How to convert AVI to MP4

Remux or transcode?

There are two ways to turn an AVI into an MP4. A remux copies the compressed video and audio streams unchanged into the new container. It is lossless and takes a few seconds. A transcode decodes the streams and encodes them again with a new codec. It takes minutes and is lossy. But it is the only option when MP4 does not allow the codec inside the AVI, or browsers cannot play it. The container vs codec explainer covers the distinction in depth.

In practice most AVI files need a transcode. The common AVI codecs are DV, Motion JPEG, uncompressed RGB, and MPEG-4 Part 2 (DivX and Xvid). Only the last is legal in MP4, and even it has patchy browser support, so re-encoding to H.264 is the reliable path. The first step is therefore to find out what you have.

Step by step

Every command below runs in a terminal on macOS, Linux or Windows. Replace the file names with your own; quote them if they contain spaces.

1. Install FFmpeg

   On macOS with Homebrew: brew install ffmpeg. On Debian or Ubuntu: sudo apt install ffmpeg. On Windows, download a build from the links on ffmpeg.org and add its bin folder to your PATH. Confirm with ffmpeg -version.

2. Inspect the AVI

   Run ffprobe -hide_banner input.avi and note four things. First, the video codec, for example mpeg4, dvvideo, mjpeg or rawvideo. Second, the audio codec, such as mp3, pcm_s16le or ac3. Third, the resolution and frame rate. Last, whether the video is interlaced: look for tff or bff in the field order.

3. Try a remux if the codecs allow it

   If ffprobe shows mpeg4 or h264 video with mp3 or aac audio, a stream copy may work: ffmpeg -i input.avi -c copy -movflags +faststart output.mp4. If ffmpeg complains about packed B-frames, add a bitstream filter: ffmpeg -i input.avi -c copy -bsf:v mpeg4_unpack_bframes -movflags +faststart output.mp4.

4. Otherwise, transcode to H.264 and AAC

   The general-purpose command: ffmpeg -i input.avi -c:v libx264 -preset medium -crf 20 -pix_fmt yuv420p -c:a aac -b:a 160k -movflags +faststart output.mp4. CRF 20 is a good quality target for archive copies. Raise it toward 23 for smaller files. -pix_fmt yuv420p ensures the output uses the chroma format every player supports, which matters for AVIs holding RGB or 4:2:2 video.

5. Deinterlace if the source is interlaced

   DV camcorder footage is almost always interlaced. Add the yadif filter before encoding: ffmpeg -i input.avi -vf yadif -c:v libx264 -preset medium -crf 20 -pix_fmt yuv420p -c:a aac -b:a 160k -movflags +faststart output.mp4. Skipping this leaves comb-like lines on motion in every progressive player and browser.

6. Check the result

   Run ffprobe -hide_banner output.mp4 and confirm h264 and aac, the expected resolution and duration. Open the file in a browser directly to confirm it starts playing immediately, which proves the faststart flag worked.

What the options mean

Handling the awkward cases

Some AVIs carry audio as uncompressed PCM at 48 kHz, which is fine to convert to AAC as above. Others carry AC-3, which MP4 permits but browsers mostly refuse. The AAC line handles that too. If an AVI has two audio tracks and you want both, add -map 0 before the codec options. Then ffmpeg keeps every stream instead of picking one of each type.

Variable frame rate AVIs, usually from screen recorders, can produce out-of-sync audio after conversion. Adding -vsync cfr (or -fps_mode cfr on recent FFmpeg versions) with an explicit -r 30 forces a constant frame rate that keeps sound and picture aligned. Corrupt indexes, common in files recovered from failing discs, are often repaired simply by transcoding, because ffmpeg rebuilds the timing from the stream itself.

Frame rate itself is worth a moment's thought. PAL DV runs at 25 frames per second and NTSC DV at 29.97. Leave those as they are. Converting between them creates judder for no benefit, as the frame rate guide explains.

Batch conversion

For a folder of files, a shell loop applies the same command to each one.

1. macOS or Linux: for f in *.avi; do ffmpeg -i "$f" -vf yadif -c:v libx264 -preset medium -crf 20 -pix_fmt yuv420p -c:a aac -b:a 160k -movflags +faststart "${f%.avi}.mp4"; done
2. Windows Command Prompt: for %f in (*.avi) do ffmpeg -i "%f" -vf yadif -c:v libx264 -preset medium -crf 20 -pix_fmt yuv420p -c:a aac -b:a 160k -movflags +faststart "%~nf.mp4"
3. Drop -vf yadif from the loop if the sources are progressive; deinterlacing progressive video wastes time and can soften the image slightly.
4. Add -n after ffmpeg to skip files whose output already exists, which makes an interrupted batch safe to restart.

Before you upload or archive

• Confirm each MP4 plays from the first second in a browser. Any delay suggests the faststart flag was missing.
• Spot-check a scene with fast motion for combing (interlacing left in) or ghosting (deinterlacing applied to progressive video).
• Compare file sizes to what you expect. A one-hour SD H.264 file at CRF 20 usually lands well under a gigabyte. A 4 GB output means something is wrong with the settings.
• Keep the AVI until the MP4 has been checked. It is the source of truth, even if it is ten times the size.
• Upload the finished MP4. Files up to 4 GB use resumable uploads, as the help page on resuming an interrupted upload explains. The large file guide covers what to expect.