How to Batch Resize Multiple Images Using Preview on macOS

When you download a batch of 50 ultra-high-resolution photographs from a digital camera, each file might be 15 Megabytes in size. If you attempt to upload these massive files to a standard WordPress blog or email them to a client, the system will instantly crash or time out. You must physically reduce the pixel dimensions of the images to shrink their file size. You do not need to open each photograph individually in Photoshop. The native macOS Preview application contains a highly efficient batch-processing engine that allows you to instantly resize hundreds of images simultaneously.

Loading the Batch Queue

To execute a batch resize, you must force the Preview application to load all the images into a single, unified workspace.

1. Open a standard Finder window and navigate to the folder containing your massive image files.
2. Press Command + A to highlight every single image in the folder simultaneously.
3. Double-click any one of the highlighted images. Preview will launch and automatically load every selected file into a single window.
4. Look at the absolute top-left corner of the Preview window. Click the Sidebar Button and select Thumbnails. You will see a vertical list of every loaded image.

Executing the Batch Resize

Before you trigger the mathematical resizing algorithm, you must explicitly tell Preview that you want the math applied to the entire queue, not just the single image currently displayed on the screen.

1. Click any thumbnail in the left sidebar to ensure the sidebar is actively selected.
2. Press Command + A. Every single thumbnail in the entire sidebar will instantly highlight in blue.
3. Look at the main menu bar at the absolute top of your Mac screen. Click Tools.
4. Select Adjust Size from the drop-down menu.

A highly detailed dimension dialogue box will appear.

• Check the Scale proportionally box to ensure you do not accidentally distort or stretch the photographs.
• Change the Width value to your desired pixel size (e.g., type 1920 for standard HD web resolution). The engine will automatically calculate the correct Height.

At the absolute bottom of the dialogue box, Preview will display a dynamic calculation showing exactly how much hard drive space you are about to save (e.g., “Resulting size: 50 MB (was 750 MB)”).

Click OK. The Quartz engine will instantly iterate through all 50 photographs, mathematically destroying the excess pixels and rewriting the files directly on your hard drive in a fraction of a second. Press Command + S to permanently lock in the changes.