Caliper Tool

Use the caliper tools to measure widths and heights.  You can also use them as plumb tool or a level to make sure things are leveled and aligned correctly.  Simply drag the handles to where you need the lines.  The measurements are instantly updated in the current system units.  If you don't have a scaling tool, the units are pixels.  You can adjust the number of decimals using the Tools->Options.  To change  the system units, use the Scaling Tool.

The caliper tools are static and will not change as you track or digitize points.