How can I find out how big my encoded file will be?

Question

I am converting a video using the following command:

ffmpeg -i "input.avi" -vcodec mpeg4 -r 15 -sameq "output.avi"

However I'm only interested in the file size of output.avi. Is there a command I can give ffmpeg so that it doesn't actually write a file, but tells me how big it would be?

Answer 1

You can't really estimate the size the of a video file before encoding. This is not possible due to the way the codec works. It allocates a certain amount of bits to every frame, but this amount usually depends on the contents of the frame itself.

However, there are some solutions.

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1. The cheap one:

Let it encode, check the file size and trash the file again

2. The "workaround":

Calculate the file size yourself using a simple calculator. This only works when using constant bit rate. Specify it with, for example, -b 500k for 500 kBit/s. You have to make sure that you specify a bit rate the codec can use. for example, there's no way to compress a 1080p video with 200k bit rate in mpeg4, because it would need much more than that.

3. The "real" solution:

The last option I had in mind was piping the output to /dev/null and measuring the piped file size. That, however, won't work for all formats, because FFmpeg needs a seekable file to produce valid output.

Still, for AVI, it works pretty well. The following command will pipe into /dev/null, thereby leaving no trace of the file itself, and finally output the encoded file size.

ffmpeg -i input.avi -vcodec mpeg4 -b 3M -f avi pipe:1 | pv > /dev/null

… for example like this:

5.42MB 0:00:10 [ 521kB/s]

What does it do?

• You have to specify the format using -f avi. Otherwise FFmpeg won't know which format to use.
• pipe:1 tells FFmpeg to write the output to a pipe.
• We will feed this output into pipe viewer, short pv.
• Pipe viewer will measure the transferred size and output your video to /dev/null.

The only minor drawback is that the output looks a bit weird until the video is finished. I haven't yet found a way to fully disable FFmpeg's output and get pv to work with this.