Date & time
Week Number Calculator
Find what week it is, or the week number of any date. The calculator gives the ISO 8601 week number (weeks start Monday) used in business and project planning, the US-style week number (weeks start Sunday), the day of the year, and a full table of week dates for the year.
How to use the Week Number Calculator
- Pick a date (today is filled in).
- Read the ISO and US week numbers.
- Choose a year to list all its weeks.
Formula
Example
November 3, 2026 is in ISO week 45, which runs from Monday, November 2 to Sunday, November 8.
ISO vs US weeks
ISO weeks start on Monday and week 1 contains the first Thursday, so January 1 can belong to the last week of the previous year. US calendars often start weeks on Sunday and count January 1 as week 1.
Related free tools
Frequently asked questions
How many weeks in a year?
52, and 53 in some years under ISO rules.
Which system does Excel use?
ISOWEEKNUM uses ISO; WEEKNUM uses Sunday by default.
Why is Jan 1 in week 53?
If it falls on Friday-Sunday, it belongs to the previous year's last ISO week.
Day of the year?
The number of days since January 1, counting it as day 1.
Results are estimates for planning. Your inputs stay in your browser and are not sent to Soft2Cloud.
Last updated: November 2026