CloudConverter
You download or record a video, double-click it, and nothing happens. Or it plays without sound. Or your TV says the format is not supported. In most cases the fix is the same: convert the file to MP4.
This guide explains why MKV, AVI and MOV files sometimes will not play, and how to convert them to MP4 in a few minutes.
Why a video will not play
A video file has two layers. The container is the file type you see, such as MKV, AVI, MOV or MP4. Inside it, the codec is the method used to compress the picture and the sound. A device can open a file only if it understands both the container and the codec.
That is why the same type of file plays on one device and fails on another. An old AVI may use a codec that modern phones do not support. An MKV may hold audio your TV cannot decode. A MOV from an iPhone may use HEVC video that some Windows programs cannot open without extra software.
Why MP4 is the safe choice
MP4 is the most widely supported video format. Phones, tablets, smart TVs, game consoles, browsers, social media platforms and video editors all accept it. If a video has to work on many devices, MP4 is the format to aim for.
Common formats and their problems
| Format | Typical source | Common problem |
|---|---|---|
| MKV | Downloads, screen recorders such as OBS | Many TVs, phones and editors will not open it |
| AVI | Old cameras and older software | Large files, outdated codecs, poor phone support |
| MOV | iPhone and Mac recordings | Some Windows apps and web forms reject it |
| WebM | Browser downloads and web recordings | Not supported on some iPhones and editors |
How to convert a video to MP4
- Open the converter for your format: MKV to MP4, AVI to MP4, MOV to MP4 or WebM to MP4.
- Drag your video onto the upload box, or tap it to pick the file from your phone or computer.
- Wait for the conversion to finish. Larger videos take longer.
- Download the MP4 and open it on the device that had trouble before.
The steps are the same on iPhone, Android, Windows and Mac, because the conversion happens on our servers and the page runs in your browser.
What to know before you convert
- The video is re-encoded. That takes more time than renaming a file, and for normal viewing the difference in quality is rarely noticeable.
- The upload limit is 200 MB per file. For a longer video, cut it into shorter parts first.
- The output is limited to 1920 pixels on the longest side. A 4K video is scaled down to 1080p. Keep the original if you need full 4K.
- MKV extras are not kept. Extra audio tracks and subtitle tracks are not included in the MP4, so keep the original MKV if you need them.
Tips for a smooth conversion
- Trim the video before you convert it. A shorter clip uploads and converts faster.
- Use a stable connection, because the whole file has to upload first.
- Play the MP4 before you delete the original, and check both sound and picture.
- If you want a small looping clip instead of a full video, see how to make a GIF from a video.
Frequently asked questions
Will converting to MP4 reduce the quality?
The video is re-encoded, so tiny differences are possible. For everyday viewing most people will not see them. If you need the exact original, keep the source file.
Why is my MKV too big to convert?
Uploads are limited to 200 MB per file. For larger videos, cut them into shorter parts, or lower the quality in the app that recorded the video.
Does the MP4 keep the sound?
Yes. The audio is converted along with the video. For MKV files, only the main audio track is kept.
Why does my iPhone video not open on Windows?
iPhone videos are saved as MOV, often with HEVC video, which some Windows programs cannot read. Converting the file to MP4 makes it open in almost any player or editor.
Ready to fix that video? Use the MKV to MP4 converter, the AVI to MP4 converter or the MOV to MP4 converter. They are free, with no sign-up and no watermark.