About the Week Number Calculator
What is the Week Number Calculator?
This week number calculator gives the ISO 8601 week used in business planning and payroll, and the US-style week number.
Early-January dates can belong to the previous year's last week – the ISO week-year shows this.
How to use the Week Number Calculator
- Date (sample: 22 Oct 2026)
- Press Find Week Number (results also refresh as you type). The result card shows the ISO Week Number, along with ISO week-year, ISO week code, Week runs and US week number (Sunday start).
Formula used
ISO week 1 = the week (Monday–Sunday) containing the first Thursday of the year
US week numbers start on Sunday, with week 1 containing 1 January.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Date | 22 Oct 2026 |
| Result | Value |
|---|---|
| ISO Week Number | 43 |
| ISO week-year | 2026 |
| ISO week code | 2026-W43-4 |
| Week runs | Mon, 19 Oct 2026 – Sun, 25 Oct 2026 |
| US week number (Sunday start) | 43 |
| Weeks in 2026 | 53 |
| Day of the year | 295 |
Tips for using the Week Number Calculator
- Use the ISO week code (2026-W40-2) in file names and reports for correct sorting.
- Years starting on a Thursday (or a Wednesday in leap years) have 53 ISO weeks.