Understanding Video Formats: MP4, WebM, and More

Most people pick MP4 because it always works, and that's a perfectly good reason. But when a download offers you the same resolution twice at different file sizes, or a 4K option arrives silent, the reason stops being obvious. A little background makes those moments make sense.

Containers and codecs are not the same thing

This is the single most useful distinction in video, and almost nobody explains it. A container is a box. A codec is the way the contents were compressed. MP4, WebM, MKV and MOV are boxes. H.264, VP9, AV1 and HEVC are compression methods.

So a file named clip.mp4 tells you about the box, not what's inside. Two MP4 files can hold completely different codecs, which is why one plays on your old TV and the other shows a black screen with working audio. The extension was never the whole story.

The containers you'll actually meet

• MP4: the universal default. Every platform accepts it, every device plays it, it handles video, audio, subtitles and chapters.
• WebM: Google's open container, built for the web. Smaller files at the same quality, less support on older hardware.
• MOV: Apple's format, common out of iPhones and Final Cut. Technically very similar to MP4 under the hood.
• MKV: the flexible one. Holds almost anything including multiple audio tracks and subtitle streams, but no social platform accepts it directly.
• AVI: a relic from the 1990s. You'll only meet it in old archives, and it should be converted rather than kept.

The codecs that matter

H.264, also called AVC

Released in 2003 and still the most compatible video codec that exists. Hardware decoders for it sit in essentially every phone, laptop, TV and set top box made in the last fifteen years. It isn't the most efficient option any more, but it plays everywhere, and that's often worth more than a smaller file.

VP9

Google's answer to the licensing fees around H.265. It delivers roughly 30 to 50 percent smaller files at similar quality, and YouTube leans on it heavily for 1080p and above. Support is good on modern browsers and phones, patchier on older TVs and some editing software.

AV1

The current state of the art, royalty free, and around 30 percent more efficient again than VP9. The tradeoff is decoding cost. Devices without dedicated AV1 hardware chew through battery playing it. Newer phones and GPUs handle it fine, older ones struggle.

HEVC, also called H.265

Very efficient, widely used by Apple for iPhone recordings, and tangled in patent licensing that stopped it taking over the web. If your iPhone footage takes half the space you expected, HEVC is why.

Bitrate is what you can actually see

Resolution gets all the attention and bitrate does most of the work. Resolution is how many pixels exist. Bitrate is how much data per second describes them. A 4K video at a starved bitrate looks worse than a well encoded 1080p file, every single time.

This is why a 1080p TikTok looks softer than a 1080p YouTube video. Same pixel count, very different data budget. It's also why fast motion suffers first: quick movement needs more data to describe, and when the budget runs out you get blocky artefacts during action while static shots stay clean.

• 720p at 2.5 to 4 Mbps: fine for talking heads, adequate for most social clips.
• 1080p at 5 to 8 Mbps: the standard sweet spot for web video.
• 1440p at 10 to 16 Mbps: worth it on a large monitor, invisible on a phone.
• 2160p at 25 to 50 Mbps: only meaningful on a genuine 4K display at close range.

Audio, briefly

AAC is the default audio codec inside MP4 and it's what almost every platform serves. Opus is the modern open alternative, noticeably better at low bitrates, and it's what you'll find inside WebM files from YouTube. MP3 still exists but is outdated for video work, and there's rarely a reason to choose it over AAC.

Most web video runs stereo audio somewhere between 128 and 192 kbps. Above that the returns are small unless you're working with music, and below 96 kbps you start hearing it on decent headphones.

Choosing in practice

If you want one rule: take MP4 with H.264 unless you have a specific reason not to. It plays on everything, every editor imports it without complaint, and every platform accepts it for upload.

Take WebM when storage or bandwidth matters more than compatibility, and you know the file is staying on modern devices. Keep MOV if you're editing iPhone footage in Final Cut and never leaving that ecosystem. Convert MKV to MP4 before uploading anywhere, because no social platform takes it.

Why you can't get quality back

Compression throws information away permanently. Converting a 720p file to 1080p does not add detail, it just spreads the same detail across more pixels and makes the file bigger. Converting between codecs re-encodes and loses a little more each time, the same way photocopying a photocopy degrades.

So always start from the highest quality version you can get, and only convert once, at the end, to whatever your destination needs. If you save clips from social platforms regularly, our how it works page covers choosing quality at download time, which is the step where these decisions actually get made.