History and Origins
AVI and MP4 represent two different eras of digital video, and their designs reflect the technological priorities of their respective times.
AVI, which stands for Audio Video Interleave, was introduced by Microsoft in November 1992 as part of its Video for Windows technology. It was one of the earliest widely adopted video container formats for personal computers. AVI was designed during a time when desktop video was a novelty, hard drives were measured in megabytes, and internet video streaming did not exist. The format's design reflects these constraints: it is simple, robust, and optimized for sequential playback from local storage.
AVI uses a RIFF (Resource Interchange File Format) structure inherited from Microsoft's WAV audio format. Data is stored in chunks identified by FourCC codes, with video and audio data interleaved so that the playback system can read both streams sequentially without seeking back and forth across the file. This interleaving approach was well-suited to the slow hard drives and limited RAM of early 1990s computers.
MP4 arrived nearly a decade later, standardized by the Moving Picture Experts Group in 2001 as part of the MPEG-4 specification. By this time, the internet was becoming a major distribution channel for video, and the demands on a video format had changed dramatically. MP4 was designed from the ground up to support streaming, random access, and modern compression codecs. Its atom-based file structure (derived from Apple's QuickTime format) is more flexible and extensible than AVI's RIFF structure, allowing MP4 to adapt to new technologies as they emerge.
The generational gap between these formats means they have fundamentally different capabilities, even though both serve the basic purpose of storing video and audio in a single file.
Codec Support and Flexibility
Both AVI and MP4 are container formats, meaning they define how video and audio data is organized in a file, not how that data is compressed. The codecs used inside the container determine quality, file size, and compatibility.
AVI is technically codec-agnostic and can hold video compressed with a wide variety of codecs, including older standards like DivX, Xvid, MJPEG, and Cinepak, as well as more modern codecs like H.264. However, AVI's RIFF-based structure was designed before modern codecs existed, and adding newer codec support involves working around limitations in the original specification. AVI lacks native support for some features that modern codecs rely on, such as B-frames (bidirectional predictive frames) in H.264. While workarounds exist (like the OpenDML extensions), they are not universally supported and can cause compatibility issues.
MP4 was designed with modern codecs in mind and natively supports H.264, H.265 (HEVC), AV1, MPEG-4 Part 2, and others. The format's extensible atom structure means new codecs can be added cleanly without architectural workarounds. MP4 also has well-defined support for advanced codec features like B-frames, variable frame rates, and multiple reference frames, which are essential for achieving the best compression efficiency with H.264 and H.265.
For audio, AVI typically uses uncompressed PCM, MP3, or AC3 (Dolby Digital). MP4 natively supports AAC (the successor to MP3), AC3, Opus, and Apple Lossless (ALAC). AAC is significantly more efficient than MP3, producing better audio quality at lower bitrates.
The practical implication is that MP4 with modern codecs produces smaller files with better quality than AVI with its typical codecs. Even when both containers use the same codec (such as H.264), MP4's better support for advanced codec features can result in marginally better compression.
File Size Comparison
File size differences between AVI and MP4 are significant and consistently favor MP4 for equivalent content quality.
The primary reason is codec efficiency. AVI files commonly use older codecs like DivX, Xvid, or even uncompressed video, all of which produce larger files than the H.264 codec typically used in MP4. A typical one-hour standard-definition video encoded as AVI with DivX might be 700 MB to 1.4 GB (the classic CD or DVD size targets from the 2000s). The same video encoded as MP4 with H.264 at comparable quality would be 300-600 MB.
For high-definition content, the disparity grows. A one-hour 1080p video encoded as AVI with Xvid at good quality might be 4-8 GB. The same content in MP4 with H.264 would be 2-4 GB. With H.265 in MP4, you could achieve the same quality at 1-2 GB.
Container overhead is another factor, though a minor one. AVI's RIFF structure has slightly more overhead per chunk than MP4's atom structure, which adds up over the duration of a video. For a one-hour video, this might amount to a few megabytes of difference, which is negligible compared to the codec efficiency differences.
AVI also lacks support for modern compression features that reduce file size without affecting quality. Variable bitrate (VBR) encoding, while technically possible in AVI, is not as well-supported as in MP4. MP4's native support for VBR means encoders can allocate more data to complex scenes and less to simple ones, achieving better overall quality at the same average file size.
For anyone concerned about storage space or bandwidth, MP4 is the clear winner. Converting existing AVI files to MP4 with H.264 typically reduces file size by 30-60 percent with no visible quality loss.
