How does JPEG compression work?
JPEG cuts an image into 8×8 pixel blocks and describes each block as a mix of patterns, from smooth (low frequency) to fine (high frequency). Quantization then rounds those values off: the quality setting scales how coarse the rounding is. At high quality the rounding is fine and almost every detail survives; at lower quality the fine patterns (texture, noise, sharp edges) are rounded to zero first, while flat areas like skies and walls cost almost nothing at any setting. Most encoders also store color at a lower resolution than brightness, because the eye notices brightness detail far more.
This is why recompressing a camera photo saves so much. Phones and cameras save JPEGs at a high quality, where the last few quality points cost the most bytes and add detail nobody sees on a screen. Bringing that down to 75 removes those bytes and leaves the picture looking the same.
What JPEG quality should I use?
JPEG quality settings and what to expect at normal viewing size.
| Quality | Use it for | What you see |
|---|---|---|
| 90–95 | Archive copies, prints, photos you will edit again | No visible change, but large files |
| 80–85 | Portfolio, product photos with zoom, hero images | No visible change at normal size |
| 75 (default here) | Blog, email, social posts, forms | Invisible on most photos; slight softening of fine texture when zoomed in |
| 60–70 | Thumbnails, previews, background images | Slight softening, possible halos around text |
| Below 50 | Only to meet a strict size limit | Blocky 8×8 squares in skies and gradients |
Quality numbers are not a universal scale: 75 in one app is not exactly 75 in another, because each encoder uses its own tables. Judge the result, not the number. For a deeper walk-through of each range, read JPEG quality settings explained.
Why did my JPG barely get smaller?
Because it was already compressed. Photos saved from WhatsApp, Facebook, a website or a content management system have usually been recompressed at quality 70–85 already, so saving them again at 75 has little left to remove. The tool then returns the original rather than a file that is bigger or only marginally smaller. To go further, lower the quality to 60–65, reduce the pixel dimensions with Limit width, or, if the image is for your own website, convert it to WebP.
How do I make a JPG smaller without lowering the quality?
Reduce the pixels instead. A 12-megapixel phone photo is 4032×3024 px, but a web page column rarely shows more than 1200 px of it. Limiting the width to 1600 px keeps 1600×1200 px, which removes about 84% of the pixels before any quality setting is applied. If the dimensions must stay exactly as they are, see how to reduce image size without changing dimensions.
Should I keep JPG or convert to WebP?
Keep JPG whenever someone else asks for it: application forms, email attachments, print services, marketplaces and anything that says “JPEG only”. For images on your own website, WebP holds the same visual quality in files that Google measures at 25–34% smaller than JPEG; JPG to WebP does that conversion, and WebP vs JPEG compares the two in detail.