Lossy vs Lossless Compression, With Real Files

Lossless compression finds patterns and writes them more briefly, like shorthand. Decompress it and you get the exact original. PNG, FLAC, Apple Lossless (ALAC) and zip work this way.

Lossy compression decides which information a person is least likely to miss, such as faint color shifts in a photo or sounds masked by louder ones, and deletes it. JPEG, HEIC, AVIF, MP3, AAC and H.264 video work this way. The deleted data does not come back.

We took one 2000 px photo and saved it four ways:

Lossless WebP was about half the size of PNG, which is a real saving with no change to the pixels. The lossy files were far smaller still. The photo is a smooth macOS wallpaper, so the lossy numbers flatter lossy formats. Detailed photos shrink less. The two lossy rows are also at different quality levels, so compare formats only at a matched quality; our benchmark does that with 24 photos.

We converted one 10 second WAV clip (882,078 bytes):

We decoded the ALAC file back to WAV and its audio hash matched the original. Lossless is the right choice for masters you will edit again. A 128 kbps lossy file is about a fifth the size of the WAV, fine for listening on earbuds, but each re-encode loses a bit more. Bitrate is explained in audio bitrate explained, and the Mac steps are in converting FLAC to MP3.

It can be, but nobody ships it. We made a lossless copy of our 6.9 MB H.264 test clip using the FFV1 codec and got 17.1 MB, about two and a half times larger than the lossy file it came from. Delivery formats such as H.264 and H.265 are lossy by design. See H.265 vs H.264 for the quality and compatibility trade.

Lossy damage shows up in predictable places. In images, look at hard edges and text: JPEG leaves faint blocky noise around them, which is why screenshots and logos belong in PNG. Smooth gradients such as skies can show banding at low quality.

In audio, low bitrates smear cymbals and quiet background detail. In video, fast motion and dark scenes break up first. The fix in every case is the same: raise the quality setting a step and compare at full size before you commit.

One more rule. Never convert lossy to lossy in a chain if you can avoid it. Going from MP3 to AAC, or JPEG to WebP, stacks a second round of loss on the first. Always go back to the source file.

• Images: save as PNG, or run a lossless optimizer such as ImageOptim. Our ImageOptim comparison shows how little lossless saves on photos.
• Audio: export Apple Lossless or FLAC. In Terminal,

  afconvert in.wav out.m4a -f m4af -d alac

  does it.
• Anything: Finder’s right-click Compress makes a zip. It is lossless but saves little on photos and video, which are already compressed.
• Metadata: removing EXIF changes no pixels; see how to remove EXIF data.

Keep a lossless or original copy of anything you will edit, print or archive. Send lossy copies to people and websites. Compress once from the original instead of compressing a compressed file. Our measurements of quality loss show how re-saving adds up.

Smol has no lossless mode for images; it always re-encodes them. For pixel-identical results use ImageOptim or save as PNG. For lossless audio,

afconvert

Smol earns $29 when you want lossy on purpose: a chosen quality across a folder of images, PDFs, video and audio, from the Finder right-click, with nothing uploaded. It writes a copy next to the original by default. Compare tools in the best image compressor for Mac, then see pricing.