The first difference is that manufacturers of media & computers use different counting rules.
Media manufacturers count a Gb as 1000x1000x1000
Computer's use binary system so a Gb is 1024x1024x1024
Your 4.7Gb disc is therefore reported as 4.37Gb on the computer.

Then to help with bit budgeting I follow the following:-

A 4.7Gbdisc has 37,600 Megabits available.
When I first started authoring DVDs I read that I should allow 4% for overheads (navigation data, still menus etc)

Take 4% from that & you have 36,096 (Make it a nice easy round number without losing too much & you have 36,000)

So we have 36,000 Megabits to divide equally amongst all the seconds in our movies that are to be included in our project. That would give us the average bitrate for all our movies (video & audio)

Because we are working in seconds (& that's a lot of numbers) we can make it easier so instead of working on 36,000/number of seconds if we divide both sides of the equation by 60 we get

600/number of minutes = average bitrate.

An example is
600/120 minutes = average combined bitrate of 5.0Mbs
If audio is ac3 at 192kbs that would leave average bitrate of 4.8Mbs for video
If audio is PCM at 1600kbs that would leave average bitrate of 3.4Mbs for video

As I said earlier the 4% ovrehead doesn't include motion menus so you'd have to add them as movies.

I don't know about you but I'm quicker dealing with small numbers than big numbers. This method provides a fairly accurate way of maximising the available space on your disc, whilst maintaining quality. It's never failed me & that's why I pass it on.


A couple of other things of note

For compatability reasons when doing small projects I don't use as high a bitrate as this formula may allow. This is because I don't want the playback device pushed to the limit (it may result in skipping during playback)
The maximum combined bitrate you can use is 9.8Mbs but I would use a combined rate lower than that even if space allowed higher. Say 9.0Mbs. At that rate you shouldn't notice any quality hit but you are taking the strain off the decoding.

If you are adding multiple language tracks & subtitles, these will have to be catered for in your original figure for the AVERAGE bitrate.

The space on the different DVD discs is as follows:- (media manufacturer counting)
DVD-5 single sided,single layer; 4.7 GB
DVD-9 single sided,dual layer; 8.5 GB
DVD-10 double sided,single layer 9.4 GB
DVD-18 double sided,double layer 17 GB