How JPG compression works
JPG is a lossy format. It divides the image into small blocks and stores less of the fine detail and color variation that the eye notices least. The quality setting controls how much is thrown away. Lower quality means a smaller file and, eventually, visible blocks and halos around edges.
This compressor also writes the file as a progressive JPEG with optimized encoding tables. That doesn't change how the photo looks, but it often trims extra bytes, and progressive JPEGs appear gradually while loading on slow connections.
Choosing a JPG quality level
For photos, 70 to 80 percent is a good range. It usually cuts the file size dramatically compared with a camera original, with no difference you would notice on screen. Use 85 to 90 percent for photos you might print or edit again. Drop to 50 to 60 percent only for small thumbnails.
If you have a size limit rather than a quality preference, use Target size and pick the limit. The tool finds the highest quality that fits.
Compressing PNG, HEIC or WebP into JPG
You can upload other formats here too. They are saved as JPG. JPG has no transparency, so transparent areas in a PNG or WebP are filled with white. HEIC photos from iPhones are decoded on the server and rotated upright before they are saved.
Compressing many JPGs or hitting form limits
To compress a whole folder of photos, the Bulk Image Compressor applies one quality level or target size to up to 20 files and returns them in a ZIP. For common form limits, pages such as Compress Image to 20KB, 50KB and 100KB preselect the target for you. They use the same quality search, so the result is always the highest-quality JPG that fits.