About the ISO Week Calendar Calculator
What is the ISO Week Calendar Calculator?
Planning documents often say "week 40" without dates. This iso week calendar turns a week number back into the seven dates it covers.
It also shows when week 1 of the year starts.
How to use the ISO Week Calendar Calculator
- ISO Year (sample: 2,026)
- Week Number (sample: 43)
- Press Show ISO Week (results also refresh as you type). The result card shows the Week Runs, along with Weeks in 2026, Month(s) and Week 1 starts.
Formula used
Week 1 Monday = Monday on or before 4 January; Week n Monday = Week 1 Monday + 7(n − 1) days
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| ISO Year | 2,026 |
| Week Number | 43 |
| Result | Value |
|---|---|
| Week Runs | Mon, 19 Oct 2026 – Sun, 25 Oct 2026 |
| Weeks in 2026 | 53 |
| Month(s) | October 2026 |
| Week 1 starts | Mon, 29 Dec 2025 |
Tips for using the ISO Week Calendar Calculator
- Week 1 can start in late December of the previous calendar year.
- Check whether your organisation uses ISO weeks or its own fiscal weeks.