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The list below shows variations of DVD. SS/DS means single-/double-sided, SL/DL/ML means single-/dual-/mixed-layer (mixed means single layer on one side, double layer on the other side), gig means gigabytes, G means billions of bytes. For reference, a CD-ROM holds about 650 megabytes, which is 0.64 gigabytes or 0.68 billion bytes. See Call for Help's article on bits and bytes for more information.
DVD-5 (SS/SL): 4.38 gig (4.7G) of data, over 2 hours of video
DVD-9 (SS/DL): 7.95 gig (8.5G), about 4 hours of video
DVD-10 (DS/SL): 8.75 gig (9.4G), about 4.5 hours of video
DVD-14 (DS/ML): 12.33 gig (13.24G), about 6.5 hours of video
DVD-18 (DS/DL): 15.90 gig (17G), over 8 hours of video
DVD-RAM (SS/SL): 2.40 gig (2.58G)
DVD-RAM (DS/SL): 4.80 gig (5.16G)
Taken from