The easiest way to do it is to get BeerSmith II, which has a tool that converts AG recipes based on your equipment and techniques, including BIAB. BeerSmith II cost $22.00 right now at Birdman Brewing.
But yeah, you can also either add up the water like the post above suggests for a full-volume mash, or you can do a BIAB sparge, where you take the sparge instructions from a normal AG recipe and sparge the water through the BIAB bag after the initial draining of the bag. You'll typically get higher efficencies with this method for BIAB becuase the sparge rinses the grains pretty thoroughly, so if you like full-bodied beers (to style), it is a good option.
Seriously, though, if you have the money, get BeerSmith II, for $22.00 it converts everything, does all calculations, and is a GREAT tool for when you start to design your own recipes. That's a serious bargin in my book.
Primary #1 - Midnight Ryeder (Midnight Wheat and Rye)
Primary #2 - Florida Weiss
Primary #3 - Kane-DOH APA (Honey Citra APA)
Secondary #1 - Downtown Flanders Brown (brewed August 2012)
Keg #1 - Raspberry Florida Weiss
Keg #2 - Cinnamon Raisin Cider
Keg #3 - NONE!
Bottled - NONE!