ISO Week Number Calculator
Find a date’s ISO week number and ISO week-year using Monday-based weeks.
Free · No signup · No watermark · On-device processing
Use the tool
How to use ISO Week Number Calculator
- Choose a real calendar date.
- Run to calculate its ISO week-year and week number.
- Check the ISO year near New Year; it may differ from the date year.
How this tool handles your input
ISO week 1 contains the year’s first Thursday. The ISO week-year can differ from the calendar year near New Year. Calculation uses UTC calendar dates.
This is ISO week numbering, not a Sunday-start regional calendar or a fiscal week system.
Example: 2021-01-01 → 2020-W53
Privacy and browser support
Your input is handled in this browser tab. Selected files and entered text are not sent to Varento Tools by this tool. Some specialized libraries load only when needed. A browser with JavaScript and sufficient device memory is required. Read our local-processing explanation.
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Frequently asked questions
Is this tool free without daily usage limits?
Yes. Varento Tools has no signup, accounts, usage-count caps, paywalls or watermarks. Device memory, format support and browser restrictions can still limit technically demanding files.
What input and output rules apply?
ISO week 1 contains the year’s first Thursday. The ISO week-year can differ from the calendar year near New Year. Calculation uses UTC calendar dates.
What are the limitations?
This is ISO week numbering, not a Sunday-start regional calendar or a fiscal week system.
Is my input uploaded?
No. This tool processes your input in the current browser tab, without a processing API or account.