Why FLV to MP4 needs a real transcode
FLV is the Adobe Flash Video container, and the codecs Flash stored inside it — Sorenson Spark (an H.263 variant), On2 VP6, and later H.264 — cannot all survive a remux into MP4. For most older FLVs the video has to be fully decoded and re-encoded. Browser ffmpeg.wasm tools attempt that single-threaded under a ~2 GB ceiling, slow and prone to failing on long lecture captures or stream dumps. We run the transcode server-side: presigned upload to the nearest Backblaze B2 POP, native ffmpeg decoding the Flash-era stream to H.264 + AAC, output delivered through Bunny CDN. A short FLV usually clears in 10–20 seconds end-to-end.
What this tool does, exactly
Native ffmpeg decodes the inner FLV stream, re-encodes the video to H.264 at CRF 23, re-encodes audio to AAC at 160 kbps, and writes the result as an MP4 with the +faststart flag so it begins playing before the download finishes. Pixel format is forced to yuv420p because old Flash encoders and capture cards often wrote odd chroma layouts that hardware decoders refuse. It's a single file in, a single MP4 out — no trim, crop, or quality presets, and no recovery of detail a low-bitrate Flash encode already discarded.
Flash is gone, but your recordings aren't
Adobe ended Flash Player support in December 2020 and browsers pulled the plugin soon after, but the .flv files people piled up never went anywhere. Camtasia and early OBS defaulted to FLV for years because it tolerated crashes mid-recording, so a lot of old screen captures, webinars, and lectures are stuck in it — alongside downloaded Flash web videos and RTMP stream dumps. Modern browsers, iOS, and most editors simply won't open them. Re-encoding the inner stream into H.264 + AAC inside an MP4 wrapper is what makes those recordings play again on current hardware.
Sister tools
For other legacy containers: WMV to MP4 converter handles old Windows Media exports, AVI to MP4 converter handles DivX and Xvid sources, and MPG to MP4 converter handles MPEG-1/2 DVD-era files. Shrinking a finished MP4 afterward? Video compressor.
Skipping the conversion entirely
FLV exists because Flash-era recorders and RTMP pipelines created it. If you are still making new recordings and somehow ending up in a legacy format, switch to a recorder that outputs MP4 directly. Clipy records your screen straight to a shareable link — no local file, no Flash, no format conversion, no watermark, no install.