How to decode Base64 to MP3
- Paste the Base64 into Base64 or data URI. It can be bare Base64 or a full
data:audio/mpeg;base64,…URI, and line breaks do not matter. - Press play in the player under the file info. Duration appears as soon as the browser has read the start of the file.
- Check the note area: if the audio is not MP3, it says what it is.
- Press Download file to save the .mp3, or Copy as data URI for an
<audio>element.
Everything happens in your browser; the page may not make network requests other than Cloudflare’s page-view count, so the recording stays on your machine.
How the tool recognizes an MP3
An MP3 file has no single fixed signature. It either starts with an ID3v2 tag, or directly with the header of the first audio frame:
| Base64 starts with | First bytes | Meaning |
|---|---|---|
SUQz | ID3 | ID3v2 tag in front of the audio: title, artist, often cover art |
//s or //o | FF FB, FF FA | MPEG-1 Layer III frame, the usual MP3 at 32 to 48 kHz |
//M or //I | FF F3, FF F2 | MPEG-2 Layer III, lower sample rates (16 to 24 kHz), common for speech |
/+M | FF E3 | MPEG-2.5 Layer III, 8 to 12 kHz |
//E or //k | FF F1, FF F9 | Not MP3: this is AAC in an ADTS stream |
Text-to-speech output and voice recordings often have no ID3 tag, so they start with //M or //s straight away. The cover art in an ID3 tag can be most of the file; MP3 to Base64 explains how to strip it before encoding.
When it is not an MP3
Data labelled MP3 is sometimes a different format, and players that trust the extension then refuse it. The tool reports the actual format with a button to save it correctly:
| Note | What it is |
|---|---|
| “This looks like M4A, not MP3.” | AAC audio in an MP4 container, as recorded by phones and many apps |
| “This looks like WAV, not MP3.” | Usually uncompressed audio: at CD quality about eleven times the size of a 128 kbit/s MP3 |
| “This looks like OGG, not MP3.” or “…Opus, not MP3.” | Ogg Vorbis or Opus, common in web and chat apps |
| “This looks like WebM, not MP3.” | A browser recording, usually Opus inside WebM |
| “This looks like AAC, not MP3.” | A raw AAC stream (.aac) |
Press Download as .m4a (or whichever extension is offered) to save it with the extension that matches the content. Converting to a real MP3 needs an encoder such as ffmpeg: ffmpeg -i input.m4a -b:a 128k output.mp3.
Duration and bitrate
The duration comes from the browser’s media engine, which reads the frame headers. For a constant-bitrate file it is exact. A variable-bitrate MP3 normally carries a Xing or VBRI header with the frame count; if that header is missing, the browser estimates the length from the first frames, and the figure can be off by a lot until playback reaches the end.
You can sanity-check the decoded size against the length: at 128 kbit/s one minute of audio is 960,000 bytes, about 1.28 million characters of Base64. The size calculator does the Base64 part for any size.
Decode an MP3 and read its duration on the command line
base64 -d voice.b64 > voice.mp3
ffprobe -v error -show_entries format=duration,bit_rate -of default=nw=1 voice.mp3
For a three-second test tone at 128 kbit/s, ffprobe prints duration=3.030204 and bit_rate=129216; the extra 30 ms and the slightly higher rate come from the encoder’s padding and the ID3 tag. More on decoding in a shell on Base64 in Bash, and on other audio formats on Base64 to audio.
FAQ
How do I convert Base64 to an MP3 file?
Paste the Base64 above, check that it plays, and press Download file. The file is saved as decoded.mp3 unless a data URI or the text file you opened supplies a name.
Why is the sample on this page silent?
It is one second of real MPEG audio frames containing silence, there to show the player and the detection. Paste your own Base64 to hear actual sound.
Is `audio/mp3` a valid MIME type?
The registered type for MP3 is audio/mpeg. Browsers accept data:audio/mp3;base64,… anyway, and the tool decodes it without a warning, but the download and Copy as data URI use audio/mpeg.
Does the tool show the title and artist?
No, the ID3 tag is not displayed. It is kept in the downloaded file, so a music player shows the title, artist and cover art as usual.