Compress JPG

It selects how coarsely the encoder rounds off the frequency data of each 8x8 pixel block. JPEG converts every block into a set of coefficients describing patterns from flat to fine, then divides them by a quantisation table: a lower quality means bigger divisors, more coefficients rounded to zero, and a smaller file. The information dropped there is gone, which is why the artifacts appear first as blocky edges and haloes around sharp lines.

Because resolution dominates file size more than quality does. A 4000 by 3000 photo carries twelve million pixels; the Recommended profile caps the longest side at 2560, which removes roughly 59 per cent of them, and Extreme caps it at 1600, which removes about 84 per cent. Encoding fewer pixels well beats encoding many pixels badly, so a downscaled photo at quality 0.72 usually looks better than a full-size one squeezed to the same weight.

No. Compression runs on your own processor, so there is no server cost for us to ration and nothing to meter. Drop as many photos as your browser can hold in memory. If a photo is already optimised and cannot be made smaller, the original comes back untouched rather than being re-encoded for nothing.

Yes, with no sign-up, no watermark and no per-file cap. The three profiles encode at quality 0.85, 0.72 and 0.5, all inside your browser, so the photos never leave your device.