Why MozJPEG makes smaller JPGs
JPEG is a 1990s format, but how well a JPG is compressed depends on the encoder that writes it. Most apps and browsers use fast, general-purpose encoders. MozJPEG, released by Mozilla in 2014, spends more effort on the same job:
- it tunes the quantisation tables, which decide how much detail each part of the image keeps, to match how people see
- it optimises the Huffman coding, the final packing step, for each image individually
- it writes progressive JPGs, which usually compress better
The result is a standard JPG that opens everywhere, typically several to 15 percent smaller than a typical encoder produces at the same visual quality, and often far smaller than the original photo.
When to compress JPG files
- Photos from a phone or camera are larger than needed for email, chat or a website
- A web page loads slowly and PageSpeed Insights suggests efficiently encoding images
- An online form or marketplace has an upload size limit
- You want to save storage on a large photo collection, keeping originals elsewhere
How to use it
Choose one or more JPG files, or drop them on the box above. Set the quality before adding files; each photo is compressed in turn with its new size shown. Click the preview thumbnail to compare the original and the compressed version side by side, then download each file or all of them as a ZIP.
Getting the best result
- Resize first if the photo is much larger than where it will be shown. Pixel count matters more than quality settings.
- Check faces and skies. Smooth areas are where compression shows first. If you see blocky patches, raise the quality.
- Need an exact size limit? Use the compress to a size tool, which finds the right quality automatically.
- Publishing on the web? WebP is usually smaller than even a well-compressed JPG.
Questions
What quality setting should I use for JPG?
75% is a good default for web pages and messaging. Use 80–85% for photos where fine texture matters, and 60–70% when the smallest size matters most. Above 90% the file grows quickly with little visible gain.
Is compressing a JPG again bad for quality?
Each lossy save discards a little more detail. One extra compression at a sensible setting is rarely visible. Avoid compressing the same file over and over, and keep your original as a master copy.
Are photo details like location removed?
Yes. The compressed JPG is written from the pixels only, so EXIF data such as GPS location, camera model and capture time is not included.
What is a progressive JPG?
A progressive JPG loads as a blurry full image that sharpens, instead of drawing from top to bottom. MozJPEG saves progressive JPGs because they are usually slightly smaller and feel faster on slow connections.
Last reviewed 30 October 2026