How much space does an MP3 need?
Bitrate, duration, and a calculator that makes the tradeoff clear.
The quick answer
For constant-bitrate MP3, approximate size in megabytes = bitrate in kilobits per second × duration in seconds ÷ 8,000. Metadata and encoder padding add a little overhead. Variable-bitrate audio will not match this estimate exactly.
Approximate decimal megabytes. Excludes metadata and container overhead.
Choose for the recording
A speech recording can often use a lower bitrate than detailed music. Start at 96 kbps for speech or 192 kbps for music, then listen. These are starting points, not a promise of transparent quality. Source quality, channels, encoder, and your listening conditions all matter.
Bigger does not mean better source quality
Converting a 128 kbps MP3 to 320 kbps cannot recreate discarded information. Repeated lossy encoding can introduce further artifacts. Keep the original or a lossless master when available.
Put it to use
Compress an MP3 or trim your audio.
Technical reference: FFmpeg codec documentation.