Compatibility and Device Support
MP4 has a commanding advantage in compatibility across the modern technology landscape, while AVI retains relevance in specific legacy environments.
MP4 plays natively on every modern platform: Windows, macOS, Linux, Android, iOS, Chrome OS, and even embedded systems. Every web browser supports MP4 in the HTML5 video element. Every smart TV, game console, streaming device, and smartphone can play MP4 files. Every social media platform, video hosting service, and messaging app accepts MP4 uploads. It is the lingua franca of video.
AVI has solid support on Windows, which makes sense given Microsoft's role in creating the format. Windows Media Player, the Movies & TV app, and most Windows-based media players handle AVI files without issues. On macOS, AVI playback depends on the codecs used inside the file; common codecs like H.264 and MJPEG work, but others may require VLC or additional codec packs. Linux handles AVI well through VLC and other open-source players.
Where AVI falls short is in the modern mobile and web ecosystem. Most smartphones can play AVI files through third-party apps, but it is not a natively supported format on either Android or iOS. Web browsers do not support AVI in the HTML5 video element, making it impossible to embed AVI files directly in web pages. Social media platforms generally do not accept AVI uploads, requiring conversion to MP4 before posting.
AVI remains relevant in some specialized contexts. Certain legacy security camera systems and industrial equipment still record in AVI format. Some older video editing software and archival systems use AVI as their native format. Scientific imaging and medical equipment sometimes produce AVI files because the format's simplicity makes it easy to implement in embedded systems.
Streaming and Web Support
Streaming capability is one of the most important differences between MP4 and AVI, and it heavily favors MP4.
MP4 was designed with streaming in mind. Its atom-based structure supports progressive download, which means a viewer can start watching a video before the entire file has finished downloading. The key technical feature is the moov atom, which contains the metadata needed to begin playback. When the moov atom is placed at the beginning of the file (a process called faststart or web optimization), the player can start decoding and displaying video as soon as it receives the first few kilobytes of data. This is essential for web video, where users expect immediate playback.
MP4 also supports adaptive bitrate streaming through standards like MPEG-DASH (Dynamic Adaptive Streaming over HTTP) and Apple's HLS (HTTP Live Streaming). These technologies allow the player to dynamically adjust video quality based on the viewer's available bandwidth, ensuring smooth playback even on variable network connections. Every major streaming service, from YouTube to Netflix, uses MP4-compatible containers for their streaming infrastructure.
AVI was designed for local file playback, not streaming. Its RIFF structure stores index information at the end of the file, which means the player needs to download or read the entire file before it can build the index and begin playback. This makes AVI fundamentally unsuitable for progressive download or web streaming. While some players can work around this by scanning the file linearly, the experience is inferior to MP4's purpose-built streaming support.
AVI also lacks support for adaptive bitrate streaming. There is no standard for segmenting AVI files for dynamic quality switching, and no major streaming platform or content delivery network uses AVI for delivery.
For any use case involving internet delivery, web embedding, or streaming, MP4 is the only viable choice between the two.
Which Format Should You Choose Today?
For the vast majority of modern use cases, MP4 is the better choice. Here is a practical decision framework.
Choose MP4 for virtually everything new. Whether you are recording video, exporting from an editing application, sharing files with others, uploading to the web, or archiving content for the future, MP4 with H.264 (or H.265 for higher efficiency) is the standard choice. It offers the best combination of quality, file size, compatibility, and streaming support available in any video format today.
Keep AVI only for legacy compatibility. If you are working with equipment or software that specifically requires AVI, such as older security camera systems, legacy industrial equipment, or archived video collections, it makes sense to continue using AVI in those contexts. However, even in these cases, creating MP4 copies for distribution and sharing is recommended.
Convert AVI to MP4 for long-term preservation. If you have a library of AVI files, converting them to MP4 is a worthwhile investment. You will reduce storage requirements by 30-60 percent while gaining universal compatibility and ensuring your content remains playable as AVI support continues to diminish in modern software and devices.
Converting AVI to MP4 is simple with browser-based tools like ConvertFree. The conversion happens entirely on your device, so your video files stay private and never get uploaded to external servers. For large AVI collections, the file size savings alone can be significant. A 10 GB collection of AVI files might compress to 4-6 GB in MP4 format without any visible quality loss.
The bottom line is that AVI is a legacy format from a different era of computing. It served its purpose well for over two decades, but MP4 has comprehensively superseded it in every meaningful dimension. Unless you have a specific technical reason to use AVI, MP4 is the format to use.