Here are the basic steps of calculating file size of a video:
First, we have to know the dimension of the video, in other words, the height and width of the video, in pixel, let's take a video that has the width of 320 pixel and the height of 240 pixel:
320 x 240 = 76,800 pixels
Next, mulitply the total pixels with the color depth, assume we have a 24-bit (3 bytes) color depth:
76,800 x 3 = 230,400 bytes
Say the video is playing at 15 frame per second (fps):
230,400 x 15 = 3,456,000 bytes
and playing for about 30 seconds:
3,456,000 x 30 = 103,680,000 bytes (or 103.68 MB)
Of course, video plays with sound, the video uses 16-bit stereo CD-quality sound, which has 44.1 kHz of sampling rate*:
44,100 x 2 bytes (16-bit) x 2 bytes (stereo) x 30 seconds = 5,292,000 bytes (or 5.292 MB)
Finally we got the total file size of the video:
103.68 + 5.292 = 108.97 MB
Note that the size of the video images is calculated separately from the size of the video sound.
*1 - The FM frequency that familiar to us uses only 11.025 kHz of sampling rate
*2 - Mono sound uses only 1 bytes per sampling
