6.1 What size and capacities of HD DVD discs are available?
6.2 How much information can actually be stored on an HD DVD disc?
What size and capacities of HD DVD discs are available?
As with CD, DVD and Blu-ray Disc (BD) formats, HD DVD-ROM discs are marketed in 8 cm (80 mm) and 12 cm (120 mm) diameter sizes. HD DVD-R, HD DVD-RW, and HD DVD-RAM are 12 cm only.
All HD DVD discs can be either single (SS) or double-sided (DS). 12 cm HD DVD-ROM, HD DVD-R and HD DVD-RW come in 15 GB per side single-layer (SL) and 30 GB dual-layer (DL) capacities and HD DVD-RAM in 20 GB (SL). 8 cm HD DVD-ROM are 4.7 GB per side SL and 9.4 GB DL.
How much information can actually be stored on an HD DVD disc?
As with DVD and Blu-ray Disc (BD), manufacturers quote HD DVD disc capacities in decimal (base 10) rather than binary (base 2) notation. Thus a 15 GB (decimal) disc stores roughly 15 billion bytes [15,076,554,752 bytes ÷ 1000 = 15,076,554.752 KB ÷ 1000 = 15,076.555 MB ÷ 1000 = 15.077 GB]. Expressed in binary notation (as is typical with most operating systems) the same disc has a capacity of roughly 14.04 GB [15,076,554,752 bytes ÷ 1024 = 14,723,198 KB ÷ 1024 = 14,378.123 GB ÷ 1024 = 14.041 GB].
Actually, discs store information in a multitude of sectors that each house 2 KB (2,048 bytes) of user data. A disc contains a maximum (HD DVD-ROM, HD DVD-R, HD DVD-RW) or fixed (HD DVD-RAM) number of sectors in a special area of the disc reserved to hold user information (Data Area) so disc capacity can be calculated by multiplying the size of the Data Area by the amount of information stored in each sector. For example, a 15 GB disc: 7,361,599 sectors x 2,048 bytes per sector = 15,076,554,752 bytes. This rounds to 15 GB (decimal notation). Real world capacity, however, may vary slightly among discs from different manufacturers (see chart).
HD DVD Disc Capacities
(12 cm, Unformatted, Single-Sided)
| Disc Format | Number of Layers | Maximum Number of Physical Sectors in Data Area | Gross Capacity (bytes) | Gross Capacity (GB decimal) | Gross Capacity (GB decimal) |
| HD DVD-ROM | 1 | 7,361,599 | 15,076,554,752 | 15.077 | 14.041 |
| HD DVD-ROM | 2 | 16,305,407 | 33,393,473,536 | 33.394 | 31.100 |
| HD DVD-R | 1 | 7,361,599 | 15,076,554,752 | 15.077 | 14.041 |
| HD DVD-R | 2 | 16,239,871 | 33,259,255,808 | 33.259 | 30.975 |
| HD DVD-RW | 1 | 7,361,599 | 15,076,554,752 | 15.077 | 14.041 |
| HD DVD-RW | 2 | 16,239,871 | 33,259,255,808 | 33.259 | 30.975 |
| HD DVD-RAM | 1 | 9,848,064 | 20,168,835,072 | 20.169 | 18.783 |
Be aware that the file system (UDF, HFS, etc.), recording method (multi-border, etc.) and any defect management system employed consume space otherwise available for user information. For example, spare sectors used by HD DVD-RAM for defect management (allocated as a Supplementary Spare Area) are taken directly out of the Data Area, which is otherwise used for user information storage. In practice, HD DVD-RAM can assign as much as 444 MB (465,567,744 bytes) of its usable capacity for defect management.
Also note that while a sector (2 KB) is the smallest element of information that can be read from a disc, the smallest that can be written is a 64 KB ECC block (32 sectors). Consequently, if less than an ECC block is supplied to the recorder by the data source, the remaining sectors in the ECC block are padded will null information. This inefficiency creates “slack” or wasted space.