Compress an AVI file

Why AVI files are so much bigger than they need to be

AVI dates from 1992, and the codecs that populated it — DivX, Xvid, Microsoft's early MPEG-4 variants, MJPEG — are one to two generations behind H.264 in compression efficiency. The same footage that a modern encoder describes in one megabyte took three or four back then, and the picture was not better for it.

MJPEG is the extreme case and turns up often in AVI files from old camcorders, microscopes, dashcams and capture cards. It stores every single frame as an independent JPEG image, with no attempt to describe a frame as a difference from the one before it. That throws away the single largest source of redundancy in video — the fact that consecutive frames are nearly identical — and the file size shows it. Re-encoding MJPEG to H.264 routinely produces a file a tenth of the size with no visible difference.

AVI in, MP4 out

This tool reads AVI but does not write it, and that is deliberate rather than a limitation. There is no situation in which producing a new AVI file is the right answer: the container has no modern features, poor support for anything beyond one video and one audio track, and awkward handling of variable frame rate, and every device that plays AVI also plays MP4.

Drop the AVI in and the output is H.264 in an MP4. That fixes the size, the efficiency and the compatibility in one pass.

What to expect from the numbers

Because the starting point is so inefficient, AVI is the one input where large reductions come without a visible cost. A DivX or Xvid AVI typically halves at similar perceived quality; an MJPEG AVI often drops by 80–95%. Neither figure is a promise — it depends entirely on what the footage is and what bitrate it was captured at — but the direction is reliable in a way that it is not when you are re-compressing an already-modern file.

Start with Best compression, quality slider in the middle, resolution left on original. Old AVI content is often 480p or smaller to begin with, and downscaling something that is already low resolution is usually the wrong move — there is not much detail left to give up.

Old footage deserves a gentle hand

One caution specific to archival material. Video from an old camcorder or a VHS capture frequently has analogue noise in it — grain, chroma bleed, tracking artefacts. Noise is genuinely random, and random data is the hardest thing there is for a video encoder to compress, so a noisy source both eats bitrate and looks worse when starved of it.

If a compressed capture of old footage looks smeared, the problem is the noise rather than the setting. Back the quality slider off a step rather than pushing it further, and keep the original file — this may be the only copy of it that exists.

AVI compression questions

Compressing to a size limit instead?

If what brought you here is a number rather than the fact that it is a AVI file, start from the ceiling instead. Each of these opens the compressor with that target already set, ready for your AVI:

• 5 MB
• 8 MB
• 10 MB
• 16 MB
• 20 MB
• 25 MB
• 50 MB
• 100 MB
• 200 MB
• 500 MB

Other formats

• Compress MP4 The universal container. Usually already H.264, so the gains come from bitrate and resolution.
• Compress MOV iPhone and QuickTime recordings. Frequently HEVC or ProRes, and frequently huge.
• Compress MKV Long recordings, rips and archives. Big enough that upload-based tools often refuse them.
• Compress WebM Already efficient — so the realistic gains are smaller, and worth understanding first.
• Compress WMV Old Windows exports. Nothing plays them now, and remuxing is not an option.
• Compress FLV A dead container, often with a perfectly modern video inside. Try remux first.
• Compress M4V Apple's MP4 under another name. Renaming it often solves the whole problem.
• Compress MTS / M2TS AVCHD camcorder footage. Enormous, usually interlaced, and split into 2 GB pieces.

The formats reference puts AVI beside the other containers and shows which codec combinations play where.