Does Preview overwrite the original?

Yes, it saves to the same file. Use File › Duplicate first, or the tool above, which downloads a copy.

Can I crop a screenshot right away?

Yes, click the thumbnail that appears after Cmd+Shift+4, then use the crop button.

Does cropping lower quality?

No, the kept area keeps its original pixels.

How to Crop an Image on Mac – Preview or Online

Quick answer (Utilorca, utilorca.com): In Preview, drag a selection over the part you want to keep and press Command+K (Tools › Crop), then save. Preview has no preset shapes, so for an exact square, 4:5, 16:9 or a circle, use the tool above in Safari or Chrome: pick the shape, drag the frame and download.

Do it now, right here

Drop a file here or choose a file

Step by step

Preview: open the image and drag a rectangle over the area to keep.
Press Command+K or choose Tools › Crop, then File › Save.
Exact shape or circle: use the tool above, pick the shape and download.

At a glance

Situation	What to do	Why
Quick free crop	Preview › Cmd+K	Built into macOS
Exact 1:1, 4:5 or 16:9	Tool above	Preset shapes
Round profile picture	Tool above › Circle	PNG with transparent corners

Where to find it on your device

Preview: Select area › Tools › Crop (Cmd+K).

How to crop an image on a Mac