← All toolsDate & Time›Date Math
Week Number Calculator (ISO)
Find the ISO 8601 week number for a date, or look up the Monday-to-Sunday dates of any week number.
Tip from Tink: Many European workplaces plan by week number.
ISO week number
Week 1
Week year
2026
Week runs
Mon, December 29, 2025 – Sun, January 4, 2026
Find the dates of a week
Monday
Mon, December 29, 2025
Sunday
Sun, January 4, 2026
ISO weeks start on Monday, and week 1 is the week with the year's first Thursday. Early January can belong to the last week of the year before.
Advertisement
How to use the Week Number Calculator (ISO)
- Pick a date to see its week number.
- Or enter a year and week to see its dates.
Everything runs in your browser. Nothing you enter is uploaded.