ISO Week Number
Enter any date to get its ISO week number, weekday name and day-of-year — today selected by default.
ISO week numbers are the international standard for business calendars and project planning: weeks start on Monday and week 1 contains the year's first Thursday. This free tool converts any date into its ISO week number, weekday name and day-of-year. Today is pre-filled for an instant answer, and everything is calculated locally in your browser.
Today is selected by default. Pick another date and press Get ISO Week.
How to use the ISO Week Number
- Today's date is already filled in — press Get ISO Week immediately, or pick any other date with the date picker first.
- Read the three results: the ISO week number with its ISO year, the weekday name, and the day-of-year count.
- Use the note underneath for a plain-English sentence, for example which ISO week a project deadline falls in.
ISO Week Number — FAQs
What is an ISO week number?
It is the international standard for numbering weeks, defined in ISO 8601. Weeks start on Monday, and week 1 is the week containing the first Thursday of the year. This system is used in business calendars, payroll and project planning across Europe and much of the world.
Why can the ISO year differ from the calendar year?
Because ISO weeks belong to the year containing their Thursday. A date like 1 January can fall in week 52 of the previous ISO year, and late December dates can belong to week 1 of the next ISO year. The tool shows the ISO year next to the week number so this is always clear.
Is this ISO week calculator free?
Yes, completely free with no sign-up and no limits. Look up the ISO week for any date, past or future, as often as you need for scheduling, reporting and planning work. Bookmark it for quick weekly reference.
Does the tool send my date anywhere?
No. The ISO week, weekday and day-of-year are all computed locally in your browser with standard date math. Nothing is uploaded to a server or stored, so the dates you check remain fully private.