Week Number Calculator – Find the Week of the Year
Find the ISO week number and week dates for any date.
About the Week Number Calculator
What is the Week Number Calculator?
A week number calculator returns the ISO week number for a date and shows the Monday-to-Sunday range of that week.
Why use this calculator?
Businesses, schools and international calendars plan by week number, which is not obvious from a plain date.
How to use it
- Enter a date.
- Read its ISO week number and the week boundaries.
Formula
ISO 8601 defines week 1 as the week containing the first Thursday of the year, and weeks start on Monday.
Around New Year a date can belong to the previous or next year's week numbering.
Frequently Asked Questions
Why does 1 January sometimes fall in week 52 or 53?
Under ISO rules a date near New Year can belong to the last week of the previous year if that week contains the year's first Thursday.
Do weeks start on Monday or Sunday?
ISO weeks start on Monday. Some countries and calendars start the week on Sunday instead.
Does it account for leap years?
Yes. The date logic follows the Gregorian calendar, including leap years and differing month lengths.
Are weekends and holidays excluded?
Only if the calculator says so. By default, results count every calendar day; use a business-day option where available.
Which time zone is used?
Calculations use whole calendar dates and are time-zone independent unless a time-of-day is involved.
Is this calculator free to use?
Yes. Every calculator on this site is completely free, with no hidden fees, no subscriptions and no limits on how many times you can use it.
How accurate are the results?
We use standard, widely accepted formulas and calculate in your browser using double-precision arithmetic. Results are rounded for readability, so treat them as reliable estimates rather than a guarantee. Real-world factors can vary the true figure.
Can I add this calculator to my own website?
Yes. Open this calculator page and use the "Add to your website" box in the sidebar to copy a ready-made iframe snippet, then paste it into any HTML page.
Does it work on a phone or tablet?
Yes. The layout is fully responsive, so it works on desktops, laptops, tablets and mobile phones, and every input adapts to touch screens.
Is the information I type kept private?
Yes. All calculations run locally in your browser. Nothing you enter is uploaded, stored or shared with anyone.