MP3 compression
MP3 compressor
Making an MP3 smaller means re-encoding it at a lower bitrate, converting it to mono, or lowering the sample rate. All three discard audio permanently — there is no lossless way to shrink an MP3.
…or drag files and folders here. Add as many as you like — everything is processed on this device.
.aac .aif .aifc .aiff .flac .m4a .mp3 .oga .ogg .opus .wav .wave
In one paragraph
Every honest MP3 compressor does the same thing: decode the file and encode it again with fewer bits. That is a second lossy generation, so the goal is to give away as little as possible for the size you need. The three levers, in order of how much quality they cost per megabyte saved, are bitrate, channel count, and sample rate.
Sonic Batch shows you the source bitrate before you choose, estimates the output size as you move the controls, and offers a target-size mode that searches for the bitrate which lands closest to a size you name. The whole process runs in your tab — useful when the files are client recordings, interviews or unreleased music.
Does this change the audio quality?
Re-encoding an MP3 is a second-generation lossy operation: the encoder is compressing audio that already has compression artefacts. Going from 320 to 192 kbps is usually inaudible on consumer playback; going from 128 to 64 kbps is not.
- There is no way to reduce an MP3’s size without discarding audio. Any tool claiming lossless MP3 compression is either stripping metadata or misdescribing what it does.
- Converting stereo to mono halves the data and is often the best trade for speech — but it is irreversible.
- Target-size mode runs a bounded search (at most five encode attempts) and reports the size it actually achieved, not the one you asked for.
A worked example
Getting a 45-minute interview under a 25 MB email limit
- Drop the MP3 in. The panel shows it is 128 kbps stereo, 41 MB.
- Switch to Target size mode and enter 24 MB.
- Set Channels to Mono first — for a single-voice interview that alone halves the data requirement, so the search lands on a higher effective quality.
- Convert. The result panel reports the bitrate chosen, the achieved size and the tolerance.
Reach for mono before you reach for a lower bitrate
For speech, converting to mono is close to free: a single microphone recording has no stereo information worth keeping, and it halves the bits needed. After that, 96 kbps mono is good speech and 64 kbps mono is acceptable. For music, keep stereo and step down gradually — 320 → 256 → 192 → 160 kbps. Below 128 kbps stereo, music starts to sound obviously processed, and no encoder setting fixes that.
Supported formats, limits and browser notes
Sonic Batch decodes with the browser when it can (faster, no engine download) and falls back to the self-hosted WebAssembly engine when it cannot. Every combination below works in every listed browser; the column only tells you which decoder does the work.
| Input | What it is | Chrome | Firefox | Safari | iOS |
|---|---|---|---|---|---|
| MP3 | The universal lossy format. Plays on essentially every device made since 1998. | Native | Native | Native | Native |
| WAV | Raw PCM samples with a small header. Perfect fidelity, roughly 10 MB per stereo minute. | Native | Native | Native | Native |
| FLAC | Lossless compression, typically 50–60% of WAV size, bit-identical when decoded. | Native | Native | Native | Native |
| M4A | Apple’s default AAC container. Better quality than MP3 at the same bitrate. | Native | Native | Native | Native |
| AAC | Bare AAC frames with no MP4 wrapper — used by broadcast streams and some hardware. | Native | Partial | Native | Native |
| OGG | Royalty-free lossy codec. Standard in game engines, Godot, Unity and Wikimedia. | Native | Native | Engine | Engine |
| Opus | The best-sounding lossy codec below 128 kbps. Used by WhatsApp, Discord and YouTube. | Native | Native | Partial | Partial |
| AIFF | Apple’s uncompressed PCM format — the WAV equivalent for Logic and Pro Tools. | Partial | Partial | Native | Native |
| Output | Encoder | Bitrates / depth | Sample rates | Tags |
|---|---|---|---|---|
| MP3 | libmp3lame | 32–320 kbps | 8–48 kHz | Yes + artwork |
- Max per file
- 400.0 MB
- Max per queue
- 4.00 GB
- Max duration
- 4:00:00
- Max archive
- 1.50 GB
These are enforced ceilings, checked before any decoding starts, not measured maximums — a file over the limit is rejected rather than allowed to exhaust the tab’s memory. Real-world capacity is lower on older phones, which is why the scheduler runs one file at a time on iOS and Safari.
Where your audio goes
Nowhere. Your file is read from disk into this browser tab with the File API, decoded and encoded by code running in that same tab, and handed back to you as a download. It is never transmitted.
That is a structural property of the application, not a policy promise: there are no API routes, no server actions and no third-party processing services in this project, so there is nothing capable of receiving a file. The page’s Content-Security-Policy also restricts connections to this origin, and the automated test suite includes a browser test that fails if any network request during import, preview, conversion or download carries media bytes.
The trade-off, stated plainly:results live in your tab’s memory until you download them. Closing or reloading the page discards them. There is nothing on a server to recover, because nothing was ever put there.
Troubleshooting
The output is bigger than the input.
You have set a bitrate above the source bitrate. Re-encoding upwards cannot add quality and only wastes space. The panel warns when the target exceeds the source.
Target size mode missed my number.
MP3 bitrate is quantised to a fixed ladder, so an exact size is not always reachable. The search picks the highest bitrate that stays under your target and reports the achieved size honestly rather than claiming a hit.
The compressed file sounds watery or metallic.
That is generational loss — MP3 artefacts being re-encoded by MP3. Try mono at a higher bitrate instead of stereo at a low one, or compress from a lossless original if you still have it.
Frequently asked questions
How do I reduce MP3 file size without losing quality?
You cannot. Removing metadata and cover art saves a few hundred kilobytes at most; beyond that, size reduction means discarding audio. What you can do is choose the reduction that costs least — usually mono for speech.
How much smaller can I make it?
The output size is roughly bitrate × duration ÷ 8. A 10-minute file at 128 kbps is about 9.6 MB; at 64 kbps mono it is about 4.8 MB. The estimate updates live as you adjust the controls.
Does this compressor work on other formats?
It accepts WAV, FLAC, M4A, OGG and Opus as input and always writes MP3. For a general compressor that keeps the source format, use the audio compressor page.
Are my files uploaded to compress them?
No. Decoding and re-encoding both happen in your browser tab, and the app has no upload endpoint at all.