Week Number Finder
See which week a date falls in, under the ISO 8601 rule used across Europe and under the US rule, or look up the dates of a week number.
INPUT
Find the dates of a week
RESULT
The result appears here as you type.
Runs entirely in your browser. Nothing you enter is sent to a server.
How to find a week number
Pick a date
Today's date is filled in. Choose another date to see its week.
Read both systems
The ISO week number, week-year and Monday to Sunday range appear next to the US week number and its Sunday to Saturday range.
Look up a week
Enter a year and a week number to get the dates of that week, or pick a week in the table for the whole year.
ISO 8601 and US week numbers
ISO 8601 weeks start on Monday, and week 1 is the week that contains the year's first Thursday, so every week has seven days and a year has 52 or 53 weeks. In the US system, weeks start on Sunday and week 1 is the week that contains 1 January, as with Excel's WEEKNUM function.
The week-year
Under ISO 8601, the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next. 3 January 2021, for example, is in week 53 of 2020. That's why ISO weeks are written with their week-year, as in 2020-W53.
Frequently asked questions
Which system does Excel use?
WEEKNUM uses the US system by default. ISOWEEKNUM, or WEEKNUM with 21 as the second argument, gives the ISO 8601 week.
Why does a year have 53 weeks?
An ISO year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year. 2026 is one of them: 31 December 2026 is in week 53.
Which system does my country use?
Most of Europe and many other countries use ISO 8601. The US, Canada and some other countries usually count weeks from Sunday. When in doubt, say which system you mean, or give the dates.