🎵 MP3 Compressor

Shrink audio files with adjustable bitrate — no upload, no signup, no watermark

✓ Free Forever · ✓ No Signup · ✓ No Upload · ✓ Works Offline

🔒 Your image never leaves your device. All processing happens locally in your browser using FFmpeg.wasm + LAME MP3 encoder (runs entirely in your browser). No upload, no server, no tracking.

Features

  • Zero Upload — Your audio file never leaves your device. Encoding runs on your CPU via WebAssembly — verifiable in DevTools Network tab.
  • Adjustable Bitrate — Pick anywhere from 64 kbps (small voice files) to 320 kbps (near-lossless music). Preview before downloading.
  • Universal MP3 Output — Output is standard MP3 — playable on every device, phone, car stereo, and DAW. ID3 metadata preserved.
  • No Signup, No Watermark — No account, no audio watermark, no time limit, no email required.

How It Works

  1. Drop your audio file — MP3, WAV, FLAC, M4A, AAC, OGG, OPUS, WMA — any common format. Up to 500 MB.
  2. Pick a bitrate — 128 kbps = streaming standard. 64-96 kbps for voice. 192-320 kbps for music you want to keep.
  3. Wait for encoding — FFmpeg.wasm loads once (~30 MB), then encodes on your CPU. Typical 5-minute song takes 10-30 seconds.
  4. Compare and download — Listen to original vs. compressed side-by-side, then download the MP3.

Use Cases

  • Shrink audio under email attachment limits (Gmail 25 MB, Outlook 20 MB)
  • Send voice memos via WhatsApp / Telegram / Discord without quality loss from auto-compression
  • Compress podcast episodes before publishing to your host
  • Save phone storage — recompress old voice recordings before archiving
  • Convert FLAC / WAV music libraries to portable MP3

Frequently Asked Questions

Does my audio file upload to your server?

No. The audio stays in your browser's memory the entire time. The only network request is loading the FFmpeg engine (~30 MB) from a public CDN the first time you use the tool. Open DevTools → Network tab to verify — you'll see the FFmpeg files load, but your audio file never sends bytes anywhere.

Which bitrate should I pick?

128 kbps is the streaming standard — Spotify Free, YouTube Music's default. 64-96 kbps is fine for spoken word (podcasts, voice memos, lectures). 192-256 kbps for music you'll keep long-term. 320 kbps is the MP3 ceiling — useful if the source is lossless and you want minimal degradation.

What's the difference between bitrate and CRF?

Bitrate (kbps) sets the size budget per second of audio. Higher = larger file, better quality. MP3 doesn't use CRF (a video codec concept) — it uses CBR (constant bitrate) or VBR. This tool uses CBR for predictable file sizes.

Why output MP3 and not FLAC, AAC, or Opus?

MP3 is the only audio format that plays on literally every device — old car stereos, smart TVs, voice recorders, DAWs, phones. Newer formats (Opus, AAC) compress better but aren't as universally supported.

Does this preserve metadata (artist, title, album)?

Yes — ID3v2.3 tags are copied from the source file (`-map_metadata 0 -id3v2_version 3`). If your source has cover art and tags, the output will too.

Why is it slower than online services?

Most online compressors run on servers with dedicated CPUs. ToolKoala runs LAME (the MP3 encoder) inside WebAssembly on your laptop's CPU. The trade-off is privacy — but the speed is still reasonable: a 5-minute song typically encodes in 10-30 seconds.

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