Rulers help you position images or elements precisely. When visible, rulers appear along the top and left side of the active window. Markers in the ruler display the pointer’s position when you move it. Changing the ruler origin (the (0, 0) mark on the top and left rulers) lets you measure from a specific point on the image. The ruler origin also determines the grid’s point of origin.
To show or hide rulers, choose View > Rulers.
You can hold down Shift as you drag to make the ruler origin snap to the ruler ticks.
To reset a ruler’s origin to its default value, double-click the upper-left corner of the ruler.
Double-click a ruler.
(Windows) Choose Edit > Preferences > Units & Rulers, or right-click the ruler and then choose a new unit from the context menu.
(Mac OS) Choose Photoshop > Preferences > Units & Rulers, or Control-click the ruler and then choose a new unit from the context menu.
Changing the units on the Info panel automatically changes the units on the rulers.
PostScript (72 points per inch)
Sets a unit size compatible for printing to a PostScript device.
Uses 72.27 points per inch, as traditionally used in printing.