Week Number Calculator (ISO 8601)
Find which week of the year a date falls in, or the start and end of any week number.
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Week Number Calculator (ISO 8601) by TheCalcTools
How the Week Number Calculator (ISO 8601) works
ISO 8601 weeks start on Monday. Week 1 is the week that contains the year’s first Thursday (equivalently, the week with 4 January). A year has 52 or 53 weeks.
Because of this rule, 1 January can belong to the last week of the previous year, and 31 December can belong to week 1 of the next year. The ISO year shown may then differ from the calendar year.
The formula
Worked example
Using the default values shown in the calculator:
| I want to | Find the week number of a date |
|---|---|
| Date | 2026-10-17 |
| ISO year | 2027 |
| Week number | 23 |
- 17 October 2026 is a Saturday in Week 42 of 2026.
- The week runs from Monday, 12 October 2026 to 18 October 2026.
Result:
| Result | Week 42 of 2026 |
|---|---|
| Week starts (Monday) | Monday, 12 October 2026 |
| Week ends (Sunday) | 18 October 2026 |
| Weeks in that ISO year | 53 |
| Check | Valid week |
Frequently asked questions
What week number is it today?
Open the calculator: the date defaults to today.
Do the US and ISO week numbers differ?
Sometimes. Some US calendars start weeks on Sunday and call the week with 1 January week 1. This calculator uses the ISO 8601 standard used in Europe and most business software.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.