This calculator is intended to make bitrate calculations for encoding movies easier. The most common use is probably to determine the required video bitrate to fill a fixed-size medium like a CDR or DVD+R, given a fixed audio bitrate and duration of the movie. To do this, enter the movie's duration, audio bitrate and target size, and press the “Time,size,audio → video” button. You can do the same for audio.
Another useful scenario is to determine whether you can fit a video into a fixed-size medium like a CD-R or DVD-R at acceptable quality. Or likewise, if you want to avoid wasting a few hours of download time on a crappy rip made by someone who has no idea how to determine a good video bitrate. To do this, do the same as above and write down or memorize the video bitrate. Then, enter the film's image dimensions and framerate in the lower part of the calculator. Then click one of the ‘MP4’ or ‘H.264’ buttons (see instructions below). If the new video bitrate is much lower than the previous one (say less than 75%), the video quality will probably be unacceptably bad. Just to give an example: there is no way to fit a normal feature-length film on a single CD-R at anything higher than DVD resolution without making it look like crap, so please don't try it.
Mind that sizes are given in two ‘flavours’. If you don't know the difference between a MB and a MiB, check out my other page that explains them. Unfortunately a lot of software still uses the ‘MB’ symbol while they actually mean MiB. In case of doubt, assume KiB, MiB and GiB values: if the software does use KB, MB and GB your file will be slightly too small, which is not as bad as too large.
Overhead is what is left of the file after removing actual video and audio data. This is heavily dependent on the container format, codecs and parameters, and includes any extra streams like subtitles. In case of doubt and if the file must fit within a certain size, it's better to use a conservatively high overhead estimate.
The ‘Suggest bitrate’ buttons will try to guess an OK minimum average video bitrate for a movie, based on the given dimensions and framerate. Use the ‘MP4’ button if you're going to encode with Xvid or DivX. For H.264, use the ‘Base’ button if you don't enable any fancy options like trellis, CABAC, RD etc. Use the ‘High Profile’ button if you enable most or all of those features. The calculation assumes you're using double-pass encoding and a ‘typical’ framerate (around 25fps).
Mind that this is a very inaccurate guess because the actual required bitrate depends heavily on the contents of the video. A few checkboxes are provided to allow tuning the guess somewhat: check the ‘CGI’ box if the film consists purely or mostly of smooth computer-generated images (like Toy Story, Ice Age etc.); check the ‘Dark’ box for films that have many dark shots in which a large part of the image is almost pure black (e.g. Dark City). Check the ‘Noisy’ button if the image is noticeably noisy throughout, has a lot of fast movement, and/or contains a lot of footage of trees, bushes or other cluttered things. For a movie like Avatar, you should check this: even though it is mostly CGI, the images are very detailed and there is a lot of action.
Keep in mind that this is all just guesswork: the main purpose of this feature is to check if the bitrate you're going to use is reasonable. If you encode a film with an actual video bitrate more than twice what this estimate gives, you're probably wasting disk space. If your bitrate is considerably lower than this estimate, the quality of the encode will probably be bad. How bad depends on the codec: H.264 and similar modern codecs are very capable at gracefully degrading the image, therefore even if you go far below what this calculator suggests it may still look OK on its own. It will however look obviously washed-out when compared to an encoding at higher bitrate. There are exceptions, for instance computer animations with very clean images and generally static backgrounds (e.g. the Toy Story series,) can fit in a surprisingly low bitrate without obvious quality loss.
If you are encoding a film and it doesn't really matter how large the resulting file is yet you don't want to waste disk space, consider using quality-based encoding, which in most cases makes a lot more sense than using a fixed bitrate. A fixed bitrate only makes sense for streaming or storage on a fixed-size medium.
If you're planning to encode multiple movies that must fit within a specific size, e.g. three movies in 4.7GB, do not just encode each movie to be 4.7/3 = 1.56GB. That does not make sense unless they are all the same length and image size. To get a sensible idea of what the relative sizes of the movies should be, use the ‘Suggest bitrate’ feature to determine their ‘ideal’ sizes, and then try to divide the total available size in chunks that have the same proportions. E.g. if the calculator says that film A should be 3GB, film B 2GB and film C 1.5GB and you want to squeeze all three on a single 4.7GB DVD-R, you should try to make them respectively 2.17GB, 1.45GB and 1.08GB.