How each row is calculated
MP3 and AAC at a fixed bitrate grow by the bitrate divided by eight, in bytes, every second. At 192 kbps that is 24,000 bytes a second, 1.44 MB a minute and 86.4 MB an hour. WAV stores every sample: sample rate × channels × 2 bytes for 16-bit audio, plus a 44-byte header. At 48 kHz stereo that is 192,000 bytes a second, eight times the 192 kbps MP3 rate.
The unchanged M4A copy is different. It keeps the source track's own bitrate, so its size depends on how the recording was made, not on PrivateMP3. Type that bitrate into the calculator. The File check panel shows it for your file after it opens, measured from the track's packets rather than read from a label.
Checked against saved files
The formulas were compared with files saved by the tool in the screen recording test and read back with ffprobe.
| Output | Formula | Saved file | Difference |
|---|---|---|---|
| MP3, 10:00 at 192 kbps | 14,400,000 bytes | 14,400,744 bytes | +744 bytes (0.005%) |
| MP3, 1:00:00 at 192 kbps | 86,400,000 bytes | 86,400,744 bytes | +744 bytes |
| WAV, 10:00, 48 kHz stereo | 115,200,000 + 44 | 115,200,044 bytes | none |
| WAV, 1:00:00, 48 kHz stereo | 691,200,000 + 44 | 691,200,044 bytes | none |
| M4A copy, 1:00:00, AAC measured at 159,937 bit/s | 71,971,650 bytes | 72,676,105 bytes | +704,455 bytes (1.0%) |
The MP3 overhead is a fixed few hundred bytes of header, whatever the length. The M4A overhead grows with length because an MP4 file indexes every audio packet: an hour of 48 kHz AAC has 168,750 packets of 1,024 samples, and four bytes per packet in the size table alone is 675,000 bytes. For planning, the calculator's figure is within about 1 percent.
Worked example: an hour of lecture into a 25 MB email
Google gives the attachment limit for personal Gmail accounts as 25 MB; above it, Gmail replaces the attachment with a Google Drive link. (Gmail Help: attachment size limits)
At 192 kbps, 25 MB holds 17 minutes 21 seconds, so a one-hour MP3 from this site will not fit, and there is no lower bitrate here to change that. Two honest options remain. Send the file as a link instead. Or split it: open the recording, select the first 15 minutes, press Keep only selection and convert, then repeat for the next stretch. Each 15-minute part at 192 kbps is 21.6 MB, under the limit with room for the message. If the recording's AAC track is 128 kbps or lower and you need the whole hour in one file, the unchanged M4A copy is 57.6 MB at 128 kbps: smaller than the MP3, but still over the limit.
Megabytes are counted two ways
This site, the File check panel and most storage makers count 1 MB as 1,000,000 bytes. Some operating systems count 1,048,576 bytes and still write MB. The same 25,000,000-byte file is then shown as about 23.8. When a limit is close, compare bytes, not the label.
When the calculator will be wrong
- Variable bitrate sources. Opus and some AAC encoders change their rate with the sound. In the test, a WebM requested at 96 kbps Opus measured 69 kbps. Use the File check figure, not the setting you remember.
- Cuts. Removed time is not written. Enter the duration that will remain, which the editor shows as Output.
- Channels. MP3 and AAC at 192 kbps are the same size in mono and stereo; WAV doubles from mono to stereo. A 5.1 track exported as WAV keeps six channels and is three times the stereo size.