How image compression actually works
Modern image compression makes a deliberate trade: throw away information the human eye can't easily perceive, in exchange for a much smaller file. JPG, WebP, and AVIF all use variations of the same idea — split the image into small blocks, run a frequency-domain transform on each block, quantize the high-frequency coefficients aggressively, entropy-code the result. Lower quality settings quantize harder, which loses more detail.
PNG works differently. Its compression is lossless: a prediction filter on each row, then deflate (the same algorithm zip uses). PNG never throws anything away — which is why you can't shrink a PNG with a quality slider. The only way to make a PNG drastically smaller is to switch to a lossy format or to reduce the image's dimensions.
What the quality slider does to your image
Quality is on a 0–100 scale, but the relationship to file size is not linear:
- 90–100: visually identical to the source for everything except specialist medical / forensic imaging. File size is only 15–30% smaller than at quality 75.
- 75–89: the sweet spot for web photographs. Reliable, fast loads, no visible artifacts on real photos.
- 60–74: noticeable softness on close inspection but acceptable for thumbnails and contexts where speed matters more than fidelity.
- 30–59: block artifacts visible in flat color regions. Color banding starts appearing in skies and skin tones.
- Below 30: deliberately stylized, broken-looking output. Not appropriate for production use.
How to use this tool
- Drop a PNG, JPG, or WebP image onto the drop zone.
- Pick the output format. "Same as input" keeps the format (best when re-compressing a JPG or WebP). Switch to JPG or WebP for real savings on a PNG.
- Drag the quality slider. Click Compress to encode. The widget shows the before/after sizes and the percentage saved.
- Adjust quality and re-compress as needed. The original file is held in memory unchanged, so each compression runs against the highest-quality source.
Beyond the quality slider — other ways to shrink images
Quality is the biggest lever, but not the only one. If you're shipping a 4000×3000 photo where the target displays at 800×600, downsizing the pixel dimensions shrinks the file far more than any quality tweak — by 90%+. Use our image resizer to drop the resolution before compressing.
Switching format can also unlock big savings. A 5 MB PNG photograph converted to WebP at quality 80 might be 400 KB — a 92% reduction with no visible quality loss. See our PNG to WebP converter for one-click format conversion.