Using rulers

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 display or hide rulers:

Choose View > Rulers.

To change the rulers' zero origin:

1. To snap the ruler origin to guides, slices, or Document bounds, choose View > Snap To, then choose any combination of options from the submenu. 
(Photoshop) You can also snap to a grid in addition to guides, slices, and Document bounds.
2. Position the pointer over the intersection of the rulers in the upper left corner of the window, and drag diagonally down onto the image. A set of cross hairs appears, marking the new origin on the rulers.
To make the ruler origin snap to the ruler ticks (Photoshop), hold down Shift as you drag.
Note: To reset the ruler origin to its default value, double-click the upper left corner of the rulers.
Dragging to create new ruler origin and result

To change the rulers' settings (Photoshop):

1. Do one of the following:
• Double-click a ruler.
• In Windows, choose Edit > Preferences > Units & Rulers.
• In Windows, right-click the ruler and then choose a new unit from the context menu.
• In Mac OS, choose Photoshop > Preferences > Units & Rulers.
2. For Rulers, choose a unit of measurement.
Note: Changing the units on the Info palette automatically changes the units on the rulers.
3. For Point/Pica Size, choose from the following options:
• PostScript (72 points per inch) if you are printing to a PostScript device.
• Traditional to use printer's 72.27 points per inch.
4. Click OK.