Week Number Calculator
Find the ISO week number for any date.
Runs in your browser. Nothing is uploaded — your input is processed on this device and discarded when you close the tab.
Week Number Calculator
How to use this tool
- Enter the values in the fields above.
- Press Generate to see the result.
- Use Load sample to try the worked example, Copy result to copy it, or Clear to start again.
Worked example
5 January 2026 is a Monday in the second ISO week of 2026, because ISO week 1 of 2026 runs from 29 December 2025 to 4 January 2026, so the next Monday starts week 2.
What this tool does not do / Limits
This uses the ISO week system, where weeks start on Monday and week 1 contains the first Thursday. Other week-numbering customs can give a different number for dates near New Year.
Frequently asked questions
Why can the ISO year differ from the calendar year?
Days in late December can belong to week 1 of the next ISO year, and early January days can belong to the last week of the previous ISO year.
Does the week start on Monday?
Yes, in the ISO system every week runs Monday to Sunday.
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