Setting measurement preferences in Photoshop

In the Units & Rulers Preferences dialog box you can set the units used to measure things on-screen (inches, pixels, millimeters, and so forth) and to define a default column size when typing text in multiple columns. In addition, you can define the resolution of the image when you choose File➪New and select Default Photoshop Size from the Preset Sizes list.

In the Units area of the dialog box, you find these options:
-->Rulers: Select the measurement units Photoshop uses for rulers. Your choices are pixels, inches, centimeters, millimeters, points, picas, or percent. The most popular sizes are inches and millimeters, but if you’re working with publications and specifying in picas, you might prefer that increment instead. If you’re prepping Web graphics, you may prefer to have your rulers incremented in pixels.

-->Type: Choose the measurement used to represent the dimensions of type. Point size is almost universally used, but pixels and millimeters are also available. You may want to use pixels if you’re trying to fit type into a specific-sized area of an image.

In the Column Size area, you can specify the following:
-->Width: The width of the column in inches, centimeters, millimeters, points, or picas.

-->Gutter: The width of the area separating columns, also in inches, centimeters, millimeters, points, or picas.

In the New Document Preset Resolutions area, you can set the following:
-->Print Resolution: The default is 300 pixels per inch, a good setting to keep. You can change to another value and use pixels per centimeter as a measurement if you want.

-->Screen Resolution: Generally, 72 pixels per inch works with most images that will be prepped for screen viewing. You can select another resolution and use pixels per centimeter if you like.

Changing the resolution of an image after it’s been created can impact the sharpness of your image and degrade quality. It’s best to choose the final resolution you want when you create a document, whether you specify the resolution manually or use these presets.

In the Point/Pica Size area, you can choose whether you want to use a measurement of 72 points per inch (which first became relevant in the Macintosh realm and spread as desktop publishing became widespread) or the traditional 72.27 points per inch definition used in the precomputer era. Unless you have a special reason to choose otherwise, use the Postscript (72 points per inch) option.