What is FLV and why is it no longer usable?

FLV (Flash Video) is Adobe's container format designed for the Flash Player plugin, which powered most web video from roughly 2005 to 2015. Adobe discontinued Flash Player on December 31, 2020, and all major browsers removed support. FLV files are now unplayable in Chrome, Firefox, Safari, and Edge without third-party software. Converting to MP4 is the only way to make FLV content accessible on modern devices.

What codecs does FLV use?

FLV files typically contain Sorenson Spark (H.263) or On2 VP6 video with MP3 or ADPCM audio. Some newer FLV files use H.264 video and AAC audio (these were sometimes called F4V). If your FLV contains H.264 + AAC, FFmpeg can rewrap it to MP4 with -c copy (no quality loss). If it contains Sorenson Spark or VP6, full re-encoding to H.264 is required.

Will converting FLV to MP4 reduce quality?

If the FLV contains H.264 video and AAC audio, FFmpeg can rewrap it losslessly with -c copy. For Sorenson Spark or VP6 encoded FLV files, re-encoding to H.264 introduces some quality loss. Keep the CRF value low (e.g., -crf 18) and set a bitrate at or above the source to minimize visible degradation. Since FLV source quality is often already limited, the output will generally look comparable to the original.

Why can't this tool convert the file directly in the browser?

FLV decoding and H.264 encoding require full codec libraries not available in browser APIs. The metadata reader uses the browser's built-in File API and runs entirely on your device. Actual conversion requires a desktop tool like FFmpeg, which has access to hardware acceleration (NVENC, VideoToolbox, QSV) and the full libx264/libvpx codec stack.

Is my video file uploaded anywhere?

No. The file inspector uses the browser's File API and URL.createObjectURL(). Your video file never leaves your device — there is no upload, no server processing, and no tracking of any kind.

How do I convert FLV to MP4 without losing quality?

First check if your FLV uses H.264 video by running ffprobe input.flv. If it does, use ffmpeg -i input.flv -c copy output.mp4 for a lossless rewrap. For Sorenson Spark or VP6 sources, use ffmpeg -i input.flv -c:v libx264 -crf 18 -preset slow -c:a aac -b:a 192k output.mp4. CRF 18 is near-lossless; lower CRF values produce larger files with higher quality.

Can I convert FLV to MP4 for free?

Yes. FFmpeg, HandBrake, and VLC are all completely free and open source. FFmpeg provides the most control and is ideal for batch jobs. HandBrake offers a polished GUI with presets. VLC is the simplest option if you already have it installed.

Why does my converted MP4 have no audio?

Some FLV files use ADPCM or Nellymoser audio codecs that need re-encoding. Force AAC output by adding -c:a aac -b:a 192k to your FFmpeg command: ffmpeg -i input.flv -c:v libx264 -crf 22 -c:a aac -b:a 192k output.mp4. If audio is still missing, check that your FLV actually has an audio stream: ffprobe -v quiet -show_streams input.flv.

FLV to MP4 Converter

Name: FLV to MP4 Converter
Availability: InStock
Author: Nham Vu

Step-by-step guide for converting FLV video files to MP4 using FFmpeg, VLC, and HandBrake, plus a client-side FLV file metadata inspector.

Video File Inspector

Select any video file to read its metadata instantly — 100% client-side, nothing uploaded.

Click to choose a video file FLV, MP4, AVI, MOV, MKV…

FLV vs. MP4 — Format Comparison

Know what you are converting and why

Feature	FLV	MP4
Developer	Adobe (Macromedia, 2002)	MPEG group / ISO (2001)
Common codecs	Sorenson Spark, On2 VP6, H.264	H.264, H.265, AV1, AAC
Browser support	None (Flash ended 2020)	Universal — every browser
Mobile support	Not supported	Full support
Streaming	RTMP only (legacy)	HLS, DASH, progressive

FFmpeg (Recommended — full codec control)

Free, cross-platform, most flexible

FFmpeg handles all FLV variants. If your FLV already contains H.264 video and AAC audio, use -c copy for a lossless rewrap. For Sorenson Spark or VP6 sources, re-encode to H.264 with libx264.

# Install on Mac via Homebrew
brew install ffmpeg

# Install on Windows via winget
winget install ffmpeg

# Check FLV codec before converting
ffprobe -v quiet -show_streams -select_streams v:0 input.flv

# Lossless rewrap (only if FLV contains H.264 + AAC)
ffmpeg -i input.flv -c copy output.mp4

# Re-encode Sorenson Spark or VP6 FLV to H.264 MP4
ffmpeg -i input.flv -c:v libx264 -crf 22 -c:a aac -b:a 192k output.mp4

# High quality + web streaming (moov atom at start)
ffmpeg -i input.flv -c:v libx264 -crf 18 -preset slow -c:a aac -b:a 192k -movflags +faststart output.mp4

CRF 18 = near-lossless (larger file). CRF 28 = smaller file with more compression. Default CRF 22–23 is a good balance for most FLV content.

HandBrake (GUI — quality-focused encoding)

Free, Windows / Mac / Linux

Download and open HandBrake from handbrake.fr.
Drag your FLV file onto the HandBrake window to load it as the source.
In the Summary tab, set Format to MP4.
Choose a preset — Fast 1080p30 or Fast 720p30 works well for most FLV files.
In the Video tab, set the encoder to H.264 (x264) and set the RF quality slider (RF 20 is a good default; lower = higher quality).
In the Audio tab, set the encoder to AAC and bitrate to at least 128 kbps.
Click Browse to set a destination MP4 file, then click Start Encode.

VLC Media Player (GUI — no command line)