Step 1: Provide your FLV files using the button above or by toss and let go.
Step 2: Click the 'Convert' button to start the conversion.
Step 3: Fetch your converted MP4 files.
FLV to MP4 Conversion FAQ
How do I convert FLV to MP4 without re-encoding the video stream?
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When the FLV and MP4 containers can both carry the same codec (H.264, H.265, VP9, AV1), the converter falls back to a remux: the elementary streams are copied byte-for-byte into the MP4 container and only the wrapper changes. A remux of a 1 GB FLV into MP4 typically finishes in 5-15 seconds with zero quality loss. FLV typically holds Sorenson Spark or H.264 video with MP3 or AAC audio, wrapped in a stream format designed for RTMP delivery. Almost every MP4 in the wild is H.264 video with AAC audio; the container also legally holds HEVC, AV1 and VP9.
Can I add selectable subtitles once the file is MP4?
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Yes. FLV has no usable soft-subtitle track, so subtitles for FLV content had to be burned into the picture or shipped as a separate sidecar file. MP4 carries subtitles as their own selectable stream, so after conversion you can attach an SRT track the viewer can switch on, switch off or restyle.
Will the MP4 play in a browser when the FLV would not?
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Yes, and that is usually the whole reason for the conversion. An FLV embedded in a web page either shows a broken player or forces a download, because browsers ship no decoder for it. MP4 plays inline through the native HTML5 video element with no plugin and no external application. MP4 is the safest possible delivery target — every phone, browser, console, editor and Smart TV built in the last decade plays it without extra software.
Is it worth converting old FLV recordings to MP4 at all?
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For anything you intend to keep, yes. FLV belongs to an earlier generation of video tooling and the software that reads it is steadily disappearing from phones, browsers and operating systems. Moving the footage to MP4 now, while a clean decoder still exists, is the difference between an archive you can open in ten years and one you cannot. No current browser, phone or TV can play FLV — anything still in this container has to be moved out of it to remain watchable.
Will multi-track audio survive FLV to MP4 conversion?
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Yes — every audio track in the FLV (director commentary, alternate languages, descriptive audio) is preserved in the MP4 when the target container supports multi-stream audio. MKV and MOV handle unlimited audio tracks; MP4 supports many but some hardware players only see track 1; WebM is capped at one Opus / Vorbis track. MP4 is ISO/IEC 14496-14, standardised in 2001 and derived from the same QuickTime box layout MOV uses.
Are subtitle tracks preserved when converting FLV to MP4?
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Soft subtitles (selectable tracks) survive when MP4 can carry them: MKV holds SRT, ASS, SSA, PGS, VobSub natively; MP4 only holds mov_text (a stripped-down format). Converting an MKV with ASS styled subs to MP4 will typically downgrade them to mov_text or burn them in. Hardsubs (burned into the video) carry over regardless.
Does FLV to MP4 keep chapter markers / DVD-style navigation?
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Chapter metadata transfers between FLV and MP4 whenever both containers support a chapter atom (MKV, MP4, MOV). WebM stores chapters in a Matroska-compatible block, so MKV <-> WebM chapters round-trip cleanly. AVI has no chapter spec at all, so an AVI source arrives with no chapters to carry across and an AVI target cannot store the ones it was handed; add markers manually afterwards if you need them.
What about MKV attachments (fonts, cover art) when going FLV to MP4?
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MKV is the only mainstream container that stores arbitrary attached files (TTF fonts for ASS subs, JPG cover art, fan-translated PDF inserts). Going from FLV to MP4 drops these attachments when MP4 is not MKV — the elementary streams convert, but the attached payload stays in the source. Keep FLV as a backup if you depend on attached fonts.
Which codec does the MP4 output use by default?
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The default codec is chosen to match the MP4 container conservatively: MKV defaults to H.265 (HEVC) for better compression; MP4 defaults to H.264 for the widest device support; WebM defaults to VP9; AVI uses MPEG-4 ASP (DivX/Xvid lineage). Override via the advanced codec dropdown — AV1 is available for MKV / WebM where you want maximum compression and can wait through the encode. MP4 only supports one widely-honoured subtitle format (mov_text), and multi-track audio is unreliable outside desktop players.
How does HDR (HDR10, Dolby Vision) survive FLV to MP4?
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HDR static metadata (HDR10 mastering display values, max content light level) carries through to MP4 when both containers and the chosen codec support it (HEVC, AV1 in MKV / MP4). Dolby Vision is more fragile — DV profile 5 / 7 / 8.1 in FLV commonly survives only into MKV with HEVC; converting to an MP4 container that does not carry the DV layer flattens to HDR10.
Will the FLV to MP4 converter keep variable framerate (VFR)?
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MKV and WebM record real per-frame timestamps and handle VFR natively, so FLV VFR survives into an MP4 of those formats with no resampling. MP4 nominally stores timestamps too but some players assume CFR; the converter exposes a "force CFR" toggle that resamples VFR FLV to 24 / 30 / 60 fps MP4 for problematic targets.
Why is my MP4 file smaller / larger than the FLV after conversion?
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Same-codec remux produces a near-identical size (container overhead differs by 0.1-2%). A codec change can swing the size by 50% or more: H.264 to H.265 typically halves the file at the same visual quality; H.264 to AV1 halves it again on a slow preset. Going from a high-bitrate FLV (Blu-ray rip) to a low-bitrate MP4 (web upload) is the most common reason for a dramatic drop.
