Inaccurate Ruler in Photoshop

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Today’s Question: In Photoshop I drew a grid  made up of one-inch squares on a transparent layer. I use it for my drawings placing the grid over a photo. However, when I place my ruler on the monitor screen to see if the squares are one inch, I find they are not and no matter what I do (fit screen, print size, actual pixels, zooming in and out), the squares are never one inch. In other words, the monitor screen inch does not match my ruler inch.

Tim’s Quick Answer: The solution here is to set the Screen Resolution in the Preferences dialog for Photoshop to the true pixel per inch resolution of your monitor display, and then make use of the Print Size display option from the View menu.

More Detail: By default Photoshop assumes a display resolution of 72 pixels per inch. That does not match the vast majority of monitor displays in use today. To calculate your actual pixel per inch (ppi) resolution, you need to know what pixel dimensions you have set as the resolution for your monitor display. You can then measure the horizontal display size for your monitor, and calculate the actual pixel per inch resolution.

For example, let’s assume a display that is operating at 1920 by 1080 pixels (full high definition resolution). Let’s further assume the width of the actual display area for the monitor is 14 inches. With 1920 pixels across and a width of 14 inches, there are 137 pixels per inch across the display.

To set the resolution you’ve calculated for the display in Photoshop, you’ll need to first bring up the Preferences dialog. To do so, choose Photoshop > Preferences > Units & Rulers on the Macintosh version of Photoshop, or Edit > Preferences > Units & Rulers on the Windows version of Photoshop. In the Units section, set the value for Screen Resolution to the number you calculated (137 in the example above) and set the popup to the unit of measure you used (“Pixels/Inch” in this example). Then click OK to close the Preferences dialog.

With this setting established, when you choose View > Print Size from the menu in Photoshop, the image will be resized based on the pixel per inch resolution so that one inch on Photoshop’s ruler display (View > Rulers from the menu) will represent one inch on your monitor, and the size of the image will match what you can expect for the image if it is printed at the current resolution setting for the image.