Lossless PNG compression on Mac

You want to shrink the PNG, but the asset is going into a design system or onto a print-ready spec, so “looks identical” isn’t good enough — you need every pixel preserved. The good news is that PNGs always have headroom for genuinely lossless compression. The honest news is that it’s much less than what a quantized PNG can do.

Why this happens

PNG already compresses using zlib (deflate), but most exporters use middling settings to save encoding time. Re-encoding with maximum effort, smarter filter selection, and tools like Zopfli or AdvPNG can squeeze out another 10-25% with zero pixel change.

That’s it. Genuinely lossless PNG compression typically gets you 10-25% reductions. If you’re seeing 80% savings, that’s quantization (technically lossy, even if invisible).

The native way

sips will re-encode at max compression:

sips -s format png —setProperty formatOptions 100 input.png —out output.png

Modest savings, but byte-perfect lossless. For more, install optipng:

brew install optipng && optipng -o7 input.png

-o7 is the highest optimisation level — slow on big files but tries every filter and stream combination. Then layer on zopflipng for one more pass:

brew install zopfli && zopflipng -m input.png output.png

-m runs more iterations. Add --iterations=50 for a heroic effort that can take minutes per file. This pipeline is what professional optimisation tools use under the hood.

The faster way (when you don’t need byte-perfect)

If “looks identical to my eyes” is enough, Download Melt and run pngquant-style quantization at quality 90. The output is visually indistinguishable but technically a different pixel set — and it’s 60-75% smaller, not 10-25%.

1. Download Melt and open it.
2. Drag the PNG in.
3. Set quality to 90 for the highest visually-lossless tier, or leave it at 80 for the standard sweet spot.
4. Click Compress.

How much smaller will it really get?

Genuinely lossless (optipng -o7 + zopflipng -m): 10-25% smaller. Visually lossless (Melt at 90): 60-75% smaller. Visually lossless (Melt at 80): 75-85% smaller.

Honestly — for most real-world use, visually lossless is what you want. Save genuine lossless for the cases where a downstream tool will re-process the file and you can’t afford colour-table snapshots. For anything destined for a screen, the difference is mathematical and invisible.