers are less common — Sunday-starting weeks (the "Broadcast Calendar") are sometimes used in media instead.

Related guides

Frequently asked questions

Is week 1 always January?
It's the week containing the first Thursday of January — so it usually starts late December or in the first week of January.
Is there a year with 53 weeks?
Yes — about every 5–6 years. The next 53-week years are 2026 and 2032.