AVI to MP4 Converter

What actually changes when an AVI becomes an MP4

Both AVI and MP4 are containers: wrappers that hold a video stream, an audio stream and a table saying where each frame lives. Neither is a codec, and that is why "AVI to MP4" can mean two completely different jobs depending on what is inside your file. AVI is Microsoft's RIFF format from 1992. MP4 is ISO base media, the same family as MOV, standardised in 2001 and built for the codecs and the players we actually use now.

The differences that matter are structural. AVI assumes one fixed frame rate for the whole file and stores audio in chunks that are meant to be constant bitrate, which is why AVIs with variable-bitrate MP3 audio famously drift out of sync towards the end. MP4 gives every sample its own timestamp, so it can hold variable frame rate footage honestly, and it can put its index at the front of the file so a browser starts playing before the download finishes. AVI has no index at the front and no real subtitle support - subtitles for an AVI almost always arrive as a separate .srt or .idx file sitting next to it.

Is this a lossless copy or a real re-encode?

It depends on the codec inside the AVI, and this converter reads that from the file header before it does anything. If the video is MPEG-4 Part 2 - what Xvid, DivX 4 and DivX 5 all produce - it is legal inside an MP4, so the tool copies the stream across instead of re-encoding it. That takes seconds no matter how long the film is, and every pixel is bit-for-bit identical. If the video is something MP4 cannot hold, the picture is decoded and re-encoded with libx264, and that is a genuine quality decision, not a repackaging job.

The converter tells you which of these it is about to do, in words, before you press Convert. If you are not sure what your file holds and you would rather know first, the guide on checking what codec a video actually uses shows you how to read it without installing anything.

Choosing quality for the re-encode

Most people who convert an AVI end up re-encoding, either because the codec forces it or because they want H.264 at the other end. This page gives you constant-quality encoding, which is the part almost no online converter exposes. Low, Medium and High map to CRF 28, 23 and 20 on x264's veryfast preset. CRF means the encoder is told how good the picture must look and then spends whatever bitrate that takes - a still interview scene comes out small, a handheld shot of a beach comes out larger, and both look consistent. A fixed-bitrate encode does the opposite: it wastes space on the easy scene and falls apart on the hard one.

For old camcorder footage, High (CRF 20) is the sensible default. The source is already soft and heavily compressed, and a second lossy pass on top of a first one is where visible mush comes from. If you specifically want to match what the phone app produces, the advanced options switch to fixed bitrates of 1, 2.5 and 5 Mb/s - the three values the app uses. Dropping the resolution or the frame rate there forces a re-encode even when a copy was possible.

What does not survive the conversion

Only the first video stream and the first audio stream come through. Extra audio tracks - a second language, a commentary - are dropped, and so is any subtitle track. AVI rarely carries subtitles internally, so in practice this bites when the AVI has more than one audio stream. Choosing MKV instead does not save them: Matroska can hold every track, but this tool writes one video and one audio stream whatever the container, so a file whose second soundtrack matters needs a desktop tool.

Creation dates and camera metadata mostly do not survive either, because AVI barely stores them in the first place. Note the recording date from the file's own modified date before you convert if it matters. One thing conversion cannot fix: if your AVI already drifts out of sync because of variable-bitrate MP3 audio, a stream copy copies the drift with it. Forcing an audio re-encode lays the sound down against real timestamps, which often straightens it out.

The honest limits of doing this in a browser tab

The engine here is FFmpeg compiled to WebAssembly, served from this domain and run in a worker inside your tab. Nothing is uploaded, which is the point when the footage is a family christening from 2004 rather than a stock clip: a file that never leaves your machine cannot be retained, scanned or leaked. It is also why there is no queue and no daily quota - there is no server to pay for. For the longer argument, read whether online video converters are safe.

The limits are real and worth stating. Files over 2 GB are refused outright, because a 32-bit WebAssembly heap cannot address them. Above roughly 300 MB you get a warning, since the conversion needs about three times the file size in free memory. The build is single-threaded, so a re-encode runs at roughly real time for 720p and slower than real time for 1080p - a stream copy, by contrast, is near-instant however long the video is.

That 2 GB ceiling is the one that catches AVI users, because DV captures from a tape camcorder run to roughly 13 GB per hour. A DV-AVI of any real length has to be handled by desktop software or by the Video Converter app, which has no size ceiling and uses your device's hardware encoder rather than a software one. The app always re-encodes and cannot stream-copy, so it is the right tool for big files and the wrong tool for a lossless repackage.

The situations that send people to this page

Old family video is the common one: a hard drive of AVIs from a camcorder or a Windows Movie Maker era PC, and a phone or a TV that shows a grey thumbnail. Editors are the other: DaVinci Resolve and Premiere both handle AVI unevenly depending on the codec inside, and the fix is usually to normalise to H.264 in an MP4 first - the same reasoning as in the guide on Resolve importing an MP4 as audio only. Then there are car stereos and older TVs that read a USB stick, where MP4 with H.264 is the safest bet by a wide margin.

If the file you are actually holding turns out not to be an AVI, the same engine handles the neighbouring jobs: MKV to MP4, which is nearly always a copy, MOV to MP4 for anything off an iPhone, and H.265 to H.264 when a modern phone recording will not open on an older Windows machine.