Week Number Calculator
Find the ISO week number for any date, plus the U.S. (Sunday-start) week number, the week’s Monday and Sunday, and the day of the year.
Result
ISO week number
43
- ISO week-numbering year
- 2,026
- ISO week
- 2026-W43
- U.S. week number (weeks start Sunday)
- 44
- Week starts (Monday)
- Mon, Oct 19, 2026
- Week ends (Sunday)
- Sun, Oct 25, 2026
- Day of the year
- 298
Common questions
How are ISO week numbers counted?
ISO 8601 weeks start on Monday, and week 1 is the week with the year’s first Thursday in it. So January 1 to 3 can belong to the last week of the previous year, and December 29 to 31 can be week 1 of the next.