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A BD-25 is the popular name for a single layer Blu-ray disc. Each layer can hold approximately 25GB of data.
More popular however, is the dual layer BD, the BD-50.
For the future however, there is the possibility of multi-layer discs which would add 25GB capacity per layer.
Due to its large size, 4.5 hours of HD video and over 12 hours of SD video can fit on a single layer BD-25.
In comparison, a dual layer DVD can hold up to 8.5GB of data and a dual layer HD DVD can hold up to 30GB of data.