What does MozJPEG do?

MozJPEG is an improved JPEG encoder. Using trellis quantization and progressive encoding, it produces standard JPEG files that are roughly 10–15% smaller than those from the default libjpeg encoder, at the same visual quality.

Are MozJPEG files different from normal JPEGs?

No. MozJPEG output is a 100% standard JPEG file — every browser, editor and device reads it. The improvement is purely in how the file is encoded, not in the format itself.

Yes. MozJPEG is open source, maintained by Mozilla, and royalty-free to use. ImageForge uses it as its JPEG encoder.

MozJPEG

Mozilla's improved JPEG encoder — same JPEG format, roughly 10–15% smaller files at equal quality.

Definition

MozJPEG is a JPEG encoder maintained by Mozilla. It improves on the standard libjpeg encoder with trellis quantization (choosing coefficients that compress better at equal visual quality) and progressive encoding, producing JPEGs that are typically 10–15% smaller at the same visual quality while remaining 100% standard JPEG files.

In practice

Because MozJPEG output is a standard JPEG, it works everywhere JPEG works — no browser or tool changes needed. The catch is that few online compressors actually use it. ImageForge uses MozJPEG for its JPEG output path.

JPEG compressor 9 ways to reduce image file size What is chroma subsampling?

Compress your images now — free and private

Drop your files and ImageForge compresses them in your browser. No uploads, no accounts, no limits. Batch process hundreds at once and download a ZIP.

Open the compressor

100% private No signup