Lossless compression software makes a file smaller and gives you back the exact original. Every pixel, sample or byte survives. On a Mac that means ZIP archives, PNG and FLAC files, and a handful of optimizers. It does not mean the top setting on a quality slider in most apps, which is usually still lossy.

I make Picmal, a Mac app that compresses images, video, audio and PDFs. Much of what it does is lossy, and I’ll say where, because the word “lossless” gets stretched a lot.

What lossless compression is, and what it isn’t

Zipping a folder is lossless. You can unzip it and every file matches the original. Repainting a picture with fewer colors is lossy: the new one is smaller, and the old one is gone.

Lossless tools find patterns that repeat and write them more compactly. That’s why a screenshot with big flat areas shrinks well and a noisy photo barely moves. There is a floor. Past it, the only way to get smaller is to throw something away.

Lossy compression (JPEG, WebP, AVIF for most settings, MP3, AAC) drops detail on purpose. You can’t get the detail back, which is why you keep your originals.

Which formats are lossless

Format is most of the answer. If the file is already in a lossless format, compressing it harder doesn’t cost you anything.

  • Images: PNG is lossless. So is a TIFF saved without lossy compression.
  • Audio: FLAC and ALAC (Apple Lossless) are lossless. WAV and AIFF are uncompressed, so they’re big and exact.
  • Anything else: a ZIP wraps any files and gives them back unchanged.

Converting between two lossless formats loses nothing. FLAC to ALAC gives you the same samples in a different container; I wrote the steps in converting FLAC to ALAC on Mac.

When you need lossless

Masters. Original photography you will edit again. Scans you’re archiving. Audio you ripped from CDs. Anything you’ll compress a second time later, because lossy compression stacks up damage each time you save.

For a file you’re sending or posting once, lossy is usually fine and it’s the only way to get a big saving. Reducing image file size on Mac covers that side.

The problem with online tools

Most pages that rank for this query are web tools. You upload the file, wait, and download the result. For a holiday photo that’s fine. For a client’s unreleased shots, a scan of your passport or a legal document, it means a copy of the file sits on someone else’s server.

Online tools also cap file size, ask for an account, and can’t take a whole folder in one go.

Local lossless compression software for Mac

A few honest options that run on your own Mac:

  • Finder’s Compress command makes a ZIP. It’s built in, lossless and free. It won’t shrink files that are already compressed.
  • ImageOptim is free and open source. It re-encodes JPEG and PNG more efficiently without changing a pixel. The saving is modest, which is the usual ceiling for lossless work on photos.
  • A FLAC or ALAC converter for audio libraries. Lossless in, lossless out.
  • Picmal, for the jobs it can do without loss, below.

Here is what Picmal does and doesn’t do losslessly, from its own docs:

  • Audio: converting to FLAC or ALAC is lossless. ALAC conversions from FLAC, WAV or AIFF are bit-for-bit.
  • Joining files: same-format audio and matching video clips are joined losslessly. Mixed formats are re-encoded.
  • PDF pages: combining and splitting copies pages losslessly.
  • Images: PNG at levels 1 and 2 is lossless. Picmal only repacks the file, and the Maximum quality preset uses level 2. From level 3 up, a lossy palette pass runs first. JPEG is lossy at every preset, Maximum quality included.

I’d rather you know that than find out from a zoomed-in screenshot. When Picmal can’t make a file smaller, it keeps your original, so a result is never larger than the input. The before/after slider shows the difference if you want to check.

What Picmal gets you is the Mac side of the job: drag in hundreds of files, run it from Finder Quick Actions, Shortcuts or Raycast, and nothing leaves your Mac. It’s a one-time purchase, with a 7-day trial of the full app.

Mistakes to avoid

Assuming “high quality” means lossless. A JPEG at the top quality setting is still a JPEG. It isn’t the original.

Using lossy compression on an archive. Keep the master lossless. Make lossy copies for sharing.

Uploading sensitive files to random sites just to shave a few megabytes.

Alberto Gallego makes Picmal, a native Mac app for converting and compressing images, video, audio and PDFs locally. More about the person behind it.