How to Convert .AVI to .MP4

Recommended workflow

1. First, check what is actually inside your AVI. AVI is a container, not a codec — it is a box, and the video and audio inside can be encoded a dozen different ways. A 2008 family video might hold DV video, an old game capture might hold a niche codec like CamStudio's, and a downloaded clip might hold Xvid. Which conversion path works depends on what is in the box, so spend sixty seconds finding out before you convert. The easiest check: open the file in VLC media player (free), go to Tools > Codec Information, and read the video and audio codec names. If you have FFmpeg installed, `ffprobe -hide_banner input.avi` prints the same information in the terminal. Why this matters: the lossless remux method below only works when the codecs inside your AVI are also legal inside an MP4 container. H.264 video or AAC/MP3 audio? Remux will almost certainly work. DV video, or something exotic like an old Indeo or Cinepak codec? You will need to re-encode instead. Knowing this upfront saves you from the classic failure mode: a "successful" conversion that produces an MP4 nothing can play, because an incompatible codec got copied into a container that does not support it. Write down the video codec, the audio codec, and the resolution — you will use all three in the steps below.
2. Try the lossless remux first — 30 seconds, zero quality loss. Remuxing means changing the container without touching the encoded video and audio — like pouring cereal from one box into another. Nothing is re-encoded, so quality is mathematically identical to the original, and it typically finishes in seconds even for large files. The command, using the free and open-source FFmpeg: `ffmpeg -i input.avi -c:v copy -c:a copy output.mp4`. The `-c:v copy` and `-c:a copy` flags tell FFmpeg to copy the video and audio streams bit-for-bit instead of re-encoding them. Download FFmpeg from ffmpeg.org if you do not have it (Windows users: download a build, unzip it anywhere, and either add it to your PATH or run it from that folder). Then open a terminal in the folder with your AVI and run the command with your real filenames. When it works — and for AVIs holding H.264 video or MP3/AAC audio it usually does — you get a perfect MP4 in seconds. When it does not work, FFmpeg will complain loudly (errors about codecs "not currently supported in container" or an unplayable output file). That is not a disaster; it just means your AVI's codecs are not MP4-legal, and you move to the re-encode step below. Either way, this attempt costs you almost nothing: keep the original AVI until you have verified the MP4 plays correctly, then decide whether to keep both.
3. Re-encode properly when remux is not an option. If remuxing failed — or if you checked the codecs in step one and already know they are not MP4-compatible — re-encoding is the reliable path. Re-encoding decompresses the video and compresses it again into a modern codec, which always loses a little quality, but done right the loss is invisible. The command: `ffmpeg -i input.avi -c:v libx264 -crf 18 -preset medium -c:a aac -b:a 192k output.mp4`. Here is what each part does, because blindly pasting commands is how people end up with terrible output. `-c:v libx264` selects the H.264 video codec — the universal standard that plays on every phone, TV, and browser made in the last fifteen years. `-crf 18` sets the quality: CRF (Constant Rate Factor) runs from 0 (lossless) to 51 (worst), the default is 23, and 18 is the sweet spot for archival work — visually indistinguishable from the source for most footage, at a reasonable file size. Lower than 18 buys you almost nothing visible and much bigger files. `-preset medium` balances encoding speed against compression efficiency; `slow` squeezes smaller files out of the same quality if you have time, `fast` if you do not. `-c:a aac -b:a 192k` converts the audio to AAC at 192 kbps, which is transparent for typical home-video audio. One warning for irreplaceable footage: never re-encode twice. Each lossy generation discards a little more detail, so convert once from the original AVI and archive the result — do not convert the MP4 again later "to make it smaller."
