HD recording take up about 8 gig of hard drive space but SD take up about half so 4gig for the SD recordings so have to do some math.if you have a 100 gig harddrive in your dvr and your recording HD content 100 divided by 8 or if SD content 100 divided by 4.this is compiled for 2 hr shows so if your show is hr long and in HD format divide 100 by 4 and if in SD format divide 100 by 2 -if they are 30 min shows, half each by half and divide so for HD content 100 gig divided by 2 if ...