# Compress Audio > Compress Audio is a browser tool that re-encodes audio files as MP3 at a chosen bitrate. The WhatsApp preset automatically calculates the highest bitrate that keeps the output under 16 MB. Small (96 kbps) and Medium (128 kbps) presets apply a fixed bitrate, and the Custom mode lets you dial in any value from 32 to 320 kbps. Processing runs entirely in the browser via the Web Audio API and lamejs — no file ever leaves your device. Live tool: https://lofttools.com/tools/audio-video-tools/compress-audio Category: Audio and Video ## What it does - WhatsApp preset — auto-fits the highest MP3 bitrate that keeps the file under 16 MB - Small preset — 96 kbps (podcast / voice-note quality) - Medium preset — 128 kbps (streaming quality) - Custom bitrate — any value from 32 to 320 kbps - Input formats — MP3, WAV, M4A, FLAC, OGG, Opus, AAC, WebM - Output — MP3 via lamejs (browser-native, no upload) - Receipt panel shows original size → compressed size + savings - Batch mode — compress multiple files at once, download as ZIP - Up to 100 MB per file ## How it works 1. **Drop your audio file** — Drag and drop one or more audio files (MP3, WAV, M4A, FLAC, OGG, Opus, AAC, WebM) onto the upload area, or click to browse. 2. **Choose a compression target** — Pick WhatsApp (≤16 MB auto-bitrate), Small (96 kbps), Medium (128 kbps), or Custom (32–320 kbps). 3. **Compress** — Click Compress — the file is decoded and re-encoded as MP3 locally in your browser. 4. **Download** — Single files download directly. Batch compressions are packaged as a ZIP. ## FAQ ### How does the WhatsApp preset work? It calculates the highest MP3 bitrate that keeps the output file under 16 MB (WhatsApp's voice note and audio message limit) using the audio duration. A 5% safety margin prevents edge-case overflows, and bitrate is clamped between 32 and 192 kbps. ### What is the output format? All outputs are MP3, encoded in-browser via lamejs. MP3 is universally accepted by messaging apps, email clients, and media players. ### Does my audio file upload anywhere? No. The Web Audio API decodes the file locally and lamejs re-encodes it in the same browser tab. No data leaves your device. ### Can I compress multiple files at once? Yes. Drop multiple files or use the file picker to select a batch. Each file is compressed in sequence and the results are bundled into a single ZIP. ### How much smaller will my file get? It depends on the source format and the target bitrate. A 320 kbps MP3 compressed to 128 kbps saves roughly 60% of the file size. WAV and FLAC sources can save 90%+ when compressed to 128 kbps. ## Tips - **Fit WhatsApp in one click** — WhatsApp rejects audio files over 16 MB. The WhatsApp preset calculates the highest bitrate that keeps the output under that limit automatically — no manual trial and error. - **128 kbps is transparent for speech** — Human speech, podcasts, and voice notes are perceptually indistinguishable at 128 kbps. Use 96 kbps for even smaller files when the audio is mono speech. - **Compress losslessly first, then to MP3** — If you're starting from a WAV or FLAC, compress it once to MP3 at the highest bitrate that meets your size target. Re-compressing an already-lossy MP3 degrades quality further — start from the lossless source when possible. ## Privacy — what we do not do This tool runs entirely in the browser via WebAssembly. Your file never reaches a Loft Tools server. Specifically: - **No upload.** The file bytes load into the browser tab's memory and process on your own CPU. Open DevTools → Network and observe zero outbound requests carrying file data while Compress Audio runs. - **No AI training on your file.** Loft does not train models. We could not train on a file we cannot see. - **No content scanning.** No virus, copyright, or content-moderation pass against your file. The bytes are not accessible to us. - **No server-side log of file contents, filenames, or EXIF metadata.** Cloudflare edge captures URL and truncated IP for abuse defense (standard CDN behaviour). Cloudflare Web Analytics records anonymous page hits, no cookies, no PII. Nothing about your file content reaches any log. - **No retention.** Close the tab and the file leaves browser memory. No backups exist on our side because no copy ever existed on our side. - **No account.** No email, no signup, no auth, no telemetry tied to you. - **Offline-capable after first visit** (PWA). Once you've loaded a tool, it caches; later sessions work without internet. For high-sensitivity files, run the tool once online to warm the cache, then disconnect before processing. Compare with upload-based services: each transmits your file to a processing server. Even over HTTPS, each has logs, retention windows, and subpoena exposure. Loft has none of these because the server architecture does not include your file. ## More - All tools: https://lofttools.com/tools - Category: https://lofttools.com/tools/audio-video-tools - LLM index: https://lofttools.com/llms.txt