4. Batch-convert a whole folder of old videos. Nobody has one AVI. You have a folder called "videos 2009" with forty of them, and converting them one at a time is a weekend you will never get back. FFmpeg plus a loop handles the whole folder unattended. On macOS or Linux, in the folder with your AVIs: `for i in *.avi; do ffmpeg -i "$i" -c:v libx264 -preset fast -c:a aac "${i%.avi}.mp4"; done`. On Windows Command Prompt: `FOR %G IN (*.avi) DO ffmpeg -i "%G" -c:v libx264 -preset fast -c:a aac "%~nG.mp4"`. (In PowerShell the syntax differs — the CMD version above is the one to use in a classic Command Prompt window.) A few practical notes from people who have done this at scale. First, run it overnight: re-encoding is CPU-heavy, and forty videos will take hours — that is normal. Second, use `-preset fast` for batches rather than `medium`; the quality difference is small and the time saving is large. Third, spot-check the first two outputs completely before letting the rest run — play them, check audio sync, confirm the length matches. If something is wrong with your settings, you want to discover it on video one, not video forty. Fourth, keep the originals until the whole batch is verified; disk space is cheap, and 2009 cannot be re-shot.
5. VLC — the no-command-line method. If the terminal makes you nervous, VLC media player (free, from videolan.org) converts AVI to MP4 with clicks instead of commands. Open VLC, go to Media > Convert / Save, click Add, and select your AVI. Click "Convert / Save" at the bottom, and in the dialog that appears set the Profile to "Video - H.264 + MP3 (MP4)". Choose a destination filename ending in .mp4, click Start, and wait. That is genuinely all of it. Under the hood VLC is doing the same re-encode described above with sensible defaults, so expect the same quality tradeoff: a small, invisible-to-the-eye loss in exchange for universal compatibility. Two things to know. First, VLC's default profile uses MP3 audio rather than AAC; MP3-in-MP4 plays everywhere in practice, but if you want strict standards compliance, click the wrench icon next to the profile and change the audio codec to AAC (MPEG-4 Audio). Second, VLC gives you no remux option in this dialog — it always re-encodes. If you specifically want the lossless remux path, FFmpeg is the tool. For most family-video purposes, though, VLC's output is perfectly good, and the zero-learning-curve factor is real: this is the method to recommend to relatives.
6. HandBrake — the friendly GUI alternative. HandBrake (free, open-source, from handbrake.fr) sits between VLC's simplicity and FFmpeg's power: a proper graphical interface with real quality controls. Open HandBrake, drag in your AVI (or use "Open Source" to browse for it), and it will scan the file. On the Summary tab, confirm the format is MP4. On the Video tab, set the Video Encoder to "H.264 (x264)" and drag the Quality slider to RF 18–20 for archival-grade output — HandBrake's RF scale is the same idea as FFmpeg's CRF, just inverted in presentation. On the Audio tab, make sure the codec is AAC. Then click "Start Encode." Where HandBrake earns its place: deinterlacing. Old footage — especially anything from MiniDV tapes or early digital cameras — is often interlaced, which shows as ugly horizontal "combing" on moving objects when played on modern progressive displays. HandBrake's Filters tab has a Deinterlace option (set it to Decomb, the adaptive mode) that fixes this during conversion; FFmpeg can do it too, but HandBrake makes it a checkbox. If your AVIs show combing artifacts, HandBrake is arguably the better tool than a plain FFmpeg re-encode. The cost is speed: HandBrake with quality settings is slower than VLC's defaults, so queue the folder (HandBrake has a built-in queue — "Add to Queue" per file) and let it run overnight.
7. When conversion fails — damaged files and weird codecs. Old AVIs fail in characteristic ways, and the fix depends on the symptom. "Could not find tag for codec" or "codec not currently supported in container" from FFmpeg means exactly what it says: the AVI holds a codec MP4 cannot contain. Drop the `-c:v copy` flags and re-encode fully (step three) — that resolves the large majority of these. Audio that drifts out of sync usually means the AVI has variable-frame-rate quirks or a damaged index; try adding `-fflags +genpts` before `-i`, or re-encode with `-async 1` for the audio. A file that will not open in anything — not VLC, not FFmpeg — may be genuinely damaged: try VLC's built-in repair (it prompts to fix broken AVI indexes on open; always let it try), and if the file came from a download rather than your own camera, re-download it. CamStudio captures deserve a special mention: very old CamStudio AVIs use a proprietary codec that modern FFmpeg builds may not decode. If `ffprobe` reports an unknown codec, install the CamStudio codec on Windows and play/convert through VirtualDub or VLC on that machine — this is one of the few cases where the Windows-only path is genuinely easier. And the golden rule for all of it: never delete the original AVI until the MP4 has been watched end to end. A conversion that "completed" is not a conversion that worked.