How much can I compress without it looking bad?

Quality 80 is the usual sweet spot for photographs — typically 60–80% smaller with no difference you can see at normal viewing size. Below about 60 you start to see blocking in skies and smooth gradients. Screenshots and flat graphics tolerate far less compression than photos do; PNG or WebP lossless usually beats JPG for those.

Does compressing change the dimensions?

No. Compression changes how the pixels are stored, not how many there are. Use the image resizer if you also want fewer pixels — resizing first and then compressing usually beats compressing alone.

Is the compression lossless?

For JPG and lossy WebP it is not — data is discarded, which is where the size saving comes from, and the discarded detail cannot be recovered from the output. Always keep your original.

Image Compressor

Drop an image in, move the quality slider, and see the new file size next to the original before you download. Most photographs lose 60–80% of their size at quality 80 with no visible difference. The compression runs in your browser, so the file never leaves your machine.

Runs in your browser — nothing is uploaded

Questions

How much can I compress without it looking bad?: Quality 80 is the usual sweet spot for photographs — typically 60–80% smaller with no difference you can see at normal viewing size. Below about 60 you start to see blocking in skies and smooth gradients. Screenshots and flat graphics tolerate far less compression than photos do; PNG or WebP lossless usually beats JPG for those.
Does compressing change the dimensions?: No. Compression changes how the pixels are stored, not how many there are. Use the image resizer if you also want fewer pixels — resizing first and then compressing usually beats compressing alone.
Is the compression lossless?: For JPG and lossy WebP it is not — data is discarded, which is where the size saving comes from, and the discarded detail cannot be recovered from the output. Always keep your original.