said by InvalidError: said by MadCow :
You're over accounting for the audio.
You are under-accounting for everything else I said the extra space may be needed for.
Most movies come with multiple audio tracks, multiple subtitle tracks, animated menus, extras, commentary tracks, etc.
Also, you need to add the MPEG framing overhead.
But most or all of those things are going to be constant. There isn't any difference in audio size between a 1080p and 4K disc, the overhead for framing is minimal, the difference in menu size is inconsequential...
If we accept that 4K content should require roughly double the bitrate (some argue less than that is required, I say 2x to keep it simple), and then consider that most blurays are single-layer (25GB) discs, 100GB for a 4K video becomes incredibly excessive, to the point where there's a danger that a longer 4K film (like three hours) would not fit on the biggest currently available quad-layer (128GB) bluray discs.
EDIT: Of course, if they try to throw in Dolby Atmos (64.1), that's another story entirely. Atmos should have significant bandwidth requirements, but when you've got that many audio channels for a single video, there are likely some pretty crazy compression tricks you can pull. If you're trying to pan a helicopter over the ceiling grid, for example, you've probably got more or less the same sound in all of them, just at different intensities.--
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