When you download a massive, high-resolution photograph of a landscape, but you only want to use a specific section of the image (like a single mountain peak) for a presentation slide, you must crop the image. You do not need to download heavy, professional editing software like Adobe Photoshop to perform basic destructive crops. The built-in macOS Preview application contains a highly intuitive, incredibly fast cropping tool that allows you to slice away unwanted pixels in seconds.
How to Select the Crop Area
Cropping in Preview is entirely driven by the standard selection tool. You must first draw a box around the specific area you want to keep; everything outside of that box will be permanently deleted.
- Double-click the image file to open it in Preview.
- Look at the top-left corner of the window. Ensure the Selection Tool (the icon shaped like a dotted rectangular box) is active.
- Click and hold your mouse button down on the image, and drag diagonally to draw a dotted box around the subject you want to keep.
Once you release the mouse button, the dotted box (the “marching ants”) will remain on the screen. If the box is not perfectly aligned, you can click and drag the blue dots on the corners of the box to adjust the exact dimensions of your selection.
How to Execute the Crop
Once your selection box is perfectly positioned, executing the actual crop is a one-click process.
- Look at the top menu bar of your Mac screen. Click Tools.
- From the drop-down menu, click Crop (or simply press Command + K on your keyboard).
The software will instantly delete every single pixel located outside of your dotted selection box. The image window will physically shrink to perfectly match the dimensions of your newly cropped photograph.
How to Crop to a Specific Aspect Ratio
If you are cropping a photograph specifically to use as an Instagram post (which requires a perfect 1:1 square) or a YouTube thumbnail (which requires a 16:9 rectangle), free-hand drawing a box is highly inaccurate. Preview allows you to lock the dimensions of your selection box.
- Draw a standard selection box on your image.
- Look at the top menu bar, click Tools, and hover over Show Inspector (or press Command + I).
- In the Inspector window, click the Crop tab (the icon shaped like a tiny ruler).
Here, you can physically type the exact pixel dimensions you want the box to be (e.g., 1080 x 1080). The dotted box on your screen will instantly snap to those exact mathematical proportions. You can then drag the perfectly-sized box over your subject, press Command + K, and execute a mathematically flawless crop. Press Command + S to permanently save the newly resized image.