Rendering: how to know final file size beforehand?

[Steve Rhoden]

No, Thats not really possible

Steve Rhoden
(Cow Leader)
Film Maker
Filmex Creative Media.
1-876-832-4956
https://filmex-creative-media.blogspot.com/

[John Cline]

Yes, it’s absolutely possible, it is a simple matter of total combined bitrate of the audio and video multiplied by the length of the program.

If it’s a constant bitrate (CBR) codec it’s quite easy. For DV, it is 3,178,117 bytes per second multiplied by the total number of seconds in the project.

In the case of MPEG2 or h.264-based AVC or MP4, where you can set either a constant or average bitrate per second, then it’s the bitrate in bits (divided by 8 to get bytes), then multiply by seconds.

However, if it’s a variable bitrate codec where you set a constant quality, like MJPEG or Cineform, for example, then you can’t determine the final size as the bitrate will change depending on varying image complexity throughout the program.

[Steve Rhoden]

Even with all that calculation John, you aint gonna get full accuracy…(a number of other factors comes into place).

Steve Rhoden
(Cow Leader)
Film Maker
Filmex Creative Media.
1-876-832-4956
https://filmex-creative-media.blogspot.com/

[Michal Bronec]

Hmmm 🙂 I need to render a 100 minutes project with 1920×1080 resolution, h264 codec in mp4. The output file shouldnt exceed 3GB. The problem is the rendering will take a few hours… and thats a bit long time for trial and error method 🙂

[David Shirey]

Well Steve gave you the simplest answer, which is that it’s really not possible to know the exact size for certain especially if you’re using variable bitrate, but you can certainly guesstimate without doing all that math in your head. I normally use a program called “vcalc” which you can google and download, but here’s a handy website that does the same thing:
https://dvd-hq.info/bitrate_calculator.php

The resolution and codec you’re rendering to doesn’t really matter, just tell the calculator the length of the project and what bitrate you’re using. Or, tell it the length and what your file size limit is, in this case 3gb, and it should figure out what bitrate you can safely render at. Once a bitrate calculator gives you the bitrate, lower it just a little to compensate for margin of error. Or just tell it your project is a few minutes longer than it really is. You don’t want to have to render it twice.

[Michal Bronec]

Thanks. BTW, the resolution doesnt matter??

[David Shirey]

What I mean is the resolution won’t have any bearing on figuring out the file size. Like John was saying, it all has to do with the amount of data you tell it to use per second, and how many seconds it is. It could be 720p or 1080i or whatever, but if you set the bitrate at something, that and the length determines the file size.

[Michal Bronec]

so if I set the bit rate at 8000, the final size will be the same for 720p and for 1080p?

[John Cline]

Determining the best encoding bitrate is a balancing act between image dimensions, image complexity, amount of motion and available storage space. A fairly static shot of a talking head with a solid background will look much better encoded at a lower bitrate than a video of a Formula 1 automobile race. Video encoding is an art and it takes experience to do it well.

In your case, Michal, your limiting factors are length of program and available space. How long is your program? If it’s relatively long then you will likely want to use 2-pass variable bitrate encoding in which the encoder will intelligently allocate fewer bits for less complex scenes and more bits to the complex scenes while hitting a set, average bitrate which will fit it all into a fixed file size.

[Michal Bronec]

Its around 100 minutes long. Mostly talking heads.