Week Number Calculator
ISO-8601 week number and ISO year. Weeks start Monday. Week 1 contains 4 January.
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How to use Week Number Calculator
- Enter date. Leave a field blank only when the page says the value is missing, not by typing 0 unless 0 is a real value.
- Read the result rows under the form. Errors appear in the status line. Nothing is uploaded.
- Use today fills the as-of or start date using this browser’s local calendar.
Week Number Calculator
ISO week
| Computes | ISO-8601 week number and ISO year. Weeks start Monday. Week 1 contains 4 January. |
|---|---|
| Includes | ISO week |
| Includes | ISO year |
| Engine | dateDiff |
| Upload | None |
| Advice | Arithmetic only |
What Week Number Calculator is
1 January 2026 is ISO week 1 of 2026 (Thursday). A date can belong to the previous or next ISO year near 1 January.
The formula runs in this tab. CZNull does not receive the numbers.
How this page computes
Weekday names are also on day of week. A full year table is year calendar.
Entities and related jobs
ISO-8601 week number and ISO year. Weeks start Monday. Week 1 contains 4 January. Named features: ISO week, ISO year.
See also Day of Week Calculator, Year Calendar, Date Difference Calculator, Age Calculator, Add or Subtract Days, Business Days Calculator.
Use cases
Payroll weeks, sprint numbering, and ISO date stamps like 2026-W01-4.
Limits
Invalid input appears in the status line. IEEE floating point applies unless a row says otherwise. CZNull does not receive the numbers.