Free converter · Lossy re-encode

Convert FLV to MP4

Runs in your browserNo uploadUp to 100 MB

Short answer

Drop your FLV file into the converter below and download the MP4 file. It runs in your browser with FFmpeg, for files up to 100 MB; nothing is uploaded. The file is re-encoded to H.264 video and AAC audio in MP4, at FFmpeg's default quality, with the source's sample rate. Re-encoding takes longer than a container swap and costs some quality; how much depends on the clip and on the quality setting, and at the converter's default setting the loss can be visible, so keep the original.

Choose fileVIDEO/X-FLV, FLV · 100 MB maximumChoose a .flv file - it stays on your device

Maximum file size: 100 MB. The first conversion loads a ~30 MB converter engine, once per visit. Nothing is uploaded - conversion runs on your device.

Flash Video re-encoded to H.264/AAC. · Lossy re-encode · Browser-side via FFmpeg WASM - nothing uploaded.

Key takeaways

FLV to MP4 re-encodes with ffmpeg defaults: H.264 (CRF 23) and AAC.
Measured on an FLV that already held H.264 and AAC: 9.18 MB became 3.36 MB (-63.4%) at SSIM 0.9707.
If your FLV already holds H.264 and AAC, a stream copy would be lossless; this converter re-encodes instead, so keep the FLV if size is not a problem.
FLV is a legacy container; MP4 is the one platforms document.

Measured: what the re-encode does

Measured on a 10 second, 1920 × 1080, 30 fps excerpt of Big Buck Bunny (animated film, not camera footage; (c) Blender Foundation, CC BY 3.0) as H.264 plus stereo AAC 192 kbps, using the same ffmpeg arguments this converter passes to its WebAssembly build, run with ffmpeg 6.1.1. Results describe this file, not every file.

The FLV input holds the same H.264 and AAC streams, copied in.

Check	Result
Input	FLV, 9,182,906 bytes (H.264 + AAC)
Output	MP4, 3,357,495 bytes (-63.4%)
Output video	H.264, yuv420p, about 2,537 kbps
Output audio	AAC, 48 kHz stereo, about 130 kbps
SSIM against the source video	0.9707
Time, native ffmpeg	4.9 s

The size fell because the default CRF 23 uses far fewer bits than the CRF 14 source. That is a quality trade, not a free saving.

FLV vs MP4

	FLV	MP4
Full name	Flash Video (Adobe)	MPEG-4 Part 14
What is inside	Sorenson Spark or H.264 video with MP3 or AAC audio	H.264 or H.265 video with AAC audio
Compression	Lossy	Depends on the codec inside
Best for	Old Flash-era recordings and streaming software exports	Uploading anywhere: the format social platforms document first

What you get

H.264 video and AAC audio in MP4, at FFmpeg's default quality, with the source's sample rate. Flash Video re-encoded to H.264/AAC.

A lossless alternative when the codecs allow it

FLV files from older tools often hold H.264 and AAC, which MP4 can store as they are. Check with ffprobe -hide_banner in.flv; if both streams are H.264 and AAC, copy them: ffmpeg -i in.flv -c copy -movflags +faststart out.mp4. That keeps every bit of quality. Older FLV files may hold other codecs that MP4 cannot copy; the converter's re-encode handles those.

Which container to upload

YouTube recommends MP4 with H.264 and AAC-LC or Opus; TikTok's API lists MP4 as recommended; Pinterest's web uploader lists MP4 and M4V only. FLV is not named in any of those pages.

Questions people ask

Is this FLV to MP4 converter free and private?

Yes. It is free, needs no account and runs entirely in your browser, so the file never leaves your device.

Will converting FLV to MP4 reduce quality?

Some, because MP4 is depends on the codec inside and the file is re-encoded. How much depends on the clip and on the quality setting, and at the converter's default setting the loss can be visible, so keep the original. For uploads, the platform re-encodes again anyway.

Is there a file size limit?

Files up to 100 MB. The first conversion downloads a converter engine of about 30 MB once per visit; larger files are better handled by a desktop app such as HandBrake or FFmpeg.

Does FLV to MP4 lose quality?

With this converter, yes: it re-encodes. On our 1080p test, SSIM was 0.9707 and the file shrank 63.4%.

Can I convert FLV to MP4 without re-encoding?

Yes if the streams are H.264 and AAC: use ffmpeg with -c copy.

What is FLV?

Flash Video, a legacy container often holding H.264 or Sorenson Spark video.

Will the MP4 work for uploads?

MP4 with H.264 and AAC is the documented-safe combination.

More converters

Sources and how we checked

MDN: Media container formats
MDN: Web audio codec guide
MDN: Image file type and format guide
FFmpeg documentation: stream copy
FFmpeg Wiki: Encode/H.264 (CRF and presets) (read 2026-10-08)
FFmpeg Documentation: ffmpeg (stream selection, -map, -c copy) (read 2026-10-08)
YouTube Help: Recommended upload encoding settings (read 2026-10-08)
TikTok for Developers: Content Posting API media transfer guide (read 2026-10-08)
Pinterest Help: Review Pin specs (Video Pin file types) (read 2026-10-08)
Blender Foundation: Big Buck Bunny (CC BY 3.0), the clip used for the measurements (read 2026-10-08)

Reviewed October 8, 2026. What the converter writes was measured with ffprobe on the same FFmpeg build your browser downloads; format descriptions follow MDN's media guides.