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FLV/WMV/AVI to MP4 Free Guide: Convert Legacy Video Formats

Published · By CutFast Team

FLV/WMV/AVI to MP4 Free Guide: Convert Legacy Video Formats

Do you still have .flv, .wmv, or .avi files sitting on your hard drive? These formats were once the standard for internet video, but in 2026, most devices and platforms only support MP4.

Your phone cannot open FLV files, messaging apps reject WMV, and video platforms will not accept AVI uploads. This guide shows you how to convert these legacy formats to universal MP4 for free.

Why These Legacy Formats Still Exist

FLV (Flash Video)

FLV was the standard web video format during the Flash era. Before 2010, nearly every online video platform used FLV. With Flash Player’s retirement, FLV became obsolete, but millions of old videos remain in this format.

Common sources: Videos downloaded from early websites, files saved by older download tools.

WMV (Windows Media Video)

WMV was Microsoft’s video format, the default during the Windows XP/7 era. Videos exported from Windows Media Player and early Windows Movie Maker were typically WMV.

Common sources: Home videos from old Windows computers, presentation-embedded videos.

AVI (Audio Video Interleave)

AVI is one of the oldest video container formats, introduced by Microsoft in 1992. It has broad compatibility but produces very large files due to lack of modern compression codec support.

Common sources: DV camcorder recordings, exports from old video editing software.

Core Concepts: Container vs Codec

Understanding the difference is important:

  • Container: FLV, WMV, AVI, MP4 are container formats that define how data is packaged
  • Codec: H.264, H.265, VP9 are video codecs that determine how frames are compressed

Two conversion approaches:

  1. Remux: Change container only, keep codec → Very fast, zero quality loss
  2. Transcode: Change container and re-encode → Slower, minimal quality change

Transcoding is usually necessary because legacy codecs (FLV1, WMV9, DivX) are not supported in MP4 containers and must be converted to H.264.

CutFast runs FFmpeg WebAssembly in your browser, supporting all legacy format conversions.

FLV to MP4

  1. Open cutfa.st/features/flv-to-mp4
  2. Upload your FLV file
  3. Select output quality (default settings recommended)
  4. Click convert, download MP4

WMV to MP4

  1. Open cutfa.st/features/wmv-to-mp4
  2. Upload, convert, download

AVI to MP4

  1. Open cutfa.st/features/avi-to-mp4
  2. Upload, convert, download

Key Advantages

  • One tool for all legacy formats: FLV/WMV/AVI/MTS/3GP/OGV all supported
  • Runs in browser: No 500MB software download
  • Local processing: Videos never leave your device
  • Free, no ads
  • Auto-optimized encoding parameters: No technical knowledge needed

Method 2: FFmpeg Command Line

FLV to MP4

ffmpeg -i input.flv -c:v libx264 -c:a aac output.mp4

WMV to MP4

ffmpeg -i input.wmv -c:v libx264 -c:a aac output.mp4

AVI to MP4

ffmpeg -i input.avi -c:v libx264 -c:a aac output.mp4

Batch Convert Entire Folder

for f in *.flv *.wmv *.avi; do
  ffmpeg -i "$f" -c:v libx264 -c:a aac "${f%.*}.mp4"
done

Method 3: HandBrake

HandBrake supports importing all legacy formats and exporting to MP4.

  1. Download and install HandBrake
  2. Drag in your legacy format file
  3. Select Preset: Fast 1080p30 (recommended)
  4. Start encoding

Conversion Parameter Recommendations

Source Format Recommended Codec Resolution Suggested Bitrate
FLV (typically 480p) H.264 Keep original 1-2 Mbps
WMV (typically 720p) H.264 Keep original 2-4 Mbps
AVI (may be high) H.264 1080p max 4-8 Mbps

Principle: Do not upscale low-resolution legacy video to high resolution — it only increases file size without improving visual quality.

FAQ

The converted file is smaller — did quality decrease?

Not necessarily. Legacy codecs (FLV1, DivX) are far less efficient than modern H.264. At equivalent visual quality, file size typically decreases 30-60% after conversion. This reflects codec advancement, not quality reduction.

Will converting large files (>1GB) be slow?

Both FFmpeg and CutFast’s WASM version handle large files, but speed depends on file size and device performance. A 1GB AVI conversion typically takes 5-15 minutes.

Can MTS and 3GP also be converted?

Yes. CutFast also supports MTS to MP4 and 3GP to MP4. MTS is the AVCHD camcorder format, and 3GP is the early mobile video format.

Next Steps

  1. Locate legacy format files on your drive
  2. Use CutFast for online conversion (simplest) or FFmpeg (most flexible)
  3. Converted MP4 files play on any device and upload to any platform

Want to learn more about video formats? Read the Video Format Conversion Guide.