Using Document Rulers
Photoshop CS6 Tutorial
How to Use Rulers in Photoshop
Rulers have several uses in Photoshop. The are used to add Guides, determine object placement and display the current curser or tool location.
Open a new 500 pixel by 500 pixel Photoshop document (File > New).
Display the Rulers by choosing View > Rulers.
Drag a Guide from the horizontal ruler, then drag another Guide from the vertical ruler.
Ruler units can be set by right clicking the ruler and choosing a unit of measurement.
By default the Ruler's 0,0 point is the upper left corner of the document. To move it to a new location, click-drag the Ruler intersection point to a new location in the document.
As you move the curser throughout the image, the rulers display the current position.