FLAC is lossless: it compresses audio to roughly half its size and gives back the original samples exactly, bit for bit. It is what people archive CD collections in and what high-resolution download stores sell. Sound quality is identical to the source; file size is not.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a FLAC file above and choose afterwards.