Why does QuickTime say "not compatible" even for an MP4 file?

The file container is MP4, but the video codec inside may not be one QuickTime supports. QuickTime only decodes H.264, HEVC (H.265 on macOS High Sierra 10.13 and later) and Apple ProRes. If the MP4 uses a different codec, you will get the error.

Can I make QuickTime play MKV files by renaming them to .mp4?

No. Renaming an MKV file to .mp4 does not change the container or codec. QuickTime will still try to read the file and fail. You must convert the file to a supported format.

Is there a free player that can open all these files without converting?

Yes, the free VLC player plays most of these formats including MKV, AVI, WMV and FLV without any conversion. It is a good alternative if you prefer not to convert your files.

What is the best format to convert to for my iPhone and Mac?

Convert to MP4 with H.264 video and AAC audio. This format gives the widest compatibility across Mac, iPhone, Windows and browsers, and it will play in QuickTime without issues.

QuickTime Can't Open Your Video? Why and How to Fix It

If QuickTime Player can't open a file you just downloaded or recorded, you are likely seeing the familiar "cannot open" or "not compatible" message. QuickTime supports a limited set of formats and codecs. The fix is usually a straightforward conversion. Here is exactly which video types QuickTime handles, which it will never play, and how to convert your file so it works.

Why QuickTime Shows a "Cannot Open" Error

QuickTime Player plays only MOV, MP4 and M4V files. Even within those formats, the video must use a codec that QuickTime can decode. Supported codecs include H.264, HEVC (H.265, on macOS High Sierra 10.13 and later) and Apple ProRes. When you receive an error, it is because either the file format is not one of those three, or the video or audio inside uses a codec QuickTime does not support.

A common confusion: a file can have an .mp4 extension but still contain a codec QuickTime does not support. The format and the codec are separate things. For example, an MP4 file might contain video encoded with a codec that QuickTime cannot read. In that case, the error will still appear.

File Types QuickTime Will Never Open

Some video formats are not supported by QuickTime at all. You cannot make QuickTime open these files without converting them first. The main examples are:

MKV (Matroska Video). QuickTime won't play MKV files.
WMV (Windows Media Video). QuickTime won't play WMV files.
FLV (Flash Video). QuickTime won't play FLV files.
AVI. AVI files often contain codecs such as DivX or Xvid that QuickTime cannot decode. Even if the format could theoretically work, the codec inside prevents playback.

The Container and Codec Problem

To understand why video won't open on Mac, you need to separate the file format from the codec. The format is what you see in Finder: .mp4, .mov, .mkv, .avi, .wmv. The codec is the compression method used for the video or audio track inside that format.

QuickTime will open .mp4 and .mov files, but only if the video codec is H.264, HEVC (H.265) or Apple ProRes. If the video inside uses a different codec, such as VP9 or MPEG-2, QuickTime will refuse to play it. That is why you might see a file with the right extension but still get an error.

How to Fix QuickTime Not Playing Your Video

The most reliable fix is to convert your video to a format QuickTime can handle. The best target for compatibility across Mac, iPhone, Windows and browsers is MP4 with H.264 video and AAC audio. Below are the steps for the two most common scenarios.

QuickTime Won't Play MKV

MKV files are common for high definition video. QuickTime won't play them, so convert to MP4.

Open your video converter tool.
Add the MKV file as the source.
Choose MP4 as the output format.
Select H.264 as the video codec and AAC as the audio codec.
Start the conversion.

QuickTime Won't Play AVI

AVI files often use DivX or Xvid codecs. QuickTime cannot decode these. Conversion to MP4 with H.264 solves this.

Load the AVI file into your converter.
Set the output to MP4.
Confirm the video codec is H.264 and audio is AAC.
Convert the file.

What About Playing WMV on Mac?

If you need to play WMV on Mac, QuickTime will not help. WMV is a Microsoft format that QuickTime does not support. You have two options. The first is to use a free player like VLC, which plays most of these formats without converting. The second is to convert the WMV file to MP4 using the same H.264 and AAC settings, making it playable in QuickTime and many other applications.

Using an Online Converter

If you do not want to install software, an online converter can handle the job. A site like hdvideoconverter-mac.com converts video to MP4, MOV or WebM online. You upload your file, choose the output format, and download the converted version. This approach works for MKV, AVI, WMV, FLV and any other file that QuickTime cannot open. After conversion, the new file will play in QuickTime without errors.

Choose the Right Output Format

For the widest compatibility, always convert to MP4 with H.264 video and AAC audio. This combination works on every modern Mac, iPhone, Windows PC and web browser. If you specifically need a format for Apple software, MOV is also an option. WebM is useful for web use but has less universal support than MP4.