How much data can you fit on a Blu-ray disc?

A single-layer disc can hold 25GB.
A dual-layer disc can hold 50GB.

To ensure that the Blu-ray Disc format is easily extendable(future-proof) it also includes support for multi-layer discs, which should allow the storage capacity to be increased to 100GB-200GB(25GB per layer) in the future simply by adding more layers to the discs.