Melt guide

How to compress a PNG on Mac

PNGs can balloon to 5–10 MB fast. Here's how to shrink a PNG on macOS — the native ways and a smarter one.

4 min read

That 6 MB PNG screenshot won’t fit in your email and is blowing past Slack’s free-tier upload limit. PNG is lossless by design, so files get big quickly — but you can usually cut the size 60–80% without anyone being able to tell the difference.

Two paths

Native macOS. Preview can re-export a PNG but it doesn’t actually compress it much — Preview keeps PNG lossless and there’s no quality slider for the format. The closest native option is the sips command. Open Terminal and run:

sips -s format png --setProperty formatOptions 50 input.png --out output.png

The formatOptions flag controls compression effort, not quality, so the savings are modest (often 10–20%). To get real savings you need a tool that quantizes the palette — reducing 24-bit colour to 8-bit indexed colour, which is what most “PNG optimisers” do under the hood. macOS doesn’t ship one.

Melt. A small Mac app ($9.99 one-time) that runs pngquant-style quantization and zlib compression in a single drag-and-drop pass. It has a “never makes a file bigger” safety check, so if the compressed version ends up larger than the original, it keeps the original.

Compress a PNG in Melt

  1. Open Melt.
  2. Drag the PNG (or a whole folder of PNGs) into the window.
  3. Leave the format on PNG and pick a quality — 80 is the usual sweet spot.
  4. Click Compress. The output saves alongside the original by default.

For most screenshots and UI mockups you’ll see a 70–85% reduction with no visible change. Photographs with lots of gradient tend to compress less because PNG isn’t designed for them — for those, converting to JPEG is the better move.

Honest tradeoffs

PNG quantization is technically lossy — it picks the best 256 colours to represent the image. On screenshots and graphics that’s invisible. On a photo with a smooth sky, you might see faint banding if you really look. If perfect fidelity matters (icon assets going into a design system, say), drop the quality slider down or keep the original. For everything else — Slack uploads, blog posts, email attachments — the savings are usually a no-brainer.

← All Melt tips