Calculate bitrate based on assumed size

[jriker1]

I want to use Expression Encoder to encode an existing MP4 file into WMV for better XBOX compatibility. The source is H.264 in the container and is 6,798Kbps 1920x798 at 23.976fps. I've seen some calculators that you give it the final size and it tells you the bitrate to use but it seems inaccurate and why wouldn't the size of the video matter? I would think a 640x480 video would require a different bitrate to create an 8GB file than the one I'm referencing here however from what I can see none of the CBR caculators include video size into the equation. Is there a formula or calculator that woudl tell me approximately if I want say an 8GB file what bitrate to use based on the characteristics of the source file?

Thanks.

JR

[J_Darnley]

bitrate = file size / length - audio bitrate

Why do you think resolution comes into it?

[jriker1]

bitrate = file size / length - audio bitrate

Why do you think resolution comes into it?

I would think that the more pixels / larger the size of the video, the bigger the filesize. For example if I save a jpg image at max quality and it's 640x480 and save a jpg image at max quality and it's 1024x768 the 1024x768 image will be larger. I would assume that same concept would relate to video.

Thanks.

JR

Edit: Also does the codec matter? I have an MP4 file that is 8GB and is 6,798Kbps. When I try encoding it with Expression at same size and used 7,300Kbps to try and assure minimal recoding loss, the file size was smaller than the source. I'm using CBR by the way although the source may be VBR. In an MKV container it doesn't specify, however when I put it into a MP4 container it comes up as variable.

[nm]

I would think that the more pixels / larger the size of the video, the bigger the filesize. For example if I save a jpg image at max quality and it's 640x480 and save a jpg image at max quality and it's 1024x768 the 1024x768 image will be larger. I would assume that same concept would relate to video.
It does, but constant quality encoding is a different case and it doesn't concern the filesize-bitrate relationship.

Edit: Also does the codec matter?
Nope, unless it's not respecting your bitrate request.

I have an MP4 file that is 8GB and is 6,798Kbps. When I try encoding it with Expression at same size and used 7,300Kbps to try and assure minimal recoding loss, the file size was smaller than the source.
Well, clearly EE didn't use the bitrate you told it to, or it skipped frames and made the video shorter.

I'm using CBR by the way although the source may be VBR.
Do you mean CFR (constant framerate) instead of VFR? That could also cause undersizing.