Every image tool has a "quality" slider. But what do the numbers actually mean? Is quality 80 twice as good as quality 40? Is quality 95 worth the file size over quality 85?
I wanted hard data, so I ran a proper test.
The Experiment
I selected 50 diverse photos: landscapes, portraits, product shots, screenshots, textures, high-contrast, low-contrast, detailed, smooth. Each one was an uncompressed PNG original at 2400x1600 pixels.
I compressed each photo as JPEG at quality 10, 20, 30, 40, 50, 60, 70, 75, 80, 85, 90, 95, and 100. Then I measured three things: file size, SSIM (perceptual quality score from 0 to 1), and whether I could visually distinguish the compressed version from the original at 100% zoom.
The Results (Averaged Across 50 Photos)
| Quality | Avg File Size | Avg SSIM | Visually Distinguishable? |
|---|---|---|---|
| 100 | 1.4MB | 0.998 | No |
| 95 | 520KB | 0.995 | No |
| 90 | 340KB | 0.991 | Barely, on some textures |
| 85 | 250KB | 0.986 | Only at 200%+ zoom |
| 80 | 195KB | 0.979 | Subtle. Most people can't tell. |
| 75 | 160KB | 0.970 | Visible on detailed textures at 100% |
| 70 | 130KB | 0.958 | Yes, on close inspection |
| 60 | 98KB | 0.936 | Yes, easily on edges and gradients |
| 50 | 78KB | 0.910 | Obvious compression artifacts |
| 30 | 48KB | 0.851 | Heavy blocking. Looks bad. |
| 10 | 22KB | 0.724 | Nearly unrecognizable detail |
The Key Insight: Diminishing Returns Above 85
Look at the file size jump from quality 85 to 100: it nearly 6x the file size (250KB to 1.4MB) for a quality improvement that nobody can see (0.986 to 0.998 SSIM). That's insane.
Quality 100 is especially wasteful. JPEG quality 100 is NOT lossless — it still applies lossy compression, just very conservatively. The file is 3-5x larger than quality 95 with zero visual benefit. If you need true lossless, use PNG.
The Sweet Spots
Quality 80-85: Best for web use. You're saving 75-85% of file size with quality differences only visible to pixel-peepers at high zoom. This is what I use for imagemochi's own images.
Quality 90: For when quality matters and file size isn't critical. Portfolio sites, photography showcases, product hero images.
Quality 72-75: The "under 100KB" zone. Where you go when portals and forms have strict size limits. Perfectly acceptable for document uploads and web forms.
Quality 50-60: Thumbnail territory. Fine for small preview images, product listing thumbnails, or any image that won't be viewed at full size.
One Thing Quality Sliders Don't Tell You
Quality numbers aren't standardized across encoders. Quality 80 in Photoshop produces a different result than quality 80 in GIMP, which is different from quality 80 in ImageMagick. They all have their own internal mapping from the 0-100 scale to actual quantization tables.
When I was building imagemochi's compression engine, I had to account for this. I use a perceptual quality mapping where "quality 80" means "SSIM of approximately 0.98" regardless of image content. It took a lot of testing to calibrate, but it means the quality slider actually behaves predictably.
See the difference yourself
Drop an image, adjust the quality slider, and watch the file size and preview update in real time.
Try Compress ToolWebP vs JPEG at the Same Quality
One more data point: at quality 80, WebP averaged 140KB versus JPEG's 195KB across my test set. Same visual quality, 28% smaller files. At quality 85, the gap was even bigger — 175KB vs 250KB (30% savings).
If your delivery target supports WebP (and in 2026, almost everything does), it's the objectively better choice for photos at any quality level.