Why WAV files are so big, and what MP3 does about it
A WAV file stores every sample as a plain number, with no compression at all. CD-quality stereo (16-bit, 44.1 kHz) takes about 10.6 MB per minute, and the 24-bit and 32-bit float files that recorders and music software export are half as big again or double. MP3 keeps what the ear notices and drops what it doesn't, so the same minute fits in 1 to 2 MB. That's the difference between a recording you can email and one you can't.
| Format | Size per minute | Our 20-second test file |
|---|---|---|
| WAV, 16-bit stereo | 10.6 MB | 3.53 MB |
| WAV, 24-bit stereo | 15.9 MB | 5.29 MB |
| WAV, 32-bit float stereo | 21.2 MB | 7.06 MB |
| MP3, 256 kbps stereo | 1.92 MB | 641 KB |
| MP3, 128 kbps stereo or mono | 0.96 MB | 321 KB |
MP3 is lossy, so keep the WAV if you'll edit the recording again. For listening and sharing, 256 kbps stereo is very hard to tell apart from the original, and that's the default here.
32-bit float WAVs can go above full scale
Field recorders that record in 32-bit float, and music software exporting a mix, can produce peaks louder than "full scale", the ceiling a normal audio file can hold. A float WAV keeps them, so nothing sounds wrong in your editor. The MP3 encoder works with ordinary samples, though, and anything above the ceiling gets cut off flat: a harsh crackle on every loud peak.
We measured it with a test file peaking 4 dB above full scale. Converted as it was, the MP3 came back with 301,850 samples pinned flat at the ceiling. With the default setting, the converter finds the loudest point first and lowers the whole file just enough to put it at −1 dB, 5 dB in this case. The resulting MP3 had no clipped samples at all. Each result says how far the peaks went and how much the volume was lowered. A WAV that never goes above full scale is left at its own level.
"Stereo" recordings that are really mono
Lots of WAV files have two channels without any stereo in them. Phone recorders, video calls and interview kits often write the same signal to both sides, and an MP3 of that spends half its bytes on a copy. The converter compares the two channels sample by sample. When they're identical, the MP3 is mono: our 20-second test came out at 321 KB instead of 641 KB, with the same sound.
The other common case is a lavalier or USB microphone plugged into one input, so the voice is on the left and the right is silent. Played as stereo, it sounds as if it's coming from one earbud. The usual fix, mixing the two sides down to mono, averages the voice with silence: on our test file the mono version came out 6.5 dB quieter, at half the signal level. Here, a silent side is detected and the mono MP3 is taken from the channel that has the sound, at its original level. You can still choose Always stereo if you need both channels as they are.
Choosing the bitrate
| MP3 bitrate | Size per minute | Good for |
|---|---|---|
| 64 kbps mono | 0.48 MB | Voice notes, lectures, meeting recordings |
| 128 kbps mono | 0.96 MB | Interviews and podcasts where quality matters |
| 128 kbps stereo | 0.96 MB | Music for background listening, upload limits |
| 256 kbps stereo | 1.92 MB | Music, mixes, demos you send to others (default) |
| 320 kbps stereo | 2.4 MB | The highest MP3 allows; little audible gain over 256 |
If a file has to fit under a limit, divide the limit by the length: a 4-minute song under 10 MB can go up to 320 kbps, while a 60-minute interview under 25 MB needs 48 kbps mono or lower.
Tags and timing
If the WAV carries a title, artist, album or date (the "INFO" fields many recorders and editors write), they're copied into the MP3's ID3 tag, and the result lists which ones came across. MP3 encoders add about 25 ms of silence at the start. This converter writes the header that tells players to skip it, so the MP3 starts on the same sample as the WAV. In our tests the start matched to the sample in stereo, mono and single-channel files.
Your recordings stay on your device
The WAV is read and encoded inside your browser, using a WebAssembly build of the LAME MP3 encoder. Nothing is uploaded, there's no account, and there's no limit on file size beyond what your device's memory can hold. Long recordings take a little longer, but they never leave your computer.