Convert AVI to MP4 — when remux works, when re-encode is forced

The short version

1. Drop the .avi onto the converter.
2. If the inner video codec is already H.264 (rare for older AVIs, common for newer ones from IP cameras), we do a container remux — no re-encode, no quality loss, a few seconds even on a 1 GB file.
3. If it's DivX, XviD, MPEG-4 Part 2, Cinepak, Indeo, M-JPEG, or any of the other legacy codecs AVI loved, we re-encode to H.264 inside MP4. Slower (roughly real-time on a modern laptop), still fully local.
4. Download. Done.

That covers 95% of traffic. The rest of this post is for when the AVI won't even open, when it has subtitles or multiple audio tracks, or when you're deciding between us and the desktop tools.

Why AVI is not MP4 (even though both are “video”)

AVI (Audio Video Interleave) is a 1992 container defined as part of Microsoft's RIFF (Resource Interchange File Format) specification — the same top-level structure you'd find in a .wav file. Its two big limitations compared to MP4:

• No modern codec support. AVI was designed before H.264 existed. It can technically carry H.264 via the AVI2 extension, but support is patchy — some players refuse, some decode badly, and Microsoft officially stopped investing in AVI's codec list around 2006.
• No variable-framerate support. AVI assumes a fixed framerate. Modern codecs and modern sources (phone video, screen recordings, Twitch VODs) are variable-framerate, which causes audio-sync drift when forced into AVI.
• No proper modern subtitle or metadata handling. AVI can embed a small text-based subtitle track, but anything modern (SRT, SSA, forced subs) needs sidecar files.
• Terrible streaming behaviour. AVI doesn't have a proper index until the end of the file, so you can't start playback over the network before most of the file has arrived. MP4 with faststart fixes this.

The codecs that actually live inside real-world AVI files, ranked by how often we see them:

• DivX — the codec that made AVI famous. MPEG-4 Part 2, highly compressed for 2002. MP4 can't carry it; re-encode required.
• XviD — open-source DivX clone. Same MPEG-4 Part 2 family, same MP4 incompatibility, same re-encode story.
• H.264 in AVI — rare; some IP cameras and older CCTV systems. Remux-eligible.
• M-JPEG — every frame stored as a full JPEG. Old camcorder format. Huge files. MP4 can carry M-JPEG but support is thin; we re-encode to H.264.
• Cinepak, Indeo, Microsoft Video 1 — 1990s codecs. Re-encode.
• Uncompressed RGB / YUY2 — raw screen captures and some scientific workflows. Re-encode and the file gets dramatically smaller.

Remux vs re-encode: the time difference, measured

The choice isn't just theoretical — it's the difference between “done in 8 seconds” and “done in four and a half minutes” on the same file. Here's a measured comparison on a 10-minute 720p clip (1 GB AVI), 2024 MacBook Air M3:

Remux is ~30× faster because we're not touching the video bits — we read the compressed samples out of AVI's RIFF chunks and write them into MP4's mdatbox. The CPU spends its time doing a disk-to-disk copy. A full re-encode decodes each frame, runs the H.264 encoder, and writes new compressed output — all of which is CPU-bound, especially on older laptops.

What AVI's structure actually looks like

An AVI file is a RIFF tree with three important chunks:

• hdrl — header list. Contains avih (global file info: total frames, width, height, suggested buffer size) and one strl per stream (video, audio). This is where we look up the inner codec.
• movi — the movie data. Interleaved video and audio chunks. A remux reads these chunks, strips the RIFF wrapper, and reassembles them in MP4 sample tables.
• idx1 — the index. Usually at the end of the file. Maps stream frames to byte offsets in movi. Crucial for seeking; MP4's moov box serves the same role but at the start of the file.

The tricky part of an AVI → MP4 remux is reconstructing the MP4's sample timing. AVI uses a fixed framerate encoded in avih. MP4 uses per-sample durations in a stts box. For a fixed-framerate AVI, the conversion is deterministic. For AVIs that lie about their framerate (some tools write 30 fps headers on 29.97 fps content), audio-sync drift creeps in over long clips. We compensate by using AVI's audio chunk count as the source of truth and stretching video timing to match — the same trick ffmpeg does under the hood.

The DivX/XviD archive story

If you've got a folder of 700 MB AVI files with titles like Movie.Name.2003.DVDRip.XviD-GROUP.avi, you've got DivX-or-XviD content. These are MPEG-4 Part 2 — the codec before H.264, designed for 2002-era hardware. Facts that matter now:

• DivX and XviD are compatible at the bitstream level. A DivX decoder plays an XviD file and vice versa. The two projects were functionally identical; XviD was the open-source response to DivX going commercial.
• MP4 cannot carry MPEG-4 Part 2 as-shipped. Technically the MP4 spec allows it via a specific profile, but essentially no modern player implements that path. Treat it as “must re-encode to H.264”.
• The file will get smaller. H.264 at visually-matched quality is roughly 40-50% smaller than DivX/XviD. Your 700 MB AVI typically lands around 380-420 MB as MP4 H.264 with no visible quality change.
• Audio is usually MP3 or AC-3 in AVI. MP4 accepts MP3 natively (remux path), but not AC-3 — that's re-encoded to AAC.

