Duration has the largest effect
At a fixed bitrate, an hour of audio needs six times as much space as ten minutes. This makes duration the first value to check when planning storage.
Choosing an MP3 bitrate
128 kbps creates the smallest of the common options, 192 kbps offers a useful everyday balance, and 320 kbps creates the largest file.
Why the final size may vary
Metadata, artwork, variable bitrate encoding and container overhead can change the finished size slightly. The result is a planning estimate.
Quality cannot be restored
A larger output bitrate cannot recover detail missing from the source. Choose a bitrate that suits the original material and your playback needs.