To change ruler measurement units, on the File menu, click Page Setup, and then click the Page Propertiestab. Under Measurement Units, click the units you want, and then click OK.
To change the spacing of ruler subdivisions, on the Tools menu, click Ruler & Grid. Under Subdivisions, click the spacing you want.
Note: For some types of drawings, you may want to change the measurement units to pixels. However, a pixel isn't a unit of measurement. A pixel is just a dot on a screen and the size of the dot varies for different screens. To simulate pixels, set the measurement units to points.