University of Oregon

How to Change the Dimensions of an Image (OS X)


Changing the dimensions of an image can be very important. Often when formatting options are limited, an image may have to be exactly the right size for a webpage, blog, or profile.

An image can be scaled by percentage, pixels, or physical measurement (inches/cm/mm). The resolution of the image can also be determined. Follow these steps to change the dimensions and/or resolution of an image file using Mac OS X.

Before altering the image, make a copy of the original file as this process is permanent.


  1. Open the desired image file using the Preview application.
  2. In the menu bar at the top of the screen, select Tools, then Adjust Size...
  3. A window should appear. On the right side of the window, click the box that states pixels and select your desired measurement.
  4. There should be three text fields along the left side of the window labeled Width, Height, and Resolution. Type in your desired values.
  5. When you're finished, select OK.
  6. Finally, remember to save the file before closing it.


There is a check box near the bottom of the "Adjust Size..." window labeled Scale proportionally. When this box is checked, it means that the photo will maintain a constant aspect ratio. This may make it impossible for you to enter your desired dimensions. You can uncheck the box, however it may result in some distortion of the image. To avoid distortion, the best thing to do is crop the image to the proper aspect ratio and then alter the size.

* Aspect ratio is the width of the image in relation to the height. Aspect ratio is written as XX:XX. For example the aspect ratio of an HDTV is 16:9.