Convert FLAC to OPUS

FLAC compresses losslessly to roughly half of raw PCM, which works out at about 5 to 6 MB a minute for CD-quality stereo — the exact figure depends on how dense the music is, not on any setting. Opus at the Balanced band writes 64 kbps, which is about 0.5 MB a minute regardless of material.

So a four-minute track goes from around 22 MB to under 2 MB, and a 500 GB collection lands near 50 GB. That is the number that decides whether a library fits on a phone, and it is roughly half what the same collection would occupy as MP3 at a comparable listening quality. Nothing else on this site produces that ratio from a lossless source without an audible penalty.

MP3 was standardised in 1993 and Opus in 2012, and nineteen years of research sits between them. The gap is not uniform: at 256 kbps the two are hard to separate, and at 64 kbps MP3 is unpleasant while Opus is comfortable. A phone copy lives at exactly the bitrates where the difference is largest, which is why this pairing is worth the trouble.

The practical translation is that Opus at 64 kbps holds its own against MP3 at 128, and Opus at 128 against MP3 at 192 or 256. Halving a library's size for the same experience is not a marginal gain when the library is the reason a phone is full. The cost is compatibility, and that is the next section rather than a footnote.

The registry marks Opus with patchy support, and it earns that. Android has decoded it natively for years, every current browser reads it, VLC and foobar2000 read it, and the self-hosted music clients — Symfonium, Substreamer, Finamp, the Jellyfin and Plex apps — all handle it because they bundle their own decoders. That covers most of the reason anyone converts a library.

What does not: car head units, older iPods and portable players, a good deal of hi-fi and network streamer firmware, and the built-in file preview on iOS in some situations. If the destination is a dashboard or a bedside streamer, this is the wrong conversion and MP3 or AAC is the pragmatic answer. Convert one album, put it on the actual device, and listen before converting two thousand.

The three bands resolve inside the encoder rather than being set here: Small file writes 38 kbps, Balanced 64 and High quality 128. Those numbers look alarming to anyone raised on MP3 and should not. Streaming services send around 64 kbps Opus on their low-data settings, and most listeners never notice on a commute.

The sensible rule is to match the band to where the copy will be heard. Podcasts, audiobooks and spoken word: Small file, and the saving is enormous. Music on a phone through earbuds or in a car: Balanced. Music on good headphones, or a copy you might transcode again for something else: High quality. The FLAC is still at home, so a wrong choice costs a re-run rather than a recording.

Opus works on a fixed 48,000 Hz clock. A decoder reports 48 kHz whatever the encoder was given, so a 44,100 Hz CD rip presents as a 48 kHz file after conversion and a 96 kHz high-resolution FLAC is brought down to the same place. This is a property of the codec rather than a setting, and there is no version of this conversion that avoids it.

For listening it is a non-issue — the resampling is done properly and no one has ever identified it by ear. It matters in exactly two situations: a cataloguing tool that records the technical sample rate from the file will disagree with the master, and anyone converting high-resolution material should understand that the extra bandwidth they paid for is discarded here, as it is by every lossy codec.

A live album, a DJ mix or a classical record with attacca movements is one continuous performance cut into tracks. FLAC reproduces the joins exactly because it reproduces the samples exactly. Any lossy encoder works in fixed blocks and has to pad the ends of each track to fill the final one, which is where audible seams come from.

Opus handles this better than MP3 does — the Ogg container records a pre-skip value that tells a decoder how much of the start to discard, and players that honour it rejoin the tracks cleanly. Whether your player honours it is the variable. If a gapless album matters, test that album specifically before converting the rest, and keep the FLAC either way so a bad result costs nothing.

A converted collection is worthless if the metadata scatters, and this is the one crossing on this site where nothing has to be rebuilt. FLAC tags are Vorbis comments; Ogg carries Vorbis comments; the fields are the same uppercase keys with the same text. Album artist, track and disc numbers, date, genre, composer, replay-gain values and anything custom all arrive as they were.

Cover art crosses as a METADATA_BLOCK_PICTURE comment, which is the standard encoding and what music servers, foobar2000, VLC and the mobile clients read. The practical consequence is that a converted library scans into a music server with its albums intact rather than as two thousand unknown tracks, which is the failure mode that makes people abandon this exercise halfway.

Treat the Opus files as disposable. They can be regenerated from the masters in an evening, at a different bitrate if the first choice was wrong, or into a different codec entirely when the next phone or the next car refuses them. The FLACs cannot be regenerated from anything.

The failure this warns against is specific and common: someone converts a collection, sees that it sounds fine, and reclaims the disk. A year later a device needs AAC, or a better encoder arrives, and every re-encode now starts from a lossy source. Keeping the masters costs a drive that is cheaper than the collection was to assemble.

Opus is the only audio target on this site that fetches nothing. Every browser implemented it for calls, so the encoder is already present, and your machine already decodes FLAC — the conversion starts the instant a file lands and contacts no server at any point. You can watch the network tab during a batch and see nothing move.

Files are capped at 100 MB each, which is around twenty minutes of CD-quality FLAC, and a hundred can be dropped together and taken back as a ZIP. For a collection that means working album by album rather than in one pass, which is also the sensible way to do it: convert one, put it on the device, confirm it plays, then work through the rest.