Size is just bitrate times duration
File size in bits equals total bitrate multiplied by seconds. Divide by eight for bytes. Everything else — resolution, codec, frame rate — matters only insofar as it determines what bitrate you need for acceptable quality.
This is why a two-hour film at 5 Mbps and a two-hour film at 50 Mbps differ tenfold in size while both running the same length.
Choosing a bitrate
As rough guidance for H.264: 1080p at 30 fps looks good around 5 Mbps, 720p around 2.5 Mbps, and 4K needs 15–25 Mbps. Modern codecs like H.265 and AV1 achieve similar quality at roughly half the bitrate, at the cost of much slower encoding.
Content matters too. Fast motion, grain and rapid scene changes need far more bitrate than a static talking head at the same resolution.
Frequently asked questions
Is variable bitrate more efficient? +
Yes. VBR allocates more data to complex scenes and less to simple ones, giving better quality at the same average size. These estimates assume a constant average, which is what VBR targets anyway.