Formula Watch: Working with week numbers

There are plenty of scenarios where you'd want to know the week number within a year. Say you are developing a schedule with a time budget, and you need to know how many hours you can commit per week over the course of a project or fiscal year. Or maybe you just want to know how many weeks are left until the holidays.

Either way, the WEEKNUM function can help. To learn what week you're on with today's date, simply enter:

=WEEKNUM(TODAY(),1)

To calculate how many weeks are left in the year, you can use:

=52-WEEKNUM(TODAY(),1)

To enter a specific date to calculate the week number, use this format:

=WEEKNUM("9/28/2010", 1)

Just substitute the date you want in the quotes, or use a cell value (e.g., A2).

If you're wondering what the ",1" means, that's used to set the start of the week to Sunday. If you want to start with Monday, change this to ",2" instead.

Now if you just want to know the number of weeks between two dates, it's much faster to simply subtract them out and divide by 7. For example:

=("6/30/2011"-"9/28/2010")/7

Suzanne