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Video Conversion Not Working? Common Problems and Fixes

By the Free Video Converter team · Published 2026-10-09

Converting video in the browser is convenient, but it relies on your computer's memory and your browser's capabilities. When something goes wrong, the cause is usually one of a handful of things. This guide lists them in the order that is most useful to try.

"Could not load the converter engine"

The first conversion downloads the engine (about 32 MB). This message means the download or startup failed. Try these:

• Check your internet connection and reload the page with Ctrl+F5 (Cmd+Shift+R on a Mac) to bypass stale cached files.
• Make sure your browser is up to date. The engine needs WebAssembly, which all current mainstream browsers support.
• Disable extensions that block scripts or workers, or try a private window.
• Corporate or school networks sometimes block large downloads or worker scripts. Try a different network.

The file shows an error or "Conversion failed"

1. Check that the original plays. If it will not open in a normal media player, it may be damaged or incomplete (for example an interrupted download).
2. Lower the resolution. Choose 720p or 480p. Smaller frames use much less memory.
3. Convert one file at a time instead of a long list.
4. Close other tabs and heavy applications to free memory, then try again.
5. Use a desktop or laptop rather than a phone or tablet.
6. Try a different output format. If MP4 fails, test MKV or WebM to see whether the problem is specific to the source.

If the file is protected with copy protection (DRM), such as many purchased or rented movies, it cannot be converted, and this tool does not attempt to circumvent protection.

The conversion is extremely slow

• The conversion engine runs on a single thread in the browser. Long, high-resolution video can take far longer than the video's own duration.
• Lower the resolution to speed things up substantially.
• Keep the tab visible. Some browsers slow down background tabs.
• For many hours of footage, a desktop converter will be much faster.

The converted file will not play or looks wrong

• No sound or wrong language: Only one audio track is kept, and it is chosen automatically. Use desktop software if you need to pick a track.
• No subtitles: Subtitles are not guaranteed to be carried over.
• Washed-out colors: Often caused by HDR or 10-bit sources being converted to standard 8-bit video without tone mapping.
• Blocky or blurry picture: The source may already have been heavily compressed, or the output setting is a speed-oriented preset. Re-encoding cannot add back lost detail.
• Played fine on the computer, not on the TV: Some TVs are picky. MP4 (H.264 and AAC) is the best format to try first; see why.

Downloads that never appear

• Check your browser's download settings and whether it blocked a download prompt.
• If you are downloading a ZIP of many large files, see the batch guide for memory and size limits.
• Results exist only on the open page. Do not reload before downloading.

When to use something else

This website suits quick, everyday conversions with no installation. It is not a replacement for professional tools. If you need precise bitrate control, multiple audio or subtitle tracks, HDR handling, hardware acceleration or very large files, a desktop application such as HandBrake or FFmpeg itself is the better tool.