Week Number Calculator

Two calculators: date → week number, and week number → dates. Both use the ISO 8601 standard, where weeks start on Monday.

Week number of a date

Enter your values and select Calculate to see the result.

Dates in a week

Week 1 of 2026Monday, December 29, 2025 – Sunday, January 4, 2026
Monday (ISO)
2025-12-29
Sunday (ISO)
2026-01-04

The ISO 8601 rule

Weeks run Monday to Sunday. Week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January. As a result, 1–3 January can belong to the last week of the previous year, and 29–31 December can belong to week 1 of the next year. A year has 52 or 53 ISO weeks.

Worked example

Week 1 of 2026

  1. 1 January 2026 is a Thursday, so it is in week 1
  2. Week 1 starts on the Monday before: 29 December 2025

Week 1 of 2026 runs from Monday 29 December 2025 to Sunday 4 January 2026.