I hesitate to include the original Room Modes calculator in this series, since it absolutely requires Visual Basic to work properly. Since it works incredibly well and, supposedly, VB was cobbled back into Office 2010, I’ve added it to the list of my old MSCM class reference materials and tools.
For you less-technical Mac users and non-VB Windows-based Office suckers, I’ve linked the more remedial Linkwitz Lab MacRoomModes calculator. It works, but no nearly so easily, since you have to resort the data manually to get the sequences right after entering new room dimensions.
More computer OS-independent is the JBL/Infinity Harmon Room Mode Calculator. I have no idea where I found this and it seems to be missing in action on the current JBL website. The thing to keep in mind with this tool is that the first tab, “WAVES,” is where the room dimensions must be entered. The 2nd page’s fields are protected.