9.1 What capacities of BD discs are available?
9.2 How much information can actually be stored on a BD disc?
9.3 How far might BD disc capacity increase in the future?

9.1 What capacities of BD discs are available?
As with CD and DVD formats, BD-ROM, BD-R and BD-RE discs are marketed in 8 cm (80 mm) and 12 cm (120 mm) diameter sizes. Most applications call for 12 cm discs while the 8 cm type will most likely find a home in portable consumer electronics devices such as digital video camcorders.

In practice, 12 cm BD discs come in 25 GB single-layer (SL) and 50 GB dual-layer (DL) capacities and 8 cm in 7.8 GB (SL) and 15.6 GB (DL). Early BD-RE discs (Japan-only version 1.0 and 1.01) were 23.3 GB (SL).

Strictly speaking, BD specifications describe a range of capacities achieved by changing the fixed rotational speed of the disc (5.28 m/s, 4.917 m/s, 4.554 m/s). For 12 cm, these are 23.3 GB, 25 GB, 27 GB (SL) and 46.6 GB, 50 GB, 54 GB (DL) and for 8 cm, 7.8 GB (SL) and 15.6 GB (DL). These alternatives accommodate past limitations, while permitting future improvements, in mastering, replication and recording technologies. For example, 23.3 GB was the practical limit when BD-RE was first introduced in 2003 but is now obsolete while 27 GB is not yet feasible to produce. In many ways the planned development path for BD parallels the history of prerecorded and writable CDs where discs eventually evolved, as technology permitted, from 63 (550 MB) to 74 (650 MB) and then to 80 (700 MB) minutes in length.

Announced in 2010, BDXL discs are 12 cm only and available in 100 GB triple-layer (TL) BD-R XL and BD-RE XL as well as 128 GB quadruple-layer (QL) BD-R XL configurations. BDXL was extended in 2014 to include a 200GB GB double-sided (DS), triple-layer (TL) BD-R XL version (BD-DSD).

Announced in 2015, Ultra HD BD-ROM discs are 12 cm only and available in 50 GB and 66 GB dual-layer (DL) as well as 100 GB triple-layer (TL) configurations.

9.2 How much information can actually be stored on a BD disc?
As with DVD, manufacturers quote BD disc capacities in decimal (base 10) rather than binary (base 2) notation so a 25 GB disc stores roughly 25 billion bytes [25,025,315,816 bytes ÷ 1000 = 25,025,315.816 KB ÷ 1000 = 25,025.316 MB ÷ 1000 = 25.025 GB]. Expressed in binary notation (as is typical with most operating systems) the same disc has a capacity of roughly 23.3 GB [25,025,315,816 bytes ÷ 1024 = 24,438,784.977 KB ÷ 1024 = 23,866 MB ÷ 1024 = 23.307 GB].

Discs store information in multitude of sectors that each house 2KB (2,048 bytes) of user data. 32 sectors are then grouped together into 64KB (65,536 byte) partitions called clusters (see table). A disc contains a fixed (BD-R, BD-RE, BD-R XL, BD-RE XL) or maximum (BD-ROM) number of clusters in a special area of the disc reserved to hold user information (Data Zone) so disc capacity can be precisely calculated by multiplying the size of the Data Zone by the amount of information stored in each cluster. For example, a 25 GB disc: 381,856 clusters x 65,536 bytes per cluster = 25,025,315,816 bytes. This rounds to 25 GB (decimal notation).

Be aware that the file system (UDF, HFS, etc.) and any defect management system employed consume space otherwise available for user information. For example, spare clusters used for defect management (allocated as inner and outer spare areas) are taken directly out of the Data Zone, which is otherwise used for user information storage. In practice, a BD-RE can assign 5% and a BD-R as much as 50% of its usable capacity for defect management (see table).

Also note that while a 2 KB sector is the smallest element of information that can be read from a disc, the smallest that can be written is a 64 KB cluster (32 sectors). Consequently, if less than a cluster is required to store a data file, the remaining sectors in the cluster are padded with null information. This inefficiency creates "slack" or wasted space.

9.3 How far might BD disc capacity increase in the future?
Generally speaking, BD disc capacity can be expanded by adding additional data layers as well as by increasing the amount of information stored per layer.

For example, in June 2010, the Blu-ray Disc Association (BDA) introduced specifications for High Capacity BD (BDXL) in 100 GB (33.3 GB triple-layer) recordable and rewritable as well as 128 GB (32 GB quadruple-layer) recordable versions. These discs are incompatible with existing BD players, drives, recorders and software.

Additionally, Sony and Pioneer have proposed possible 200/204 GB (8/34-layer) and 400/500 GB (16/20-layer) BD-ROMs respectively while TDK has announced that it has created inorganic 100 GB (4-layer), 200 GB (6-layer), 400 GB (16-layer) and 512 GB (16-layer) BD-R prototypes. Again, these discs would be incompatible with the installed base of BD hardware and software.

If or when such complex multi-layer types could be cost effectively mass-produced or if there is sufficient market need or interest to warrant their development and introduction are, of course, additional considerations.

On Dec. 4, 2008, in response to widespread erroneous reports that Pioneer’s experimental 400/500 GB disc designs are to be released to market shortly and will be compatible with existing devices, Pioneer provided this author with the following official statement:

"This is a technology exhibit only, and is not being proposed as a candidate for addition to the Blu-ray Disc format at this time. Current BD players and drives would not be able to play/read these discs. Current Blu-ray optical pickup designs could be utilized in new players/drives designed to read these discs. At present, this is a read-only technology. Recordable versions could eventually be possible."