A phone with 8 GB free does not care how strongly you feel about FLAC. Give it a collection that needs 30 GB and something has to go. You can delete albums, carry fewer of them, or make a smaller listening copy. We would usually try the third option before doing the first.
Keep the archive somewhere with room for it. Put the travelling copy on the phone. MP3, AAC and FLAC make much more sense once you give each copy a job.
First, don't throw away what you have
Suppose your only copy of an album is an MP3 downloaded years ago. Turning it into FLAC will not bring back the information removed when the MP3 was encoded. Keep the MP3. If a better source becomes available, you can decide whether that particular album is worth replacing.
The reason is in the encoding, rather than the filename. MDN's audio documentation explains the distinction between lossy and lossless compression. MP3 and AAC belong to the lossy side of that distinction; decoding and re-saving them does not undo the earlier losses.
FLAC is useful for keeping a lossless source. The FLAC project describes its approach as compression that can reproduce the input audio. Notice the word “input.” A poor source remains a poor source, however neatly it has been packed.
For an archive, that ability to retain the input is valuable. You can make a portable copy later without having to start from a copy that has already been through lossy encoding. Whether you personally hear a difference on today's headphones is a separate question.
The storage bill: a four-minute example
Let's put some actual numbers on the problem. A constant-bitrate file at 320 kbps contains 320,000 bits per second. Four minutes means 240 seconds. Multiply those numbers and divide by eight: 9,600,000 bytes, or 9.6 decimal MB of audio payload.
Here are the same calculations at a few settings. They leave out tags and container overhead; they are estimates, not measurements of songs from a download service.
| Setting | Four-minute payload | Calculation |
|---|---|---|
| 128 kbps constant bitrate | 3.84 MB | 128,000 × 240 ÷ 8 |
| 256 kbps constant bitrate | 7.68 MB | 256,000 × 240 ÷ 8 |
| 320 kbps constant bitrate | 9.60 MB | 320,000 × 240 ÷ 8 |
| Stereo PCM at 44.1 kHz and 16 bit | 42.34 MB | 44,100 × 16 × 2 × 240 ÷ 8 |
A thousand of those four-minute, 320 kbps tracks would need about 9.6 GB before overhead. That is something you can compare with the free space on a phone. “FLAC is bigger” is less useful, and a fixed FLAC size would be misleading: its compression depends on the audio.
Variable-bitrate MP3 and AAC files will vary too. Don't promise yourself a precise library size from a setting that deliberately lets the bitrate change. Check a representative batch instead.
Try the difficult album first
Pick an album with a bonus track, a compilation with several artists and one long recording. Make the portable copies, then open them on the intended device. Check the order, artwork and search results as well as playback.
It is a dull little job. It is also considerably less dull than discovering that 8,000 newly converted tracks have the wrong tags. Our Picard checking guide deals with that part of the library.
Three situations, three reasonable choices
At home, the awkward question is usually the backup. Two folders on the same failing drive are not much of an escape route. Keep the lossless originals if you have them, and make sure you can get to another copy if that drive stops working.
An older player gets a different answer. Its manual sets the limits. A format that works on your laptop may not work there, so put a few tracks on the player and actually open them. Then decide whether to convert the rest.
And for that crowded phone? A compatible MP3 or AAC copy may be all you need. You have changed the copy you carry, not voted against good sound or cancelled your lossless archive.
Our bitrate, sample-rate and bit-depth explainer unpacks the PCM calculation above. If you want to investigate what you can actually hear, try the ABX procedure using matching sources and a consistent volume.