[3.3] What are the sizes and capacities of DVD?

There are many variations on the DVD theme. Discs come in two physical sizes: 12 cm (4.7 inches) and 8 cm (3.1 inches), both 1.2 mm thick, made of two 0.6mm substrates glued together. These are the same form factors as CD. A DVD disc can be single-sided or double-sided. Each side can have one or two layers of data. The amount of video a disc can hold depends on how much audio accompanies it and how heavily the video and audio are compressed. The oft-quoted figure of 133 minutes is apocryphal: a DVD with only one audio track easily holds over 160 minutes, and a single layer can actually hold up to 9 hours of video and audio if it's compressed to VHS quality.

At a rough average rate of 5 Mbps (4 Mbps for video and 1 Mbps for two or three tracks or audio), a single-layer DVD can hold a little over two hours. A dual-layer disc can hold a two-hour movie at an average of 9.5 Mbps (close to the 10.08 Mbps limit).

A DVD-Video disc containing mostly audio can play for 13 hours (24 hours with dual layers) using 48/16 PCM (slightly better than CD quality). It can play 160 hours of audio (or a whopping 295 hours with dual layers) using Dolby Digital 64 kbps compression of monophonic audio, which is perfect for audio books.

For reference, a CD-ROM holds about 650 megabytes, which is 0.64 gigabytes or 0.68 billion bytes. In the list below, SS/DS means single-sided/double-sided, SL/DL/ML means single-layer/dual-layer/mixed-layer (mixed means single layer on one side, dual layer on the other side), gig means gigabytes (2^30), BB means billions of bytes (10^9). See note about giga vs. billion in section 7.2. 

* Formatted DVD-RAM discs have slightly less than stated capacity. For example, the contents of a completely full DVD-R will not quite fit on a DVD-RAM.

Tip: It takes about two gigabytes to store one hour of average video.

The increase in capacity from CD-ROM is due to: 1) smaller pit length (~2.08x), 2) tighter tracks (~2.16x), 3) slightly larger data area (~1.02x), 4) more efficient channel bit modulation (~1.06x), 5) more efficient error correction (~1.32x), 6) less sector overhead (~1.06x). Total increase for a single layer is about 7 times a standard CD-ROM. There's a slightly different explanation at <www.mpeg.org/MPEG/DVD/General/Gain.html>.

The capacity of a dual-layer disc is slightly less than double that of a single-layer disc. The laser has to read "through" the outer layer to the inner layer (a distance of 20 to 70 microns). To reduce inter-layer crosstalk, the minimum pit length of both layers is increased from 0.4 um to 0.44 um. To compensate, the reference scanning velocity is slightly faster, 3.84 m/s, as opposed to 3.49 m/s for single layer discs. Longer pits, spaced farther apart, are easier to read correctly and are less susceptible to jitter. The increased length means fewer pits per revolution, which results in reduced capacity per layer.

Note: Older versions of Windows that use FAT16 instead of UDF, FAT32, or NTFS to read a DVD may run into problems with the 4 gigabyte volume size limit. FAT16 also has a 2 gigabyte file size limit, while FAT32 has a 4 gigabyte file size limit. (NTFS has a 2 terabyte limit, so we're ok there for a while.)

See 4.3 for details of writable DVD. More info on disc specifications and manufacturing can be found at Disctronics, Cinram. Technicolor, and other disc replicator sites. 

These super-size discs are used for data but are not commonly used for movies. The first commercial DVD-18 title, The Stand, was released in October 1999. A DVD-18 requires a completely different way of creating two layers. A single-sided, dual-layer disc (DVD-9) is produced by putting one data layer on each substrate and gluing the halves together with transparent adhesive so that the pickup laser can read both layers from one side. But in order to get four layers, each substrate needs to hold two. This requires stamping a second data layer on top of the first, a much more complicated prospect. Only a few replicators can make DVD-18s, and the low yield (number of usable discs in a batch) makes it more difficult and expensive than making DVD-9s.

(My prediction in this FAQ, in December 1998, was that we wouldn't see commercial DVD-18 discs until fall 1999, in spite of many rumors that they would appear sooner.)