Reset to defaults
Need help?Compress MP3 files online, free. This MP3 compressor re-encodes your audio at a lower data rate and keeps the .mp3 extension: upload one file or a batch, then choose how to shrink it — to a percentage of its present size, to an exact size in MB, or to a bitrate you name. Switch to mono or lower the sample rate for speech, and download the smaller files.
How to compress MP3
- Upload the MP3 files: Drag your files onto the page or click Add Files. Several files can be compressed in one job with the same settings.
- Choose how much to shrink: File Size Percentage is selected by default at 80 % of the original. Move the slider, or switch to Specific file size to type a target in MB, or to Constant Bitrate, Variable Bitrate or Custom Bitrate to set the data rate yourself.
- Compress and download: Optionally set Audio Channel to Mono or lower the Audio Sample Rate for spoken-word recordings, then click Compress and download each file when it is ready.
Three ways to shrink an MP3
| Setting | What you enter | What the encoder does | Visible cost |
|---|---|---|---|
| File Size Percentage (default, 80 %) | A share of the current size, 1–100 % | Works out the bitrate that lands at or under that share | Proportional: 50 % roughly halves the bitrate |
| Specific file size | A number in B, KB or MB (8 MB is pre-filled) | Derives the bitrate from the file's duration so the result fits; a request above the codec's ceiling is capped at 320 kbps | Depends on length — a long recording forced into a small target gets a low bitrate |
| Constant / Variable / Custom Bitrate | A fixed rate (8–320 kbps), one of ten VBR ranges from 45–85 kbps to 220–260 kbps, or any typed value | Encodes at that rate; VBR spends more bits on complex passages and fewer on quiet ones | You decide: 128 kbps is clean for most music, 64 kbps mono is enough for speech |
A Quality Preset ladder (Highest to Lowest) is also offered.
Audio Channel, Audio Sample Rate and Trim
Audio Channel is Original by default; Mono halves the data a stereo file carries and is the single biggest saving for interviews, lectures and voice memos. Audio Sample Rate is Original by default, with 8, 12, 16, 24, 44.1 and 48 kHz on offer; a lower rate removes high frequencies that speech does not need and music does. If the bitrate you ask for is too low for the current sample rate or channel count, the compressor lowers them for you and says so in the result's notes; if it is too high, it is capped at the codec's maximum. Trim, Unchanged by default, takes a start time and a duration — in seconds or HH:MM:SS.sss — to compress only part of a file.
Hitting a size limit
When an attachment is refused for size, Specific file size is the direct route: enter a figure a little under the limit so that tags and container overhead do not push the file over. Mail services generally allow about 25 MB per message, Discord's free tier about 20 MB, and WhatsApp allows up to 2 GB when the file is sent as a document. For long recordings, combine the size target with Mono so the bitrate stays usable.
When compression hurts
MP3 is already lossy, so every re-encode discards more detail. Music stays clean at 128 kbps and above; below about 96 kbps cymbals, reverb and dense mixes begin to smear. Speech tolerates far more: 64 kbps mono, even 32 kbps mono at a lower sample rate, remains intelligible. If you still have the WAV or FLAC master, encode from that instead of compressing twice. Compression cannot add detail back, so a low-bitrate file set to a higher bitrate only grows.
Frequently Asked Questions
How much smaller will my MP3 get?
With the default 80 % setting, about a fifth is removed. Choosing a bitrate half the original roughly halves the size; switching stereo speech to mono halves it again.
Will my ID3 tags survive?
Title, artist, album and the other tags are copied from the source into the compressed file; only the audio stream is re-encoded.
CBR or VBR?
Constant Bitrate gives a predictable size and suits platforms that expect a fixed rate. Variable Bitrate gives better quality for the same average size and is the better choice for music you keep. The exact-size and percentage modes choose the rate for you.
Can I compress several files at once?
Yes. All files in the job use the same settings and are downloaded individually when done.
Is anything stored on your servers?
Files are processed on our servers and deleted automatically after a few hours.
Related tools
- Audio Compressor — the same controls for WAV, M4A, FLAC, OGG and other formats
- Compress WAV — shrink WAV files while keeping the .wav extension
- Compress M4A — for Apple AAC audio
- FLAC to MP3 — encode an MP3 from a lossless master instead of re-compressing
- Audio Cutter — cut a long file into pieces before compressing
References
- ISO/IEC 11172-3 (MPEG-1 Audio, Layer III): https://www.iso.org/standard/22412.html
- LAME MP3 encoder documentation: https://lame.sourceforge.io/