Why your JPG is too big for the email or upload form
A modern phone or camera saves 12 to 48 megapixel photos at 3 to 10 MB each, because the device encodes fast and conservatively rather than efficiently. Then you hit the real world: Gmail and Outlook cap attachments at 20 to 25 MB total, job portals and government forms often demand files under 1 or 2 MB, and forum avatars can require even less. Re-encoding with MozJPEG, Mozilla's improved JPEG encoder, typically cuts those photos by 60 to 80% at quality 80 with no visible difference, which is usually enough to clear the limit in one pass. If you need to hit an exact number, the compress to 100KB tool targets a specific file size, and for images going onto your own website, JPG to WebP squeezes out even more.
The MozJPEG advantage, running in your browser
Most compressors, including the encoder built into your browser's canvas API, use a standard libjpeg-style pipeline. MozJPEG improves on it with trellis quantization, better progressive scan ordering and tuned quantization tables, producing files that are noticeably smaller than standard encoders at equal visual quality. Normally that requires a server; here it is compiled to WebAssembly and runs on your own device, so nothing uploads and there is no reason for the 20-file, 5 MB caps that server-based services impose. Quality guidance: 80 is the sweet spot, 65-75 is the aggressive range for hard size limits, and 50 is the floor before artifacts get obvious. For screenshots and graphics that live in PNG files, use the lossless Compress PNG tool instead, or Compress Image if your batch mixes formats.
Common questions
How much smaller will my JPG get?+−
Photos straight from a camera or phone typically shrink by roughly 60 to 80% at the default quality of 80. That is because originals are encoded quickly and conservatively by the device. MozJPEG spends more computation per image and produces noticeably smaller files than standard JPEG encoders at the same visual quality, so most photos have a lot of headroom.
What quality setting should I use?+−
The default of 80 is the sweet spot for almost everything: files shrink dramatically and the result looks identical to the original in normal viewing. If you are fighting a strict email or form upload limit, go down to 65-75; small artifacts may appear in smooth gradients but photos still look fine. Values above 90 grow files quickly for little visible gain.
Does compressing a JPG again ruin it?+−
One re-encode at quality 80 is visually harmless for photos; the losses become noticeable only after many repeated save cycles. This tool reads your original once and encodes once with MozJPEG, so you get a single, well-optimized generation. Keep your original if it is an archival master, and avoid compressing the same file over and over across different tools.
How is this different from TinyPNG?+−
TinyPNG is excellent at what it does, but its free tier caps you at 20 images of 5 MB each, and every file is uploaded to their servers. This tool has no count or size caps because there is no server doing the work: MozJPEG runs as WebAssembly in your own browser, and your photos never leave your device.