Forumula for working out estimated filesize

[nkarnold]

If i know, how long a clip is, what Video and Audio bitrate's im using,
what formula do I need to try and estimate (doesn't have to be 100% bang on) what the output filesize will be

thanks in advance

N

[Doom9]

what the output filesize will beuhh.. and your container would be? and what about your audio codec and encoding mode?There are lots of variables here.

[nkarnold]

uhh.. and your container would be? and what about your audio codec and encoding mode?There are lots of variables here.
sorry.. forgot those.

MP4 container, H264/AVC video codec, AAC audio codec

[Caroliano]

The basic one is: bitrate x lenght = size
Of couse there is container overhead and so on, but this shoud be razoably acurate. Remember of using the correct metric system for each thing. Eg: bit is diferent than byte, etc.

[nkarnold]

The basic one is: bitrate x lenght = size
Of couse there is container overhead and so on, but this shoud be razoably acurate. Remember of using the correct metric system for each thing. Eg: bit is diferent than byte, etc.
thanks
i thought it was that, but i can't seem to fathom something out, probably doing something real dumb.

working with
216kbits/sec video bitrate
76 seconds of video

(216 * 1024) * 76

doesn't comeanywhere near.

[foxyshadis]

That should get you the number of bits. Try:

(216/8)*76 = 2052 kbytes (or /1024 to get 2 mbytes)

[Oline 61]

You could always use the built in calc in MeGUI. IMHO that is the easiest solution.

[Multimon]

Isn't 1 kbps usually 1000 bits and not 1024? (I think I read that once, not as if it would matter for the estimation)

216kbits/sec <-- just the video stream or is audio already added? (200 kbps vid+16 audio?)
Also your result will most likely be a few % larger because of the overhead.

If my guess for the bitrate made above is right you most likely typed that into the encoding apps. But ratecontrol is almost never optimized for such short clips so it might be totally off from the desired bitrate.
If your filesize is nowhere near the calculated result that's probably the reason.
(try with a longer vid?)

Megui's result is a little less than 2 Mb.

[foxyshadis]

There's another thing, in that most codecs can't hit too low (or too high) a bitrate. On a given source, you'll always have upper and lower boundaries. I'm assuming if you're aiming for 216 that you have a small, low-framerate clip, however. 200 is about the lowest I can hit (acceptably) for ntsc-res cartoons heavily filtered and reduced to minimum vfr, with every quality option maxed. Nothing else would even encode at that bitrate with normal resolution.

[JarrettH]

I found that using the MeGUI bitrate calc I could add another 100kbps to whatever value it gave me. It told me 835kbps whereas I could do 950kbps with enough room for the audio. I used HQ-SLOW and touched nothing! Ever since then I've been experimenting, lol

[Doom9]

that calculator works and is very accurate.. provided you configure it properly of course. After encoding, there's some output in the log that tells you what bitrate was requested from the encoder, and what was obtained.. these values should be very close to one another.. if they are not.. blame the encoder itself.. MeGUI does the plain math.. if the encoder doesn't stick to the bitrate it is being fed, then a bugreport for the encoder would be in order.