Source: https://mactoolbox.app/glossary/container-format

# What is a container format?

By Nikhil Barik · Published November 3, 2026

## Quick answer

A container format is the file type that holds video, audio and sometimes subtitles together. MP4, MOV, MKV and WebM are containers, and each can hold several codecs.

## What a container is

Think of a container as a box. Inside are separate streams, each compressed by its own codec. The container also stores timing, chapters and metadata.

## Why it matters on a Mac

MP4 and MOV are the most compatible on Apple devices. MKV and AVI often will not open in QuickTime Player, even when the codecs inside are fine.

## How to check it on macOS

Select the file in Finder and press Command-I to see its Kind. Media Info in Toolbox shows the container format and each track inside.

## How Toolbox handles it

Convert Video saves MP4, MOV, WebM or M4V. WebM files get VP9 video and Opus audio. When the codecs already fit the new container, Copy streams when possible rewraps the file quickly without re-encoding, so nothing is lost.

## Frequently asked questions

**Does changing the container reduce quality?**

Not if the streams are copied. Re-encoding is what costs quality.

**Is MOV the same as MP4?**

They are closely related containers. MP4 is more widely supported outside Apple devices.

**Why will my MKV not play in QuickTime?**

QuickTime does not open MKV. Convert it to MP4 and it usually plays.

## Tools mentioned

- [**Convert Video**: Change videos to MP4, MOV, WebM or M4V.](https://mactoolbox.app/convert-video-to-mp4-mac)
- [**Media Info**: See the codec, resolution, frame rate, bitrate and metadata of videos and audio.](https://mactoolbox.app/video-codec-info-mac)

## Keep reading

- [Convert Video](https://mactoolbox.app/convert-video-to-mp4-mac)
- [Media Info](https://mactoolbox.app/video-codec-info-mac)
- [Codec](https://mactoolbox.app/glossary/codec)