請問現時市面上賣的dvd碟,有多少種容量?

Please simply see the page of wikipedia :

DVD disc capacity
圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Size comparison: A 12 cm Sony DVD+RW and a 19 cm pencil.
Single layer capacity Dual/Double layer capacity Physical size GB GiB GB GiB 12 cm, single sided 4.7 4.37 8.54 7.95 12 cm, double sided 9.4 8.74 17.08 15.90 8 cm, single sided 1.4 1.30 2.6 2.42 8 cm, double sided 2.8 2.61 5.2 4.84 The 12 cm type is a standard DVD, and the 8 cm variety is known as a mini-DVD. These are the same sizes as a standard CD and a mini-CD.
Note: GB here means gigabyte, equal to 109 (or 1,000,000,000) bytes. Many programs will display gibibyte (GiB), equal to 230 (or 1,073,741,824) bytes.
Example: A disc with 8.5 GB capacity is equivalent to: (8.5 × 1,000,000,000) / 1,073,741,824 ≈ 7.92 GiB.
Capacity Note: There is a difference in capacity (storage space) between + and - DL DVD formats. For example, the 12 cm single sided disc has capacities:
Disc Type Sectors bytes GB GiB DVD-R SL 2,298,496 4,707,319,808 4.7 4.384 DVD+R SL 2,295,104 4,700,372,992 4.7 4.378 DVD-R DL 4,171,712 8,543,666,176 8.5 7.957 DVD+R DL 4,173,824 8,547,991,552 8.5 7.961
[edit] Capacity nomenclature The five basic types of DVD are referred to by their approximate capacity in gigabytes.
DVD type Name Single sided, single layer DVD-5 Single sided, dual layer DVD-9 Double sided, single layer DVD-10 Double sided, dual layer on one side, single on other DVD-14[5] Double sided, dual layer on both sides DVD-18
[edit] Technology
圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Internal mechanism of a DVD-ROM Drive DVD uses 650 nm wavelength laser diode light as opposed to 780 nm for CD. This permits a smaller spot on the media surface that is 1.32 µm for DVD while it was 2.11 µm for CD.
Writing speeds for DVD were 1x, that is 1350 kB/s (1318 KiB/s), in first drives and media models. More recent models at 18x or 20x have 18 or 20 times that speed. Note that for CD drives, 1x means 153.6 kB/s (150 KiB/s), 9 times slower. DVD FAQ

In the market had double layer capactor 9.4G