What happens when you convert AAC to MP4
- MP4 is a video format. The audio is encoded as AAC audio, and by default the file has no picture.
- Tick "Add a black video track" if the MP4 is going to YouTube or another site that rejects audio-only video.
How to convert AAC to MP4 online
- Press "Choose AAC files" and select one or more files, or drag them onto the box. You can add files of any size your device can hold in memory.
- Adjust the settings if you need to: resolution and quality control the size of the result. The defaults suit most people.
- Press "Convert to MP4". The first audio or video conversion downloads the converter engine, about 32 MB, which your browser then keeps for next time.
- Save each MP4 with its download button. Close the tab and every trace of the files is gone from memory.
Why convert AAC to MP4?
AAC files usually come from iTunes and Apple Music, YouTube audio and streaming. People convert them to MP4 because MP4 with H.264 plays on every phone, laptop, smart TV and website, and it is what YouTube, Instagram, TikTok and WhatsApp expect.
In short, AAC is good at what it was made for, and MP4 is the more practical choice when the file has to play, upload or edit somewhere else. AAC plays in all current browsers and on Apple and Android devices, while MP4 plays in every browser, phone and smart TV.
How to convert AAC to MP4 on Windows, Mac, iPhone and Android
On Windows
Open this page in Chrome, Edge or Firefox, drag your AAC files in from File Explorer, and press Convert. Each MP4 is saved to your Downloads folder. If you prefer a desktop program, VLC can also convert AAC to MP4: choose Media, then Convert / Save.
On a Mac
Safari, Chrome and Firefox all work. Drag the AAC files from Finder onto the drop zone and the MP4 files land in Downloads.
On iPhone and iPad
Open the page in Safari and tap Choose AAC files. You can pick from Photos or from the Files app. The converted MP4 appears in Files, in the Downloads folder; to put it in your photo library, open it there, tap Share and choose Save Video.
On Android
Open the page in Chrome, tap Choose AAC files and pick them from Files or your gallery. The MP4 is saved to the Download folder, where any file manager can find it. Keep the screen on while a long video converts, because Android may pause a tab in the background.
Best settings for AAC to MP4
Leave Resolution on Keep original and Quality on Standard if the video only needs to change container: that is what lets the tool copy the H.264 stream straight into the MP4, which is fast and loses nothing.
If the file is too big, change either setting and the video is re-encoded with H.264. Quality uses the CRF scale: High is 20, Standard 23 and Smaller file 28. As a rule of thumb each step of about 6 on that scale halves or doubles the size. Dropping from 1080p to 720p usually saves more than lowering the quality, and it also makes the conversion much faster.
AAC vs MP4: the differences
| AAC | MP4 | |
|---|---|---|
| Full name | Advanced Audio Coding | MPEG-4 Part 14 video |
| Type | audio | video |
| Created by | the Moving Picture Experts Group | the Moving Picture Experts Group |
| First released | 1997 | 2001 |
| Compression | Lossy | Lossy |
| Usually contains | AAC audio in a raw ADTS stream | H.264 or H.265 video with AAC audio |
| In a browser | plays in all current browsers and on Apple and Android devices | plays in every browser, phone and smart TV |
| Common uses | iTunes and Apple Music, YouTube audio and streaming | phone videos, YouTube uploads, social media and streaming |
| MIME type | audio/aac | video/mp4 |
About AAC and MP4 files
What is an AAC file?
AAC was designed as the successor to MP3 and sounds better at the same bitrate, especially below 128 kbps. A bare .aac file is a raw stream with no chapters or cover art; the same audio in a container is usually called M4A.
What is an MP4 file?
MP4 is the most widely supported video format, and H.264 video with AAC audio inside it plays on essentially every device made in the last fifteen years.
AAC to MP4 not working? Common problems
- The conversion is slow. Audio and container changes take seconds, but re-encoding video runs on your own processor. Lower the resolution to 720p, close other tabs, and keep the device plugged in.
- The tab stops or reloads on a very large file. Browsers limit how much memory one tab can use, around 2 GB and less on phones. Split long recordings or convert them on a computer.
- The MP4 will not play on an old device. Try the other formats offered for AAC, or choose 720p, which older players handle more reliably.
Convert AAC to MP4 with ffmpeg
This converter runs ffmpeg inside your browser. If you would rather use the ffmpeg command line on your own computer, this command produces an equivalent MP4 with the same kind of settings:
ffmpeg -i input.aac -c:v libx264 -crf 23 -preset veryfast -c:a aac -b:a 128k -movflags +faststart output.mp4Replace input.aac with the name of your file. For a whole folder of AAC files, run the command in a loop, or drop them all on the converter above, which processes them one after another.
AAC to MP4 converter FAQ
Will I lose quality converting AAC to MP4?
A little. Both formats are lossy, so re-encoding compounds the loss slightly. Keep the default quality or higher and the difference is rarely noticeable.
Why is the MP4 a different size from the AAC?
Without a picture, the MP4 is about the size of the audio. Adding a black video track adds a little, because a still black frame compresses to almost nothing.
Are my AAC files uploaded anywhere?
No. The conversion runs inside this browser tab using ffmpeg compiled to WebAssembly, so your files stay on your device. You can disconnect from the internet after the page and converter engine have loaded and it still works.
Can I convert several AAC files to MP4 at once?
Yes. Select or drop as many files as you like; they are converted one after another and each gets its own download button.
How do I open an MP4 file?
MP4 plays in every browser, phone and smart TV. MP4 is common for phone videos, YouTube uploads, social media and streaming.
Is this AAC to MP4 converter free?
Yes. There is no sign-up, no watermark and no limit on how many files you convert. The site is paid for by advertising, and because conversions run on your device there is no server cost per file.
Can I convert AAC to MP4 on my phone?
Yes. The converter works in Safari on iPhone and iPad and in Chrome on Android. Long HD videos convert more slowly on a phone than on a computer.