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What Week Number Is It?

Find the ISO 8601 week number for any date, along with its week year and the Monday–Sunday range it belongs to.

What is an ISO week number?

An ISO week number is the week-of-the-year under the ISO 8601 standard, where weeks start on Monday and week 1 of any year is the week containing the first Thursday. A year has 52 or 53 ISO weeks. Because of this rule, dates in early January or late December can belong to a different ISO week year than calendar year.

Calcadian uses ISO 8601 weeks. They start on Monday, and the ISO week year can differ from the calendar year around January.

Result

ISO week
Week 45
ISO week year
2026
Day of week
Monday
Week range
2026-11-02 → 2026-11-08

Monday, November 2, 2026 is in ISO week 45 of 2026 (2026-11-02 to 2026-11-08).

How to find the ISO week number for a date

  1. Pick any date.
  2. Read the ISO week number (1–53) from the result card.
  3. Check the ISO week year, this can differ from the calendar year near January.
  4. Use the week range (Monday–Sunday) for scheduling or sprint planning.

Example

January 1, 2024 falls in ISO week 1 of 2024. That ISO week runs from Monday, January 1, 2024 to Sunday, January 7, 2024.

December 31, 2023 falls in ISO week 52 of 2023.

ISO week numbering reference

Week numbering rules: ISO 8601 week date (Wikipedia).

Related tools

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Frequently asked questions

Which week system does Calcadian use?
ISO 8601 weeks. Weeks start on Monday, and week 1 of any year is the week containing the first Thursday.
Why does the ISO week year sometimes differ from the calendar year?
Late December dates can belong to ISO week 1 of the following year, and early January dates can belong to ISO week 52 or 53 of the previous year. The week year column makes this explicit.
How many ISO weeks are in a year?
Either 52 or 53. Years that start or end on a Thursday have 53 ISO weeks.
When does an ISO week start and end?
Monday to Sunday. The result shows both endpoints for the week containing your date.
Can I use this for project sprints?
Yes, ISO week numbers are widely used in scheduling, payroll periods, and project timelines.

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Calcadian calculators are for general planning and informational use. Please verify critical deadlines, legal, payroll, tax, HR, or official requirements with the relevant authorities or a qualified professional.