Since a lot of pack makers are using liters, it's useful to know the conversion.

BPL has done some reviews of backpacks that show that most manufacturer's volume measurements aren't very accurate anyway, so the 1 liter = 60 cubic inches is close enough. (As we used to say when I worked for a government contractor, "Close enough for government work.") However, it's far, far better to try the pack out with the actual gear you'll be putting in it. The pack has to be comfortable for you carrying your gear, and there's only one way to find that out!