The DVD sizes can be a bit confusing. There are basicly 4 different DVD Sizes,

DVD-5, holds around 4 700 000 000 bytes (4.7 GB) and that is 4.37 computer GB* where 1 kbyte is 1024 bytes* . DVD+R/DVD+RW and DVD-R/DVD-RW supports this format. Also called Single Sided Single Layered. This is the most common DVD Media, often called 4.7 GB Media.

DVD-10, holds around 9 400 000 000 bytes (9.4 GB) and that is 8.75 computer GB*. DVD+R/DVD+RW and DVD-R/DVD-RW supports this format. Also called Double Sided Single Layered.

DVD-9, holds around 8 540 000 000 bytes (8.54 GB) and that is 7.95 computer GB*. DVD+R supports this format. Also called Single Sided Dual Layered. This media is called DVD+R9, DVD+R DL (dual layer) or 8.5 GB Media.

DVD-18, holds around 17 080 000 000 bytes (17.08 GB) and that is 15.9 computer GB*. DVD+R supports this format. Also called Double Sided Dual Layered.

* In the computer world is 1 KB data = 1024 bytes so,
4 700 000 000 bytes / 1024
= 4 589 843KB / 1024
= 4482MB / 1024
= 4.37GB