Calculating a Project’s Disc Space Requirements

While the video bit rate is the single biggest factor in controlling the size of your project, a number of other factors determine how much disc space will be required to hold your project:

  • The number of and types of audio assets

  • The number of slideshows and their audio

  • The number of subtitles

  • Any DVD-ROM content to be included

The steps below guide you through the process of estimating your project’s disc space requirements. For this example, the project has:

  • Video asset 1 with a duration of 01:26:13:12 and two audio streams: an AC-3 5.1 surround and an AC-3 stereo

  • Video asset 2 with a duration of 00:45:34:07 and one PCM 16-bit 48 kHz audio stream

  • Motion menu video with a duration of 00:00:16:20 and one AC-3 stereo audio stream

Stage 1: Calculating the Total Video Play Time

You first need to determine the total play time of all video assets and motion menus in your project. To make calculations easier, you need to convert the time from hours, minutes, and seconds to the total number of seconds. For frames, round up to the next second when there are frames left over.

In the example above, video asset 1 is 5174 seconds long, video asset 2 is 2735 seconds long, and the motion menu contains 17 seconds of assets, for a total of 7926 seconds.

Note: Slideshows do not count as video play time, and are discussed later.

Stage 2: Calculating Disc Capacities

To calculate the appropriate bit rate for your content, you need to know each disc type’s capacity in bits. The capacity of a DVD-5 disc is 4.7 GB (or 4.37 binary gigabytes). For the purposes of this calculation, you should use the 4.7 GB value because it does not have to be corrected to account for the difference that counting in binary adds. To make the calculation, you multiply the byte capacity by 8 (the number of bits in a byte).

DVD name
Byte capacity
Bit capacity
DVD-1
1.46 GB
11.68 gigabits (Gbit)
DVD-2
2.66 GB
21.28 Gbit
DVD-3
2.92 GB
23.36 Gbit
DVD-4
5.32 GB
42.56 Gbit
DVD-5
4.7 GB
37.6 Gbit
DVD-9
8.54 GB
68.32 Gbit
DVD-10
9.4 GB
75.2 Gbit
DVD-14
13.24 GB
105.92 Gbit
DVD-18
17.08 GB
136.64 Gbit

Stage 3: Calculating the Bit Rates for Each Disc Size

Once you know the play time in seconds and the bit capacity of the different disc sizes, you can calculate a basic bit rate for each disc size. To do this, divide the disc size by the play time (7926 seconds in this example).

DVD name
Bit capacity
Bit rate
DVD-1
11.68 gigabits (Gbit)
1.47 megabits per second (Mbps)
DVD-2
21.28 Gbit
2.68 Mbps
DVD-3
23.36 Gbit
2.95 Mbps
DVD-4
42.56 Gbit
5.37 Mbps
DVD-5
37.6 Gbit
4.74 Mbps
DVD-9
68.32 Gbit
8.62 Mbps
DVD-10
75.2 Gbit
9.49 Mbps
DVD-14
105.92 Gbit
13.36 Mbps
DVD-18
136.64 Gbit
17.24 Mbps

These bit rate values should not be considered final at this point because you still have to take into consideration the audio and DVD-ROM content. However, these values do provide an indication as to which DVD disc size you will need to use. If your project uses compressed audio, such as the Dolby Digital AC-3 format, and has little or no DVD-ROM content, you would need to decide whether encoding at 4.74 Mbps (or slightly less to ensure the video and audio all fit on the disc) would provide satisfactory results. If so, a DVD-5 disc should work for this project. If the project includes multiple soundtracks, a lot of slides or subtitles, and some DVD-ROM content, you may decide it is best to use a DVD-9 disc size so that everything you want to include will fit on the disc.

Another point to keep in mind is that video assets are not required to use the same bit rate. For some assets, you may decide that quality is not as important as it is for others, and choose to use a lower bit rate for them, allowing more disc space for the other video assets.

Choosing a bit rate that provides satisfactory results depends on the video content and the encoder you use. In general, bit rates below 4.0 Mbps are marginally acceptable. For bit rates between 4 and 6 Mbps, you should get good results when using a good variable bit rate (VBR) encoder, such as the integrated MPEG encoder, or, for more options, Compressor directly. For rates between 6 and 8 Mbps, you should have no trouble getting good results even when using a constant bit rate (CBR) encoder. Eight Mbps is usually considered the highest usable bit rate—the DVD specification allows up to 9.8 Mbps, but this can cause playback problems on some DVD players and limit your audio options.

For more information on encoding, see Encoding Video Materials for DVD.

Stage 4: Calculating the Audio Allowances

Depending on your project, the audio may have a relatively minor effect on disc space. It can also be challenging to determine the audio allowances because each video asset can have up to eight audio streams, and each stream can be a different type and length. The following table lists the typical bit rates for the common audio formats.

Audio format
Bit rate
AC-3 stereo
192 kilobits per second (kbps) to 224 kbps
AC-3 5.1 surround
384 to 448 kbps
DTS 5.1 surround
754.5 kbps or 1509.75 kbps
PCM stereo 16 bits at 48 kHz
1536 kbps
PCM stereo 24 bits at 96 kHz
4608 kbps

In the earlier example, video asset 1 has two audio streams, each 5174 seconds long. The first stream is AC-3 5.1 surround, and the second is AC-3 stereo. The video asset 2 has one audio stream 2735 seconds long, using 16-bit 48 kHz PCM audio. The motion menu has an AC-3 stereo audio stream 17 seconds long. If your slideshow includes audio, you also need to include that in the formula.

