do an online search according to model number of sub use the maximum sealed air space ie: 1.25 etc to calculate the the box air space you multiply length times width times height divided by 1728 also if you use 3/4 mdf you must subtract 1.5 from the length/width/height for example you have a box that measures externally 18 L X 15 Wx 14 H so you true measurements for the air space would be 16.5 L x 13.5 W x 12.5 H keep in mind speakers have displacements meaning the size of the airspace the speaker it self occupies let's say as an example .10 so this box's air space is 1.611- .10 gives you 1.50 cu hope this helps