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ISO Week Number Calculator
Enter a date to instantly find its ISO 8601 week number, corresponding ISO year and Monday-to-Sunday period.
- 100% local
- Instant
- Free
- No account
- Works offline
Enter a valid date.
ISO week
Week
ISO year
Period :
Why use this tool?
Find the ISO 8601 week number for a date, its ISO year and the exact Monday and Sunday that define the corresponding week.
Smart processing
Generate, convert, or validate technical data with purpose-built controls.
Supported formats
Technical formats and syntaxes suited to each generator, converter, or validator.
Save time
Get a clean, ready-to-use result in seconds without installing software or configuring a complex workflow.
Fonctionnement
How does this tool work?
The engine follows ISO 8601 rules: weeks start on Monday and week 1 is the week containing the first Thursday of the year, which is also the week containing January 4. From the selected date it determines the ISO week number and ISO year, then calculates the corresponding Monday and Sunday. This method correctly handles dates around New Year that may belong to the neighboring ISO year even when their calendar year is different.
Use cases
Planning
Use this calculation to prepare a deadline or schedule with a value that can be checked immediately.
Verification
Check a result produced elsewhere and identify an input or method mistake more easily before using it.
Everyday use
Get an immediate answer for a common timing need without building a spreadsheet or repeating manual arithmetic.
Guide
How to use this tool
- 1
Enter the values
Provide the dates, times or durations requested by the tool.
- 2
Adjust the options
Choose the units or parameters that match your situation.
- 3
Review the result
The result recalculates automatically whenever an input changes.
Tips and best practices
- Check the entered values before using the result in a document, schedule or decision.
- The calculation runs locally in your browser and your data is not sent to a server.
- Reset the tool whenever you want to start a new scenario with clean inputs.
Frequently asked questions
Is the calculation automatic?
Yes. The result updates as soon as the required values are valid, without sending your data to a server.
Does my data stay local?
Yes. The entered information is processed directly in your browser by the shared Dates & time engine.
Can I change values after a calculation?
Yes. Change any input at any time and the result is immediately recalculated using the new values.
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