To calculate the storage required, you determine each video asset’s total audio bit rate and multiply that by the duration.

Section
Duration
Bit rate
Disc space
Video 1, AC-3 5.1
5174 seconds
448 kbps
2.32 gigabits (Gbit)
Video 1, AC-3 stereo
5174 seconds
224 kbps
1.16 Gbit
Video 2, 16-bit 48 kHz PCM
2735 seconds
1536 kbps
4.20 Gbit
Menu, AC-3 stereo
17 seconds
224 kbps
0.004 Gbit
Total: 7.684 Gbit

As you can see, using PCM audio requires substantially more disc space than using a compressed format like AC-3. See Preparing Audio Assets for more information on the different audio formats.

Stage 5: Calculating the Effect of Audio on the Video Bit Rate

Now that you know how much disc space is required for the audio, you can more accurately determine the video bit rate you can use. Because the DVD-5 and DVD-9 discs were the only practical ones based on the previous calculations, you will continue experimenting with them in this example.

For the DVD-5 disc, you need to subtract the total audio amount from the total disc size: 37.6 gigabits – 7.684 gigabits = 29.916 gigabits for the video. Dividing the available space by the video duration provides a new bit rate: 29.916 gigabits ÷ 7926 = 3.77 Mbps. You can see that the audio has considerably affected the original bit rate for the DVD-5 disc (4.74 Mbps). The use of PCM audio for the second video asset in this example has forced the video bit rate to drop below 4 Mbps, making the use of a DVD-5 disc questionable because a bit rate that low may not provide suitable quality.

For the DVD-9 disc, you end up with 60.636 gigabits available for the video (68.32 gigabits – 7.684 gigabits). The adjusted video bit rate is now 7.65 Mbps (60.636 gigabits ÷ 7926 seconds)—easily high enough to provide good quality.

Stage 6: Calculating DVD-ROM Allowances

If you intend to include DVD-ROM content on the disc, you need to subtract it from the disc space before coming up with your video bit rate.

Because the size of your DVD-ROM content is expressed in binary megabytes, you must convert it to the same standard as used for the DVD disc size. In this example, there is a total of 36 MB of data. Each computer MB is equal to 1,048,576 actual bytes. To convert the DVD-ROM content, you need to multiply its size (36 MB) by 1,048,576 to get the actual size of 37,748,736.

The next step is to convert this number into bits by multiplying it by 8, providing a final size of approximately 0.3 gigabits. While this is not a large amount of space, you should take it into account if you are trying to maximize usage of the disc’s available space.

Stage 7: Calculating Other Allowances

There are several other factors that should be taken into account when determining available disc space.

  • Slideshows: Each still in a slideshow requires approximately 200 kilobytes (KB) of disc space. A full slideshow with 99 stills requires about 20 MB of disc space. If the slideshow contains audio, you will find that the audio can require as much or more disc space than the stills. If your disc contains many stills, you will need to take them into account.
  • Still menus: Each standard still menu requires about 300 KB of space, depending on the number of buttons.

    Layered menus can require a lot more space than standard menus because a new menu is created for each button in each of its separately specified states (normal, selected, and activated). This can mean as many as 12 menus are created to support 4 buttons. See Creating Menus Using the Layered Method for more information on layered menus.

    Note: Menus that use button shapes with motion assets assigned are considered to be motion menus, and need to count as a video asset.

  • Subtitles: The amount of space required by subtitles can vary widely, depending on how many of the 32 streams you use, the type of content you use (simple text characters or full-screen graphics), and how often you change them (several times a second, or more typically, once every four or five seconds).

    Simple subtitles average about 10 kbps—roughly 2 megabits of space per hour, which is negligible on most titles. If you intend to use subtitles more as an animation tool, with full-screen graphics that change often, you must make significant allowances for them. (The maximum bit rate allowed for a subtitle stream is 3.36 Mbps—almost as much as the video stream.)

  • Transitions: The amount of disc space required by each transition varies depending on the transition’s length and type. In general, for SD projects, which use a bit rate of 6 Mbps for transitions, you should allow 750 KB of disc space for every second of transition time in the project. For HD projects, which use a bit rate of 20 Mbps for transitions, you should allow 2.5 MB of disc space per second.

    Note: Keep in mind that adding a transition to a menu results in a separate transition clip for each button—a menu with 18 buttons can require 18 transition clips.

Unless you are using an exceptional number of stills, menus, or subtitles, you can usually account for the amount of space required by these items by allowing a five percent overhead. The easy way to apply this overhead amount is to multiply the bit rate you have figured out by 0.95. In the earlier example, the DVD-5 disc bit rate drops from 3.77 Mbps to 3.58 Mbps; the DVD-9 bit rate drops from 7.65 Mbps to 7.27 Mbps.

Important: It is much better to be conservative and find yourself with some disc space left over than to get to the end of the project and find it will not fit on the disc.