Files never leave your browser

How it works

Every conversion on Konverton runs through FFmpeg WebAssembly — a compiled version of the FFmpeg media processing library that runs directly in your browser tab as a WebAssembly module.

When you upload a file, it is read locally by the browser's File API and passed to FFmpeg's virtual in-memory filesystem. FFmpeg processes it — applying the codec, bitrate, and format settings you chose — and writes the output back to the same virtual filesystem. The converted file is then read back into a browser Blob and made available for download. At no point does the file travel over a network connection.

Each media type (audio, video, image) uses its own FFmpeg instance that loads once and is reused for subsequent conversions in the same session. The version-pinned processing engine is hosted with Konverton and loads on first use; your media file never travels with that request.

Converted files are stored in IndexedDB(your browser's built-in local database) with a 24-hour expiry so you can revisit a download without reconverting. These entries are never transmitted anywhere and are deleted automatically after 24 hours, or immediately when you clear your browser data.

Privacy by design

Most online file converters — CloudConvert, Convertio, OnlineConvert, and others — work by uploading your file to their cloud servers, processing it there, and giving you a download link. This means a third party receives a copy of your file, stores it on their infrastructure for hours or days, and processes it using their compute.

For many files this is fine. For others it is not: private photographs, confidential business documents, unreleased audio recordings, legal filings, medical imagery. The standard privacy policy of these services grants them a broad license to use uploaded content for service improvement, and data breaches affecting cloud storage are a documented, recurring event.

Konverton was built specifically to eliminate this tradeoff. Because WebAssembly makes it possible to run a full-featured media processing pipeline in the browser, there is no technical reason to upload files to a server. The privacy guarantee here is architectural, not just a policy claim: the server has no endpoint that accepts file uploads because one does not exist.

Technology stack

• FFmpeg — the core media processing engine. Open source under LGPL/GPL. The definitive cross-platform tool for audio, video, and image format conversion. ffmpeg.org
• @ffmpeg/ffmpeg — the WebAssembly wrapper that compiles FFmpeg to run in the browser. Maintained by the ffmpeg.wasm project.
• WebAssembly — the browser execution environment that makes it possible to run near-native-speed code in a sandboxed tab. Supported in all modern browsers since 2017.
• Next.js — the React framework powering the frontend. All converter components are server-rendered HTML for fast initial load, with converter UI loaded lazily client-side only when needed.
• Tailwind CSS — utility-first CSS framework for styling.
• IndexedDB / idb — browser-native structured storage used to temporarily hold converted files with a 24-hour TTL.

Browser compatibility

FFmpeg WebAssembly requires SharedArrayBuffer, which browsers only expose in cross-origin isolated contexts. Konverton sets the required Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: credentialless headers on all pages.

Mobile support is functional but constrained by available RAM. Browsers on iOS and Android impose memory limits of roughly 1–2GB per tab. Very large video files (above 500MB) may cause out-of-memory errors on older or lower-RAM devices. Desktop browsers with more available system memory handle larger files reliably.

Limitations

• File size: Files above ~500MB may exceed browser tab memory limits on devices with less than 4GB RAM. Desktop Chrome handles files up to 1–2GB in practice.
• HEIC output not supported: Writing HEIC files requires libhevc, which is not included in the @ffmpeg/core@0.12.6 UMD WebAssembly build. HEIC input (reading iPhone photos) is supported for conversion to JPEG, PNG, or WebP.
• Conversion speed:FFmpeg WebAssembly runs on the browser's main thread using Web Workers. Speed depends on CPU performance. Video conversion is significantly slower than native FFmpeg — a 10-minute 1080p video may take 3–8 minutes depending on hardware.
• DRM-protected files: M4P files purchased from iTunes prior to 2009, and any audio or video with active DRM, cannot be converted — the content is encrypted and cannot be decoded by FFmpeg.
• No server fallback: If your browser does not support WebAssembly or SharedArrayBuffer (which affects Internet Explorer and some very old browsers), Konverton will not function. There is no server-side fallback.