Free converter · Lossy re-encode
Convert AAC to Opus
Short answer
Drop your AAC file into the converter below and download the Opus file. It runs in your browser with FFmpeg, for files up to 100 MB; nothing is uploaded. The file is re-encoded to Opus audio. Re-encoding takes longer than a container swap and costs some quality; how much depends on the clip and on the quality setting, and at the converter's default setting the loss can be visible, so keep the original.
Maximum file size: 100 MB. The first conversion loads a ~30 MB converter engine, once per visit. Nothing is uploaded - conversion runs on your device.
AAC to Opus; both lossy. · Lossy re-encode · Browser-side via FFmpeg WASM - nothing uploaded.
Key takeaways
- The 245 KB AAC file became a 117 KB Opus file (-52.3%), about 93 kbps for the 10 second clip.
- The result holds: Audio: opus 48000 Hz 2 ch.
- In your browser the same conversion of a 3 second clip took 0.07 seconds on the single-thread engine; natively, the 10 second clip took 0.10 seconds.
- Next to it: AAC to FLAC (+567%), AAC to M4A (-0.3%), AAC to MP3 (-34.2%).
Measured: AAC to Opus
The converter's own ffmpeg arguments, run on a 10 second 1920 × 1080 excerpt of Big Buck Bunny (an animated film, not camera footage) saved in AAC. The browser column is the same arguments on the single-thread WebAssembly core with a 3 second excerpt, in a headless run; your device will differ. Results describe this clip and these encoders.
| Check | Result |
|---|---|
| Mode | Audio re-encoded |
| Input | AAC, 245 KB |
| Output | Opus, 117 KB (-52.3%) |
| Output streams | Audio: opus 48000 Hz 2 ch |
| Time, native, 10 s clip | 0.10 s |
| Time, browser engine, 3 s clip | 0.07 s |
A re-encode decodes and encodes every frame or sample, so the time grows with the length of the file.
AAC vs Opus
| AAC | Opus | |
|---|---|---|
| Full name | Advanced Audio Coding (raw stream) | Opus audio in Ogg |
| What is inside | AAC audio without a container | Opus audio |
| Compression | Lossy | Lossy |
| Best for | Players that take a bare AAC stream; M4A is the more common wrapper | Voice and music at low bitrates; browser video calls (WebRTC) use it |
What you get
Opus audio. AAC to Opus; both lossy.
Planning a bigger file
These are estimates scaled up from the 10 second clip. Real files differ with content, resolution and bitrate, so treat them as an order of magnitude, not a promise.
| Length of the video or recording | Input size (like ours) | Opus size | Time in your browser |
|---|---|---|---|
| 1 minute | 1.46 MB | 698 KB | 1 seconds |
| 10 minutes | 14.65 MB | 6.98 MB | 13 seconds |
| 1 hour | 87.90 MB | 41.89 MB | 79 seconds |
An AAC file of this kind reaches the 100 MB limit of the converter after about 72 minutes; converting it would take roughly 1.6 minutes on the single-thread engine, and the Opus would be about 49.97 MB.
AAC and Opus next to their neighbors
The same clip through other conversions with the same source or the same target.
| Conversion | How | Size change | Time (native) |
|---|---|---|---|
| AAC to Opus | Re-encode | -52.3% | 0.10 s |
| AAC to FLAC | Re-encode | +567% | 0.11 s |
| AAC to M4A | Copy | -0.3% | 0.05 s |
| AAC to MP3 | Re-encode | -34.2% | 0.24 s |
| AAC to OGG | Re-encode | -51.2% | 0.22 s |
| AAC to WAV | Re-encode | +686% | 0.09 s |
| FLAC to Opus | Re-encode | -92.8% | 0.09 s |
Choosing Opus
Opus: Opus audio in Ogg. Voice and music at low bitrates; browser video calls (WebRTC) use it. AAC: Advanced Audio Coding (raw stream). Players that take a bare AAC stream; M4A is the more common wrapper.
- Keep your original: a re-encode is a copy of a copy.
- If the destination states the format it accepts, use that and nothing else.
Questions people ask
Is this AAC to Opus converter free and private?
Yes. It is free, needs no account and runs entirely in your browser, so the file never leaves your device.
Will converting AAC to Opus reduce quality?
Some, because Opus is lossy and the file is re-encoded. How much depends on the clip and on the quality setting, and at the converter's default setting the loss can be visible, so keep the original. For uploads, the platform re-encodes again anyway.
Is there a file size limit?
Files up to 100 MB. The first conversion downloads a converter engine of about 30 MB once per visit; larger files are better handled by a desktop app such as HandBrake or FFmpeg.
How do I convert AAC to Opus?
Choose the file in the converter above and download the Opus. It runs in your browser with no upload.
Will converting AAC to Opus reduce quality?
A little, because Opus is lossy. Keep the original file.
Why did the file size change by -52.3%?
The target codec and settings decide the size. This 10 second clip came out at 117 KB, about 93 kbps.
How long does it take?
A 3 second clip took 0.07 seconds in the browser engine in our test; time grows with the length of the file.
How long will an AAC file of 100 MB take?
At the speed we measured in the browser engine, about 1.6 minutes for a file like our clip. Your device and the content will change it.
Is my file uploaded?
No. The conversion runs in your browser.
Is there a size limit?
Files up to 100 MB. The first conversion downloads a converter engine of about 30 MB once per visit.
More converters
Sources and how we checked
- MDN: Media container formats
- MDN: Web audio codec guide
- MDN: Image file type and format guide
- FFmpeg documentation: stream copy
- FFmpeg Documentation: ffmpeg (stream selection, -map, -c copy) (read 2026-10-09)
- MDN: Web audio codec guide (read 2026-10-09)
- Blender Foundation: Big Buck Bunny (CC BY 3.0), the clip used for the measurements (read 2026-10-09)
Reviewed October 8, 2026. What the converter writes was measured with ffprobe on the same FFmpeg build your browser downloads; format descriptions follow MDN's media guides.