Week Number Calculator
Enter a date to see its ISO-8601 week number, the day of the week, the day of the year and how many weeks remain in the year.
ISO week number
Week 3 of 2026
Day of the week
Thursday
Week runs
12 January 2026 → 18 January 2026
Day of the year
15 of 365
Days remaining in the year
350
How this was worked out
- Step 1ISO weeks start on Monday and week 1 is the week containing the first Thursday.
- Step 215 January 2026 falls in week 3
ISO-8601 week numbering can put the first days of January in the previous year’s final week, and vice versa.
Formula
ISO week 1 is the week containing the first Thursday of the year
Example
Input
15 January 2026
Result
Week 3
Frequently asked questions
ISO weeks start on Monday, so the first few days of January can belong to the previous ISO year.
Most of Europe and many international standards. The US uses a different convention.
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