Compress MP3 files without uploading them
This MP3 compressor makes MP3 files smaller by re-encoding them at a lower bitrate. Pick a bitrate such as 64 kbps for a lecture or 128 kbps for music, or type the size you need — say 5 MB for an email attachment — and the tool works out the highest bitrate that fits each file. Mono and a lower sample rate squeeze speech recordings further, and the ID3 tags and cover art are copied into the new file.
Everything happens in your browser: each MP3 is decoded with Mediabunny (MPL-2.0), resampled, and encoded on your device with LAME (LGPL), the open-source MP3 encoder, compiled to WebAssembly. Batches download one by one or as a ZIP.
How to use the MP3 Compressor
- Drop one or more MP3 files on the box (WAV, FLAC, M4A and Ogg come out as MP3 too). Each row shows the length, sample rate, channels, bitrate, size, tags and cover art.
- Choose Bitrate and pick a value from the list, or choose Target file size and type how many megabytes each file may use.
- Set Channels to mono for speech and leave Sample rate on Auto unless a device needs a particular rate. The summary shows the estimated result before anything is encoded.
- Press Compress. The progress bar follows each file, and Cancel stops at any time.
- Play the original and the compressed version side by side, then download each file or press Download all (ZIP).
MP3 Compressor features
Bitrate or exact size
Constant-bitrate MP3 from 32 to 320 kbps, or a size in MB per file. Every file in a batch gets the highest standard bitrate that fits its own length.
Clean resampling
Lower sample rates go through a windowed-sinc filter that removes everything above the new limit first, so treble does not fold back as harsh aliasing.
Speech mode
Mono gives the encoder half as much to describe, so at the same bitrate a voice recording sounds noticeably cleaner than in stereo.
Tags and cover art kept
Title, artist, album, track number, genre, comments, lyrics and other ID3 frames are copied, and the cover image is embedded once.
Honest results
Files already at or below your bitrate are left alone, and if a result would be bigger than the original you are told to keep the original.
Which MP3 bitrate should I choose?
Size per minute is the bitrate × 60 seconds ÷ 8 bits per byte: 128,000 × 60 ÷ 8 = 960,000 bytes, just under 1 MB. The audio file size calculator does the same sum for any length and format.
Re-encoding is lossy: each pass through an MP3 encoder discards detail for good. Taking a 320 kbps song to 160 kbps is rarely noticeable; taking 128 kbps stereo to 64 kbps stereo is. Speech is far more forgiving — mono at 48–64 kbps usually sounds as clear as the original.
| Bitrate | Size per minute | Good for |
|---|---|---|
| 32 kbps mono | 0.24 MB | Voice notes, phone calls, dictation |
| 64 kbps mono | 0.48 MB | Lectures, audiobooks, spoken podcasts |
| 96 kbps | 0.72 MB | Talk radio, background music |
| 128 kbps | 0.96 MB | Most music for phones and earbuds |
| 192 kbps | 1.44 MB | Music on good headphones |
| 320 kbps | 2.4 MB | The highest bitrate MP3 allows |
How the target size is reached
An MP3’s size is almost exactly bitrate × duration ÷ 8, plus the ID3 tag and cover image. The compressor subtracts the tag, divides the rest by the file’s length and rounds down to a standard MP3 bitrate (32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 or 320 kbps), so a file lands at or a little below the target.
Example: a 10-minute podcast must fit a 5 MB email limit. 5,000,000 × 8 ÷ 600 s ≈ 66.7 kbps, so it is encoded at 64 kbps and comes out at about 4.8 MB. A result still over the target is encoded again one step lower.
Sample rate, mono and the Auto setting
At low bitrates an encoder cannot afford to describe the highest frequencies anyway, so lowering the sample rate spends the bits where they matter. Auto is modelled on LAME’s own lowpass table: the original 44.1 or 48 kHz from 112 kbps (stereo), 32 kHz at 80–96 kbps, 24 kHz at 56–64 kbps, 22.05 kHz at 48 kbps and 16 kHz below that. A mono file counts double, so 64 kbps mono is treated like 128 kbps stereo. The rate is never raised above the original.
MP3 has two families: 32–48 kHz files use 32–320 kbps, while 16–24 kHz files use 8–160 kbps, so 192 kbps at 22.05 kHz becomes 160 kbps (the plan says so). The AAC compressor makes M4A files instead, and the audio cutter trims silence before you compress.
MP3 Compressor FAQ
How do I compress an MP3 file?
Drop the MP3 on this page, choose a bitrate — 128 kbps suits most music, 64 kbps mono suits speech — or a target size in MB, and press Compress. Listen to the result in the player, then download it.
Does compressing an MP3 reduce its quality?
Yes, a little. MP3 is lossy, so re-encoding at a lower bitrate removes detail that cannot be brought back. Keep the original if you may need full quality later; at 128 kbps and above most people hear no difference on earbuds.
How do I make an MP3 small enough to email or share?
Choose Target file size and enter the limit with a little headroom, for example 4.5 MB for a 5 MB limit. For long recordings also choose mono; the bitrate and sample rate are picked to fit.
Is this MP3 compressor free and private?
Yes. It is free with no sign-up, watermark or file limit, and nothing is uploaded: decoding and encoding run in your browser.
Why didn’t my MP3 get smaller?
If it is already at or below the bitrate you chose, re-encoding could only lower the quality, so it is kept as it is. If the new file comes out bigger — say 160 kbps from a 128 kbps original — the tool says so and offers the original instead.
Are ID3 tags and album art kept?
Yes. Title, artist, album, track, genre, comments, lyrics and other ID3 frames are copied, and the cover is embedded once. Untick Keep tags and cover art to strip them, which also saves the size of the image.
Can it make VBR MP3s?
No — the LAME build used here writes constant bitrate only. Very small targets are met at 16 kHz, where MP3 allows 8–24 kbps.