M4A is a file extension for MPEG-4 Audio files — essentially an MP4 container that holds only audio, typically encoded with AAC (Advanced Audio Coding). It is the native format for iTunes purchases, iPhone voice memos, and recordings made on Apple devices. Renaming .m4a to .mp4 works in most players because they share the same container format.

Why is Opus better than the AAC inside M4A at low bitrates?

Opus uses more advanced psychoacoustic models and combines the SILK (speech) and CELT (music) codecs, switching between them based on content. At 48 kbps, Opus typically sounds transparent for speech and very good for music, while AAC at the same bitrate sounds noticeably compressed. Multiple independent listening tests place Opus ahead of AAC-LC at bitrates below 96 kbps.

Why can't I convert M4A to Opus directly in the browser?

Browsers can decode many audio formats via the Web Audio API, but they do not ship an Opus encoder in JavaScript. Encoding Opus requires libopus, a native compiled library. FFmpeg bundles libopus and handles the full M4A-decode → Opus-encode pipeline on your local machine.

What bitrate should I choose for Opus?

32 kbps is excellent for voice and speech content. 64 kbps is transparent for most listeners in everyday listening. 96 kbps is near-transparent for music. 128 kbps is indistinguishable from lossless for virtually all listeners on any playback system. For archival purposes keep your original M4A — Opus is lossy too.

What is the basic FFmpeg command for M4A to Opus?

ffmpeg -i input.m4a -c:a libopus -b:a 96k output.opus. The -vbr on flag enables variable bitrate mode, which is recommended for music. For a specific output name: ffmpeg -i "My Song.m4a" -c:a libopus -b:a 96k -vbr on "My Song.opus".

Does converting M4A to Opus lose quality?

Yes — this is a lossy-to-lossy transcode. Each re-encode discards additional audio data. At 96 kbps Opus the difference is minimal on most playback systems, but for archival purposes you should keep the original M4A file (or obtain a lossless source such as FLAC or WAV before encoding to Opus).

M4A to Opus Converter

Inspect any audio file in your browser to see its duration, size, and sample rate — then copy the ready-to-run FFmpeg command to convert it from M4A to Opus.

Audio File Inspector

Drop or select any audio file. It is decoded locally in your browser — no data is uploaded to any server.

Click to select or drag an audio file here

M4A, AAC, MP3, FLAC, WAV, OGG, or any format your browser supports

FFmpeg Command Generator

Input File Name

Also accepts .mp4 or any M4A-containing MPEG-4 file.

Target Opus Bitrate

Enable VBR mode (-vbr on) — recommended for music

ffmpeg -i "input.m4a" -c:a libopus -b:a 96k -vbr on "input.opus"

Install FFmpeg free at ffmpeg.org. Run this command in your terminal after replacing the file name.

Batch convert M4A files (Linux / macOS)

for f in *.m4a; do ffmpeg -i "$f" -c:a libopus -b:a 96k -vbr on "${f%.m4a}.opus"; done

Batch convert MP4 audio files (Linux / macOS)

for f in *.mp4; do ffmpeg -i "$f" -c:a libopus -b:a 96k -vbr on "${f%.mp4}.opus"; done

Output File Size Estimator

Enter audio duration below, or inspect a file above to auto-fill.

hrs

min

sec

Estimate: bytes = bitrate (kbps) × duration (s) × 1000 / 8; displayed KB/MB use binary units.

32 kbps

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64 kbps

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96 kbps

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128 kbps

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48 kbps

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Duration: 3 min 30 sec (210 s)

Why No In-Browser Conversion?

Browsers expose the Web Audio API for decoding audio, but they do not include an Opus encoder. Opus encoding requires libopus — a native library compiled for your operating system. FFmpeg bundles libopus and handles the full pipeline:

1 FFmpeg opens the M4A container and reads the compressed AAC audio stream.
2 The AAC decoder decompresses the stream to raw PCM audio in memory.
3 libopus re-encodes the PCM data to Opus at the bitrate you specify.
4 FFmpeg wraps the Opus stream in an OGG container and writes the .opus file to disk.

Install FFmpeg at ffmpeg.org (free, open-source). On Linux: sudo apt install ffmpeg. On macOS: brew install ffmpeg.

Opus Bitrate Reference

Bitrate	Quality	Size / min	Best For
24 kbps	Acceptable	~0.18 MB	Voice calls, minimal bandwidth
32 kbps	Good	~0.24 MB	Podcasts, audiobooks, speech
48 kbps	Very Good	~0.36 MB	Speech, transparent for voice
64 kbps	Excellent	~0.48 MB	Casual music listening
96 kbps	Near-transparent	~0.72 MB	General music, headphones
128 kbps	Transparent	~0.96 MB	Audiophile, archival Opus

M4A vs. Opus — At a Glance

M4A (MPEG-4 Audio / AAC)

Default format for iTunes, iPhone voice memos, Apple devices
Very broad hardware and software support
Supported by all major streaming platforms
Lower quality than Opus at equivalent low bitrates
Larger file sizes for the same perceived quality

Opus (RFC 6716, libopus)

Outperforms AAC at most bitrates, especially below 128 kbps
Royalty-free and open-source (Xiph.org / IETF)
Native support in all major browsers and WebRTC
Not supported by older car stereos or dedicated audio players
Apple devices have limited native Opus support outside browsers

Copied!

Summary

Inspect any audio file in your browser to see its duration, size, and sample rate — then copy the ready-to-run FFmpeg command to convert it from M4A to Opus.

How it works

Drop or select an audio file in the inspector panel — the file never leaves your device.
The Web Audio API decodes the file header and reports duration, file size, and sample rate.
Enter your file name and choose a target Opus bitrate in the command generator.
Copy the generated FFmpeg command and paste it into your terminal.
FFmpeg decodes the M4A stream, re-encodes it as Opus inside an OGG container, and writes the output file.
Play the output file to verify quality, then delete the original M4A if no longer needed.

Use cases

Convert iPhone voice memos or Apple Music downloads from M4A to the more efficient Opus format.
Reduce podcast episode file sizes significantly with Opus at 32–64 kbps versus AAC at 128 kbps.
Prepare audio for WebRTC or web streaming where Opus is natively supported by all browsers.
Batch-convert an entire M4A library to Opus to save storage space.
Inspect an unknown audio file to confirm its duration and sample rate before processing.
Generate ready-to-paste FFmpeg commands without memorizing Opus encoder flags.
Compare M4A and Opus quality characteristics before choosing a target bitrate.
Convert audiobooks or lecture recordings from M4A to Opus for efficient long-term storage.

Frequently Asked Questions

Last updated: 2026-09-24 · Reviewed by Nham Vu

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