This is a basic, non-axial Sabin calculator for RT60, the length of time in seconds it takes for a signal to drop 60db. For a recording studio, this should be about .20-.40s at 1000Hz, for a vocal boot it should be < .20s. A home theater or music room has slightly looser tolerances.
There are more complex (and accurate) methods of determining RT60 along various axes, or with assumptions of less evenly-distributed absorption (because who has a perfectly symmetrical room?) but for the most part, this will help determine just how much acoustic treatment your room will need.
Basic assumptions for these calculations: This is a standard household room, primarily drywall walls and ceiling, carpeted floor. The treatment described is 2″ mineral wool/fiberglass panels, of the variety available from many DIY sources.