yt-dlp audio: WebM/Opus, M4A/AAC or MP3 — which one should you keep?

By Tunelio team·Published Sep 7, 2026

YouTube does not store MP3s. Its audio streams are Opus in a WebM container (up to ~160 kbps) and AAC in M4A (~128 kbps). yt-dlp -x gives you one of those untouched; asking for MP3 makes ffmpeg re-encode it, losing a little quality and taking CPU. Keep WebM/Opus for quality and size, M4A for Apple devices and broad compatibility, MP3 only when the target player understands nothing else.

What YouTube actually serves

yt-dlp -F "https://youtu.be/dQw4w9WgXcQ"
# ID  EXT  ... ACODEC  ABR
# 249 webm     opus    ~50k   (low)
# 250 webm     opus    ~70k   (medium)
# 251 webm     opus    ~130–160k (high)  ← usually "bestaudio"
# 140 m4a      aac     ~128k

Those are the only audio-only tracks for a normal video (some videos also expose higher-bitrate AAC or multi-language tracks). Whatever you end up with — MP3, FLAC, WAV — is derived from one of these, so it cannot be better than ~160 kbps Opus or ~128 kbps AAC.

The three choices

Keep the original (WebM/Opus or M4A/AAC) — best quality, no transcode

yt-dlp -x --audio-format best "URL"     # keeps whatever bestaudio is (usually .webm/.opus)
yt-dlp -f bestaudio "URL"               # same stream, no post-processing at all

Zero quality loss, instant, smallest file for the quality. Opus plays natively in every modern browser, Android, VLC, Windows 10+ and most desktop players. --audio-format best also remuxes .webm to .opus when possible so tagging tools recognise it.

M4A (AAC) — Apple-friendly, still no transcode if the source is AAC

yt-dlp -f 'ba[ext=m4a]' "URL"           # take the AAC track as-is (itag 140)
yt-dlp -x --audio-format m4a "URL"      # AAC in .m4a; transcodes only if the chosen source was Opus

iPhones, iPads, iTunes/Music and many car systems handle M4A but not Opus. Selecting the AAC track directly avoids any re-encode; the ~128 kbps AAC is fine for spoken content and most music.

MP3 — maximum compatibility, always a re-encode

yt-dlp -x --audio-format mp3 --audio-quality 0 "URL"

Use MP3 when the destination truly requires it: old car stereos, cheap MP3 players, some podcast platforms and legacy tools. The transcode is lossy-to-lossy and costs ffmpeg CPU time; VBR 0 keeps the loss small. Full details in the MP3 guide.

Side-by-side

• Quality: WebM/Opus > M4A/AAC ≳ MP3 (at the same bitrate Opus is the most efficient codec of the three; MP3 the least).
• File size for a 4-minute song: Opus 160k ≈ 4.7 MB · AAC 128k ≈ 3.8 MB · MP3 VBR 0 ≈ 6–8 MB.
• Compatibility: MP3 everywhere · AAC/M4A everywhere except a few very old players · Opus everywhere except older Apple devices and some car systems.
• CPU: original = none · M4A from AAC = none · MP3 = a few seconds per track (matters at scale).
• Metadata and cover art: all three support tags via --embed-metadata --embed-thumbnail.

Picking by use case

• Personal library on phone/desktop → keep the original (--audio-format best).
• Apple ecosystem, car via USB → M4A (-f 'ba[ext=m4a]').
• Podcast feed, old hardware, unknown players → MP3 VBR 0.
• Speech for transcription or ML → keep Opus, or ask for the transcript directly instead of the audio.
• Archival → keep the original stream; you can always transcode later, never the other way round.

Common mistakes

• “--audio-format mp3 --audio-quality 320K for best quality” — the source is ~160 kbps; you are enlarging the file, not improving it.
• Converting Opus to M4A when the AAC track exists — select itag 140 instead and skip the re-encode.
• Hard-coding .mp3 in -o — use %(ext)s so yt-dlp can rename after conversion.
• Expecting FLAC/WAV to be lossless — they are lossless containers of a lossy source; larger, not better.

In a product

If your app hands users audio, the format decision becomes a server cost: every MP3 is an ffmpeg job, every Opus is a passthrough. A hosted API lets you ask per request — quality=mp3 for a ready MP3, quality=opus for the original stream — and returns a URL either way. Tunelio is our own service; for personal downloads the commands above are all you need. New accounts get 100 free credits.

Sources

• yt-dlp README — Format selection (-f, -S, bestaudio), --audio-format best.
• yt-dlp -F output — the itag/codec table for any video.
• Opus codec (opus-codec.org) and ffmpeg libmp3lame documentation — bitrate and quality scales.