FLAC is lossless compression: it shrinks audio to roughly half the size of WAV while keeping every sample exact, which makes it the format of choice for ripping CDs and for hi-res downloads. It is also why FLAC libraries are big, and why many car stereos, older players and some phone apps refuse to play it. MP3 at 320 kbps is the usual answer, cutting a typical album from 300 to 400 MB down to around 120 MB while sounding the same to nearly every listener. This page opens with MP3 as the output and 320 kbps already selected, since the source is lossless and there is no reason to start lower; drop the files and convert. If space matters more, 192 or 256 kbps are still very good for music. Keep the FLAC files as your archive. Converting from lossless is a one way step, and if a better format comes along you will want to convert again from the original rather than from the MP3. Hi-res FLAC at 24 bit or high sample rates is converted down to what MP3 supports. Tags and cover art are not carried over, only the audio. For uncompressed output from FLAC choose WAV in the format buttons, and for WAV sources see WAV to MP3 Converter.
The split that matters most in audio formats is lossy against lossless. A lossless format such as WAV or FLAC stores enough information to reconstruct the original waveform exactly, which is why WAV files run around 10MB per minute of stereo CD quality audio. Lossy formats such as MP3, OGG Vorbis, AAC, and Opus use psychoacoustic models to discard sound that listeners are unlikely to perceive, typically hitting a tenth of the size for audio most people cannot distinguish in normal listening. The important consequence is that the discarded information is gone permanently. Converting MP3 to WAV inflates the file back to full size without recovering anything, and each further lossy conversion compounds the loss, an effect known as generation loss. The practical rule is to keep a lossless master and export lossy copies from it, rather than converting between lossy formats repeatedly. Bitrate is where quality and size are actually traded. At 128 kbps most listeners cannot reliably distinguish MP3 from the source on laptop speakers or earbuds, though the gap becomes audible on good headphones with dense music. At 256 to 320 kbps MP3 is effectively transparent for nearly everyone. OGG Vorbis reaches similar quality at a somewhat lower bitrate, because it is a newer design than MP3, and Opus does better still, which is why messaging apps and voice chat use it. Sample rate and bit depth apply to the uncompressed side. This tool writes WAV at 16 bit and 44.1 kHz, the CD standard, which is the format editing software and mastering workflows expect, and which every audio application on every platform can open without conversion.