Why are dual-layer DVDs not twice the capacity of Single Layer?

[plokm]

A single layer DVD holds 4483 MB, but a dual layer DVD only holds 8150 MB rather than the 8974 MB you would expect. Why is this?

[unskinnyboy]

There are technical limitations to exactly double the storage capacity just by doubling the layers, which is why you can't have 9.4 GB on a single-side, dual-layer disc. It is also easier to read the 2nd layer with the current configuration as opposed to if both the layers had the same size. What you are thinking of is there, but in the form of the dual-side, single-layer disc a.k.a DVD-10.

Here is what you need to keep in mind:

DVD-05 -> SSSL -> 04.70 GB.
DVD-09 -> SSDL -> 08.50 GB.
DVD-10 -> DSSL -> 09.40 GB.
DVD-18 -> DSDL -> 17.08 GB.

[r0lZ]

And don't forget that the sizes above as expressed in "false GB", where a GB is considered as being 1,000,000,000 bytes. A real GB is 1024 x 1024 x 1024 bytes. Therefore, for example, the real capacity of a DVD-5 is only around 4.37 GB.