Subtitles and multi-audio reality

AVI has two ways to handle subtitles. Neither is great:

• Sidecar files. The most common setup in scene releases — a .srt or .sub next to the AVI. These don't travel with the AVI's bytes, so they won't magically become embedded in the MP4 unless you explicitly add them. Our converter accepts a sidecar SRT drop and bakes it in as an MP4 text track.
• DivX Subs (DXSA). An embedded bitmap subtitle track some DivX tools used. Poor compatibility; we decode and re-encode to mov_text (MP4's native subtitle format).

Multiple audio tracks: AVI supports them via multiple strlstreams. MP4 does too. We carry across up to 8 audio streams by default — enough for the weird case of commentary-plus-dubs-plus-original language. If your AVI has more than that, use the general video converter for per-track selection.

When you want to re-encode deliberately

Even when a remux would work, sometimes re-encoding is the better call:

1. The AVI is way bigger than it needs to be

Early-2000s DivX rips tend to be 700 MB for 90 minutes of content — a 1.0 Mbps bitrate. Modern H.264 gets to the same perceived quality at roughly 1.6× smaller (~450 MB), and HEVC at roughly 2.5× smaller (~290 MB). If storage matters, re-encode on purpose.

2. The AVI is uncompressed

Raw-RGB AVIs from screen capture tools like CamStudio or early OBS are enormous — 50-100 GB per hour. Remux preserves that. You almost certainly want to re-encode.

3. Playback target doesn't play the inner codec

If your AVI is H.264 + AC-3 audio, remux puts H.264 into MP4 fine, but AC-3 won't play on older iPhones. Re-encoding audio to AAC fixes it. This is why the “Force AAC audio” checkbox exists on our converter.

How our tool compares (honestly)

Honest summary: batch of 100+ AVIs with precise encoder control — install HandBrake. Already live in ffmpeg — use ffmpeg; we use ffmpeg.wasm in the browser ourselves. One AVI you want converted in 60 seconds with no upload — our AVI to MP4 tool is the shorter path. See also our detailed comparison with CloudConvert.

Where our tool works well — and where it doesn't

Works well

• DivX/XviD AVIs from 2000s archives (forced re-encode, but fully local)
• IP camera and CCTV AVIs with H.264 inside (fast remux path)
• Old camcorder AVIs with M-JPEG or DV inside
• AVIs with embedded MP3 audio (the most common case)
• Up to ~2 GB files on the free tier in the browser

Doesn't work (well) yet

• Uncompressed 4K AVIs — they're typically 100+ GB and browser memory gives out
• DRM-protected AVIs (rare but they exist) — legally and technically blocked
• Frame-accurate cut-before-convert — use HandBrake or an NLE for trim-first pipelines
• Decades-old proprietary codecs (Sorenson, Indeo 5) — we don't ship every legacy decoder; for those, VLC still wins

Tips for the best result

• Leave defaults on. We remux when we can, re-encode to H.264 when we can't, and you don't have to guess.
• If the AVI has an .srt sidecar, drop them together. We bake the subtitles into the MP4 as a toggleable text track.
• For DivX/XviD re-encodes, default CRF 23 is right. The original was already lossy at a lower bitrate, so a second generation at CRF 23 is visually transparent.
• For really long archives, batch. Run ten 90-minute DVD rips as a batch; they queue while you do something else.
• Audio-only? Skip the MP4 step entirely. Use extract audio from video or MP4 to MP3.

Common questions

Is AVI obsolete?

For new recordings, yes — nothing modern writes AVI by default. For existing archives, AVI isn't going anywhere. Billions of files exist; conversion tools like this one are the bridge.

Will AVI to MP4 lose quality?

Remux: no loss. Re-encode (the common case): a small generational loss, visually transparent at the default CRF 23. If the source was already low-bitrate DivX, the re-encode noise is basically invisible because the encoder spends most of its bitrate preserving the existing artefacts.

Why is my AVI not playing on my phone?

Most phones have no AVI decoder at all. iPhone doesn't. Android varies but generally ships with MP4 and WebM only. Converting to MP4 with H.264 is the only reliable fix — installing a third-party player (VLC, MX Player) also works but doesn't help you share the file.

Does your tool strip DRM?

No — and not just because of legality. The AVI container doesn't have a native DRM system. DRM'd AVIs are rare and usually use wrapper DRM from old Windows Media licensing; those we refuse to touch. For everything else, nothing to strip.

Can I convert back from MP4 to AVI?

Technically yes, practically almost never what you want. MP4 is newer, smaller at matched quality, more widely supported, and handles variable framerate. The only reason to go back is compatibility with a specific old device, and in that case, check our general video converter and pick AVI as the output.

Why is my converted MP4 bigger than the AVI?

Usually because the AVI was DivX at a low bitrate (say 700 kbps) and you re-encoded at a higher default (say 1.5 Mbps). Lower the CRF value if size matters more than quality. For uncompressed or M-JPEG AVIs, the MP4 is almost always dramatically smaller.

Is this really free?

Yes. No watermark, no sign-up, unlimited per day. Free tier caps per-file size; paid tiers lift the ceiling. Quality is identical.

Ready?

AVI to MP4 →. Drop a file, we figure out remux vs re-encode, you download the result. Free, in your browser, no sign-up.