Compress PNG images
This makes PNGs smaller without changing the format. Two things happen: colour quantisation reduces the image to a palette of 256 colours or fewer, and OxiPNG then rewrites the file, storing it as indexed colour rather than truecolour. Together these routinely cut 60-80% with no visible difference — the same approach the well-known paid PNG compressors use.
What to know
- Quantisation is slightly lossy. Turn it off for a purely lossless OxiPNG pass, which typically saves 10-20% instead.
- Floyd-Steinberg dithering is applied to hide banding in gradients.
- Screenshots, logos, icons and UI assets compress spectacularly well because they contain few distinct colours to begin with.
- Transparency is fully preserved throughout.
Frequently asked questions
- How does PNG compression work without changing the format?
- Two passes. First the image is quantised to a palette of at most 256 colours using Wu's algorithm with Floyd-Steinberg dithering. Then OxiPNG losslessly rewrites the file, which lets it store the image as indexed colour — roughly a quarter of the bytes per pixel of truecolour. The output is still an ordinary PNG that opens anywhere.
- Is compressing a PNG lossless?
- With quantisation off, completely — OxiPNG only rewrites the compression, never the pixels, typically saving 10-20%. With quantisation on it is slightly lossy, because reducing to 256 colours discards some, but this is rarely visible and saves far more.
- Why is my PNG still large after compressing?
- Photographic PNGs with thousands of distinct colours resist palette reduction — quantising them to 256 colours would be visible. PNG is simply the wrong format for photographs. Convert to WebP or AVIF instead for a dramatically better result.