Week Number Calculator
Find the ISO week number for any date.
This week
Week 42 of 2026
Monday, October 12 – Sunday, October 18, 2026
Day 285 of 365 · 53 weeks in 2026
The weeks either side
- Week 40Sep 28 – Oct 4
- Week 41Oct 5 – 11
- Week 42NowOct 12 – 18
- Week 43Oct 19 – 25
- Week 44Oct 26 – Nov 1
- Week 45Nov 2 – 8
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Frequently Asked Questions
What is the ISO week number?
The ISO week number is defined by ISO 8601. Weeks start on Monday, and week 1 is the week containing the first Thursday of the year.
Can the ISO year differ from the calendar year?
Yes, dates in late December may belong to ISO week 1 of the next year, and dates in early January may belong to the last ISO week of the previous year.