Week Number Calculator
Find the ISO 8601 week number for any date, plus the day of year and the date range of that week. Useful for planning and scheduling. Free, private, instant.
Last reviewed by the Radiatus Cloud team
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Find the week number for any date
Businesses and schedules often refer to the ISO week number, and knowing which week a date falls in is a common need. This calculator finds the ISO 8601 week number for any date, plus the day of year and more.
Why ISO week numbers are specific
The ISO 8601 standard defines week numbering precisely: weeks start on Monday, and week one is the week containing the year’s first Thursday, which means early January can fall in the last week of the previous year and vice versa. This precision matters because businesses, especially in Europe, schedule and report by week number, and a casual count gets the boundaries wrong. The calculator applies the ISO rules correctly, so the week number matches what standardised systems use.
A calculation, not advice
It runs entirely in your browser, so nothing you enter is uploaded and it works without an account, keeping your figures on your own machine.
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Frequently Asked Questions
What is the ISO 8601 week number?
A standardised week numbering where weeks start on Monday and week one contains the year’s first Thursday, used widely in business and scheduling.
Why can early January be in the previous year’s last week?
Because ISO week one is defined by the first Thursday, so days in early January before it belong to the final week of the previous year.
Why do businesses use week numbers?
Because they give a compact, standard way to refer to a period, common in scheduling and reporting, especially in Europe.
What else does it show?
The day of year and related date information alongside the ISO week number.
Is my data uploaded?
No. The calculation runs entirely in your browser.
Privacy & Security
All processing happens locally in your browser — nothing is uploaded.
How to Use
Pick a date to see its ISO week number, day of year and week range.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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