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Week Number Calculator

Find the ISO 8601 week number and week year for any date.

ISO week number for a date

How it works

  1. The ISO 8601 week number is computed directly from the calendar: weeks start on Monday, and week 1 is the week containing the year’s first Thursday.
  2. The week-year belongs to the year of the week’s Thursday, which is why a late-December date can be week 1 of the next year (2025-12-29 is 2026-W01) and an early-January date can belong to the previous year.
  3. Alongside the number, the tool shows the exact Monday–Sunday span of that ISO week and the ISO weekday (Monday = 1 … Sunday = 7).

Good to know

  • ISO 8601 is the international standard used by most software and planning tools; US-style week numbering (Sunday-first) differs and is not used here.
  • When the week-year differs from the calendar year, the note calls it out, preventing a common source of off-by-one-year reports.
  • The computation is exact across leap years and centuries (the shared calendar engine is unit-tested).
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Frequently asked questions

Which week numbering standard is used?
ISO 8601: weeks start on Monday; week 1 is the week containing the year’s first Thursday; and the week-year can differ from the calendar year at the boundaries.
Why does late December sometimes show week 1?
Because ISO weeks belong to the year of their Thursday. A Thursday in January pulls the surrounding week (including its late-December Monday) into the new year.
Do all countries use ISO weeks?
No: the US commonly numbers weeks from Sunday-based rules, and fiscal calendars differ. If your report needs a specific standard, this tool gives the ISO value.