MP4, MOV, WebM or MKV: Which Video Format Plays Everywhere?

Updated 27 October 2026 · by the My Gadgets team

If you want a video to play everywhere, send an MP4 file with H.264 video and AAC audio. It opens on every phone, computer, TV, browser and messaging app in common use. MOV, WebM and MKV are fine formats, but each has places where it won't play. And the reason a video "won't open" is often not the format at all, but the codec inside it. This article explains the difference in plain terms and tells you which to use for what.

Format and codec: two different things

A video file has two layers:

• The container is the file type you see in the name: .mp4, .mov, .webm, .mkv. It is the box that holds the video, the sound, subtitles and information such as length.
• The codec is how the picture and sound inside the box are compressed: H.264, HEVC (H.265), VP9, AV1 for video; AAC, Opus, MP3 for audio.

A device must understand both. That is why two .mp4 files can behave differently: one holding H.264 plays everywhere, one holding HEVC may not play on an older Windows PC. And why renaming .mov to .mp4 sometimes "works" (same codec inside) and sometimes doesn't.

Which format plays where?

At the time of writing, for the common combinations:

"Messaging apps" here means sending the file itself. When you send a video as a normal video in most chat apps, the app converts it to its own format anyway, at a lower quality.

Why does my iPhone video not play on Windows?

Recent iPhones record in High Efficiency mode by default, which stores video as HEVC in a .mov file. HEVC files are roughly half the size of H.264 at similar quality, which is why Apple uses it, but not every Windows PC can play HEVC without an extra codec.

Two fixes:

• For a video you already have: convert it to MP4 (H.264) with Convert Video Format.
• For future videos: on the iPhone, go to Settings → Camera → Formats and choose Most Compatible. The camera then records H.264. Files are larger, but they open everywhere without conversion.

Which should I choose?

Sending to someone, or uploading to a form: MP4 with H.264 and AAC. Nothing else is as safe.

Putting a video on your own website: MP4 (H.264) as the main file plays in every browser. WebM can be smaller for the same quality and is a good second source if your site serves both.

Editing later: keep the original file from the camera. Every conversion to a compressed format loses a little quality, so convert once, at the end.

Archiving films with several audio tracks or subtitles: MKV is the most flexible container, but convert to MP4 for anything you share.

A short loop without sound: a muted MP4 is many times smaller and smoother than a GIF. Use GIF only where nothing else is accepted.

How do I convert or shrink a video?

• Convert Video Format writes MP4 (H.264 / AAC), MOV (H.264 / AAC), MKV (H.264 / AAC), WebM (VP8 / Opus), AVI (MPEG-4 / MP3) and animated GIF (10 frames per second, 320 pixels wide). It re-encodes the video, so the result plays on far more devices than a simple rename would.
• Compress Video makes an H.264 MP4 smaller. Choose a resolution (1080p, 720p, 480p or 360p) and a quality level (CRF 18 for high quality up to 40 for the smallest file). For a phone video that will be watched on another phone or laptop, 720p at CRF 28 is a good starting point.
• If you only need part of the video, cut it first with Trim Video: every second you remove is a second you don't have to convert.

These tools run in your browser using a video engine that is downloaded the first time you use them (about 32 MB). Your video is processed on your own device, so a private clip never goes to a server. The trade-off is speed: a browser is slower than desktop software, so a long or 4K video can take several minutes. For hour-long recordings, a desktop converter will be faster.

Quick answers

Is MP4 always H.264? No. MP4 can hold HEVC, AV1 and others. If an MP4 won't play, the codec is usually the reason; converting to MP4 with H.264 fixes it.

Does converting lose quality? Re-encoding always loses a little. At sensible settings you won't see it once, but avoid converting the same file again and again.

Why is my converted file bigger than the original? HEVC and VP9 compress better than H.264, so converting from them to H.264 at the same quality makes a bigger file. That is the price of compatibility; compress it afterwards if size matters.

Which format is smallest? For the same quality, AV1 and HEVC are smallest, then VP9, then H.264. But the smallest file is no use if the other person can't play it.

What to do next

To make a video that plays everywhere, convert it to MP4 with Convert Video Format; to make it small enough to send, use Compress Video at 720p. If it is going by email, what to do when an attachment is too big covers the limits and the alternatives.