Your files stay on your Mac.
AVI files are usually old, and old is the problem: legacy codecs macOS abandoned, audio that drifts out of sync, recordings split at two gigabytes. Convertessa runs them through FFmpeg on your Mac and writes a modern MP4 at a quality you choose. Same frames, same footage, in a container that QuickTime Player, iPhones, Android devices and browsers all accept without argument.
AVI is 1992 technology. Microsoft built it on the RIFF chunk format and it shows: the header assumes one fixed frame rate, there is no field for pixel aspect ratio, and there is no concept of subtitles, chapters or attachments. What modern containers describe with per-sample timestamps, AVI describes with an index of evenly spaced frames.
The AVI files people still have tend to fall into three piles: DivX and Xvid rips from the early 2000s, Motion JPEG clips from compact cameras and camcorders, and captures made with old cards using Cinepak, Indeo or DV. That last pile explains the error message on a current Mac, because those decoders lived inside QuickTime 7, which was 32-bit, and macOS Catalina dropped 32-bit support entirely in 2019. FFmpeg still decodes all of them, and FFmpeg is what Convertessa calls, so the footage is not lost. It is stranded in a wrapper macOS walked away from.
Audio moves from MP3 or PCM to AAC, and the two-gigabyte ceiling of the original AVI specification, the reason long recordings arrive as numbered parts, stops mattering.
Drag old camcorder clips, capture-card recordings or a folder of downloads onto the window. FFmpeg decodes formats macOS itself gave up on, Xvid and DivX among them, along with Cinepak, Indeo, MJPEG and DV.
Pick MP4 and set the quality. A transfer from tape deserves a high setting because the source cannot be recaptured; a 2000s Xvid rip already lost that detail years ago and gains nothing from one.
Play the finished MP4 from the middle rather than the start. AVI files with variable-rate MP3 audio drift progressively later, and only the middle of a clip tells you whether the new timestamps fixed it.
Images, audio, video, documents, archives, eBooks and fonts — all converted natively, right on your Mac.
Pick a format you have — see everything Convertessa can turn it into.
Convert hundreds of files at once with per-type group defaults and individual overrides.
First-class support for HEIC, AVIF and WebP — encode and decode, both ways. Read JPEG XL and a dozen more modern formats too.
Drop files anywhere on the window. Native, instant, and exactly what you’d expect on a Mac.
Dial in compression, resolution and bitrate. Keep originals pristine or shrink for sharing.
Never overwrite a file by accident. Convertessa appends safe suffixes automatically.
Every job is logged locally. Re-run a previous conversion or revisit recent outputs in a click.
Bundle your converted files straight into a single ZIP, TAR or 7z — packaged and ready to share the moment a job finishes.
Right-click any file in Finder and choose Convert. A Quick Action handles it on the spot — no need to open the app first.
Shrink Convertessa to a tidy mini window that tucks into a corner — just a drop zone and a format picker for quick, one-off conversions.
No cloud. No account. No upload. Every conversion happens entirely on-device, using the power already in your Mac. What you convert is nobody’s business but yours.
Script conversions, wire them into your build, or batch a folder from the terminal. The same engine, no GUI required.
Read the full CLI documentation →Shrink Convertessa down to a small, focused window — drop, convert, done, with your recent conversions one click away.
macOS 14+ · Apple Silicon & Intel
New formats, new features, and fixes — every release, on the record.
Almost always the codec, not the container. Cinepak, Indeo, Sorenson and the older DV decoders shipped inside QuickTime 7, and macOS Catalina removed the 32-bit frameworks they depended on in 2019. QuickTime Player X only handles a narrow slice of what an AVI might contain. FFmpeg carries decoders for essentially all of it, so a file that Finder shows as a blank preview usually converts without any complaint.
Frequently, yes. AVI stores audio and video as interleaved streams at an assumed constant rate, and a variable-rate MP3 track breaks that assumption, so the two separate further and further as the file plays. MP4 timestamps every sample and FFmpeg reconstructs the timing during conversion. It is not a guaranteed repair for a badly damaged file, but it fixes the ordinary case.
A DivX and Xvid quirk. To smuggle B-frames through a container that cannot express them, those encoders packed two frames into one chunk and inserted dummy frames to keep the count right. FFmpeg reports this and unpacks the stream. The symptom in the original file is stuttering or an occasional duplicated frame; in the MP4 the frames land at their proper times, because MP4 describes B-frames natively.
Not by converting. The original AVI specification tops out at two gigabytes, and although OpenDML raised that ceiling plenty of capture software kept splitting anyway, which is why long recordings arrive in numbered pieces. Convertessa converts each piece into its own MP4. Joining them is an editing job for iMovie, Final Cut or FFmpeg's concat feature, and it is easier once they are MP4s.
No, and it cannot. Conversion re-encodes what is already there. It cannot restore detail a 2003 Xvid encoder discarded, undo blocking, or invent resolution that was never captured. What improves is compatibility and, usually, file size. Set a high quality so the second encode does not stack its own damage on top of the first, which is the only real quality decision this conversion involves.
AVI has no subtitle track at all; the format predates the idea. If a download came with subtitles they live in a sidecar .srt or .sub file next to the video, untouched by the conversion. Rename that file to match the new MP4 and VLC or IINA will load it automatically. Subtitles burned into the picture by an old encoder stay burned in, since they are part of the image.
Drop the folder onto the window, or use the terminal: convertessa ./*.avi --to mp4 --quality 92 -o ~/Movies/Transfers. Each AVI becomes an MP4 of the same name in the destination folder, processed one after another. A single file is convertessa tape03.avi --to mp4. A tape transfer is worth doing at a high quality setting, and worth doing once.
Two suspects. If the source was a modest Xvid rip and you asked for a very high quality setting, H.264 is being told to reproduce the original's compression artefacts faithfully, and that costs space. If the source was DV, MJPEG or uncompressed, it should not happen at all; expect a large reduction instead. Lower the quality a notch and convert the original again.