What is a FLAC file?

FLAC is a codec: the encoding itself rather than the box it travels in. It is used for archiving.

The extension is .flac, and the full name is Free Lossless Audio Codec. Both matter less than what the file can hold, which is what the rest of this page is about.

Xiph.Org published it in 2001. The specification is RFC 9639.

Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

FLAC stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.

Up to 32 bits per channel.

More than eight bits per channel is what stops a gradient from stepping when it is edited, which is why it matters in a working file and rarely in a finished one.

Up to 8. Two covers stereo and is what almost everything is; more than two is surround, and it is the part most likely to be folded down when converting to a format that does not define it.

FLAC carries a checksum, so a damaged file is detected rather than silently mis-read.

A checksum will not repair anything. It tells you the file is wrong, which is the difference between finding out now and finding out from whoever you sent it to.

Audacity, foobar2000 and VLC read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

Every current browser reads it.

That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.

FLAC can hold document properties.

Whether any of it survives a conversion depends entirely on the target, and the honest answer is usually "some of it".

FLAC is meant to be opened and changed. Keep the file in this format for as long as the work is going on, and export from it whenever a finished copy is needed.

Decompress a FLAC and you get back exactly the samples that went in — every one of them, the same values in the same order. This is not a claim about how it sounds; it is a claim about the data, and it can be checked by comparing the decoded output against the original byte for byte.

That is the whole distinction from MP3, AAC and Opus, which model what human hearing misses and discard it. Those are good engineering and the loss is usually inaudible, but it is permanent. FLAC makes no such trade: it compresses the way ZIP does, by finding structure, not by deciding what you will not notice.

A FLAC typically lands between 50 and 60 per cent of the equivalent WAV. A CD album that occupies 600 MB uncompressed becomes something in the region of 300 to 350 MB. That is a genuine saving and it is far short of what a lossy codec achieves, where the same album is 60 MB.

The ceiling is mathematical rather than a limitation of the implementation. Audio is not very repetitive, and lossless compression can only exploit real redundancy. Anything claiming lossless audio at a tenth of the size is either not lossless or not audio.

FLAC was released in 2001 by Josh Coalson and is maintained by the Xiph.Org Foundation. The format specification and the reference implementation are both free, with no patents asserted and no licensing to arrange.

Apple Lossless — ALAC — does the same job at very similar sizes and was proprietary until 2011. FLAC had a decade of open availability first, which is why it became the default for music archiving, for CD ripping tools, for hi-res download stores and for every open-source music player, while ALAC remained largely inside the Apple ecosystem.

Encoders offer levels from 0 to 8. The audio that comes back out is identical at every one of them — this is lossless, so it must be. What changes is how hard the encoder searches for redundancy, and therefore how long encoding takes and how small the result is.

The spread between the fastest and slowest settings is only a few per cent of file size, while the time difference is substantial. Level 5 is the default for good reason, and level 8 is mostly of interest to people archiving a large library once. Decoding speed is essentially the same regardless.

Nearly everything except Apple, historically. Android has supported it natively for years, Windows and Linux handle it, every browser plays it, VLC and every serious music player read it, and most modern network audio hardware expects it.

Apple was the long holdout — iTunes and iOS did not play FLAC for well over a decade, which is the single biggest reason it never reached general consumers. Recent iOS versions handle it in the Files app, though the Music library still prefers ALAC. If a FLAC library needs to live in Apple Music, converting to ALAC is the lossless path across, with no quality change in either direction.

FLAC uses Vorbis comments — free-form name-and-value pairs with no fixed field list and no size constraints. Anything can be stored, including long values, and unlike ID3 there is no legacy structure to work around.

Cover art has a proper home here too, in a dedicated picture block rather than smuggled into a comment field as it is in Ogg. Combined with an embedded checksum of the audio, that makes FLAC unusually well suited to a library that has to stay organised for years.

Nearly every FLAC stores an MD5 signature of the original uncompressed audio in its header, and a decoder can verify that what it produced matches. If a file has been corrupted on a failing disk or in a bad transfer, the check fails and says so. Sixteen zero bytes are legal there and mean the encoder did not know the signature — the reference encoder writes them for `--no-md5-sum` and for anything it could not seek back over.

This is why FLAC is the format of choice for archiving a CD collection: a library can be verified in bulk years later, and silent bit rot is detectable rather than something discovered when a track plays wrong. It detects damage; it cannot repair it.

FLAC handles up to 32-bit samples at 655 kHz, so hi-res downloads — commonly 24-bit at 96 or 192 kHz — are distributed in it. The files are three to four times the size of CD-quality FLAC.

Whether that extra resolution is audible is genuinely contested, and the honest summary is that controlled listening tests have not reliably shown a difference on playback. Where the extra bit depth does unambiguously help is in editing, for the same reason it helps in a raw photograph: headroom before adjustment causes damage.

To MP3 or AAC when the destination is a phone with limited storage, a car, or anything that will not play FLAC — this is the normal case and there is nothing wrong with it. Use a high bitrate; the source is perfect, so the only loss is the one you choose.

To WAV when a tool demands uncompressed input, which many audio editors and speech pipelines do. To ALAC when the library needs to live in Apple Music, which is lossless to lossless and changes nothing. And keep the FLAC in every case — it is the archival copy, and every lossy file made from it can be remade, while the reverse is impossible.