The JPEG quality percentage is one of the most misunderstood numbers in digital imaging. It's not a percentage of the original image data retained. It's not a simple scale where 80% looks 20% worse than 100%. It's a tuning parameter for a mathematical compression model, and its effect on file size and visual quality is non-linear and content-dependent.
What the quality number controls
JPEG encodes images by breaking them into 8×8 pixel blocks, transforming each block into a set of frequency components using the Discrete Cosine Transform (DCT), and then quantising those components. The quality setting controls the quantisation matrix — essentially, how aggressively high-frequency details (fine textures, sharp edges) are reduced in precision.
At high quality settings, the quantisation is gentle — fine detail is mostly preserved. At low quality settings, high-frequency components are rounded heavily, causing the characteristic JPEG "blocking" artefacts around edges and in smooth areas.
The non-linear relationship between quality and file size
Going from quality 100 to quality 90 typically reduces file size by 50–60%. Going from quality 90 to quality 80 might only reduce size by an additional 25%. The curve flattens at lower quality settings — most of the compression gains are in the 100→80 range. Below quality 60, you're trading significant visible quality for diminishing returns on file size.
| Quality | Typical file size (vs original) | Visual result |
|---|---|---|
| 100 | 100% (no compression) | Lossless-equivalent for most content |
| 90 | ~45% | Imperceptible difference for most viewers |
| 80 | ~25% | Near-imperceptible; excellent sweet spot |
| 70 | ~18% | Tiny artefacts visible in smooth gradients |
| 60 | ~13% | Visible artefacts in close inspection |
| 50 | ~10% | Clearly visible blocking and colour banding |
Content-dependent results
These numbers vary significantly by image content. Photographs with complex textures (grass, bark, skin pores) compress well at lower quality settings because the texture masks artefacts. Images with smooth gradients (sky, skin close-ups, flat backgrounds) show artefacts at lower quality settings because there's less texture to mask the compression noise.
The practical recommendation
Quality 78–82 is the practical sweet spot for most web images. At this range, file sizes are typically 25–30% of the uncompressed original, and the quality difference from quality 100 requires side-by-side comparison at full resolution to detect. For content that will be displayed small (thumbnails, list images), quality 65–70 is fine. For large hero images or product shots, quality 80–85 is appropriate.
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