Get the ISO week number for any date

Find a calendar date’s ISO week index and week-based year.

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ToolWeek Number
Input
Output
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How it works

The date is shifted to Thursday of its ISO week, then counted from the Thursday in the week containing 4 January. Weeks begin Monday, and the week-year comes from the shifted Thursday rather than automatically from the date’s calendar year.

  • ISO rules are applied consistently.
  • Sunday-start locale calendars are a different convention.

Worked example

Get the ISO week number for any date
Find the ISO week number for a specific date
Input
											2025-03-15
Output
												Date: 2025-03-15
ISO Week: 2025-W11
Week Number: 11
Week Year: 2025
Day of Week: Saturday (6)

When to use this

Production schedules label work by ISO week, warehouse reports group Monday-through-Sunday shipments, and calendar integrations verify week-year boundaries.

Edge cases

  • 1 January can belong to week 52 or 53 of the preceding ISO week-year.
  • 29 December can belong to week 1 of the following ISO week-year.
  • A Sunday-start locale calendar produces different boundary weeks and is not the convention used here.

References