I do a lot of video encoding via Handbrake. Beginning with version 0.9.6 (way back in 2012!), Handbrake decided to do away with Target Size Mode - a great feature that determined your codec's average bitrate based on how large you wanted the file to be. So if you knew you were burning something to a 4.7 GB DVD, you could encode it to specifically fit that entire space without going over.
The math to calculate your own bitrate is pretty easy, to be honest. But I wanted a quick tool to just do it for me, so I whipped this together.
You need a couple pieces of information in order to calculate a target bitrate:
The length of the media.
The target size.
Audio bitrate to include in reaching that target size.
Bitrate Calculator allows you to specify the duration of your media in minutes (you are free to use decimals, so if your video file is 10 minutes and 30 seconds, you'd put 10.5 in this field), or you can use the "Browse" button and select a media file. Assuming your system has the proper codecs to read it, the duration will be typed into the "Specify Minutes" field automatically.
Then you can specify your Audio Bitrate. If your video has no audio, put 0 in this field. If it has more than one audio track, put the sum of all the bitrates in this field.
Finally, you can type in a Target Size in MB. If you'd rather figure out how big your file will be at a particular bitrate, you can type a bitrate into the Video Bitrate field and the Target Size field will update accordingly, taking the audio bitrate into account as well.
Here is a quick video demonstration to show you how the program works.