When you take a photograph with a modern smartphone or download a high-resolution image from the internet, the resulting file can be massively large, often exceeding 5000 pixels in width and taking up several megabytes of storage space. If you need to upload that image to a web portal as a tiny profile picture, or embed it into an email without crashing the recipient’s inbox, you must physically scale the image down. You do not need professional photo editing software to achieve this; the built-in macOS Preview application contains a highly precise image resizing tool.
How to Open the Dimension Tool
Unlike cropping (which violently cuts away the edges of a picture), resizing mathematically shrinks or expands every single pixel in the entire image while maintaining the overall picture.
- Double-click your image file to open it in Preview.
- Look at the top menu bar (next to the Apple logo) and click Tools.
- From the drop-down menu, select Adjust Size…
A new, detailed window will pop up over your image, displaying the current dimensions and providing fields to manipulate the scale.
How to Resize an Image proportionally
By default, Preview will show you the image’s dimensions in either pixels, inches, or centimeters. For web usage, always ensure the drop-down menu next to the “Width” and “Height” boxes is set to pixels.
Before you change any numbers, look for the checkbox labeled Scale proportionally (it is usually checked by default). This is the most critical setting in the window. When this box is checked, a small padlock icon connects the Width and Height boxes together.
If you type “800” into the Width box, Preview will instantly, automatically calculate the exact required Height to ensure the image does not become stretched, squished, or horribly distorted. The software does the math for you.
How to Force an Exact Resolution
Sometimes, a strict web portal requires an exact, perfect square resolution (e.g., exactly 500×500 pixels for a logo). If your original image is rectangular, attempting to force it into a square will cause massive distortion.
To force the dimensions, you must uncheck the Scale proportionally box. The padlock icon will break apart. You can now type “500” into the Width box, and manually type “500” into the Height box independently.
Click OK. Preview will violently stretch and squash the image to force it into those exact dimensions. (Note: If the distortion is unacceptable, press Command + Z to undo the action, and use Preview’s Crop tool to cut the rectangle into a square first, before attempting to resize it again).