How to Resize and Compress Images Using the Preview App on macOS

When you try to email a high-resolution photograph or upload a headshot to a corporate directory, you will frequently encounter strict file size limits (e.g., “Image must be under 2MB”). Modern smartphone cameras and DSLRs take massive photos that are often 10MB or larger. While you could use complex, expensive photo editing software to shrink the file, macOS includes a highly capable resizing engine built directly into the default Preview application.

How to Change the Physical Dimensions

The most effective way to drastically reduce the file size of a photograph is to reduce its physical pixel dimensions. A photo that is 6000 pixels wide is unnecessary unless you are printing a billboard.

1. Double-click your image file (JPEG or PNG) to open it in Preview.
2. Click Tools in the top menu bar.
3. Select Adjust Size… from the drop-down menu.
4. A new dialog box will appear. Ensure the padlock icon next to the Width and Height boxes is closed; this ensures the image retains its original aspect ratio and does not become stretched or distorted.
5. Change the measurement drop-down menu from “inches” to “pixels”.
6. Type a new number into the Width box. For standard web uploads and email attachments, a width of 1200 or 1920 pixels is usually more than enough.

As you type the new number, look at the bottom of the dialog box. Preview will calculate the “Resulting Size” in real-time, showing you exactly how many megabytes you are saving before you even click OK.

How to Compress the File (Without Changing Dimensions)

If you absolutely must maintain the original 6000-pixel width of the photograph, but you still need to reduce the file size, you must use JPEG compression.

1. With your image open in Preview, click File in the top menu bar.
2. Select Export… (Do not select Export as PDF).
3. In the save dialog box, ensure the Format drop-down is set to JPEG.
4. You will see a slider labeled Quality, ranging from “Least” to “Best”.

This slider controls the compression algorithm. If you drag the slider slightly to the left (towards “Least”), Preview will begin discarding invisible color data from the image. As you drag the slider, look at the “File Size” indicator directly below it. It will update in real-time. You can visually balance the quality of the image against the exact kilobyte or megabyte target you need to hit for your upload requirement. Once you hit your target, click Save to generate the compressed copy.