I'm using ffmpeg to transcode mp4 file, I got the below command from: http://www.wowza.com/forums/content.php?192-Encoding-Suggestions-for-Video-on-Demand.
ffmpeg -y -i Butterfly_HD_1080p.mp4 -s 320x180 -y -strict experimental -acodec aac -ab 64k -ac 2 -ar 48000 -vcodec libx264 -vprofile main -g 48 -b 270000 -threads 64 butterfly_400.mp4
based on what they decide the bitrate for above output file will be 400(400000)? what the equation that they use for bitrate value?
-ab 64k
), so the total bitrate for the assembled file would be 270k+64k=334k. So I have no idea where you're getting 400(400000) from, and I would have to agree with Neckbeard: telling us what you want to accomplish makes it possible to provide answers.