By default, the Measure toolbar provides ready access to the Measure and Calibrate tools as well as the measurement modes.
If the toolbar is currently hidden, go to View > 
Toolbars and select it to show it.
Measure (M): Launches the Measure tool and opens the Measure tab.
Calibrate: Calibrates the document's scale to allow accurate measurements to be taken.
Length (SHIFT+ALT+L): Calculates a singular linear measurement.
Area (SHIFT+ALT+A): Calculates the area of a simple rectangle or of an irregular, polygonal shape.
Perimeter (SHIFT+ALT+P): Calculates the length of all the sides of a given shape or multi-segment line.
Diameter (SHIFT+ALT+D): Calculates the diameter of a circular shape.
Angle (SHIFT+ALT+G): Measures a given angle as defined by three points.
Radius (SHIFT+ALT+U): Measures the radius a circle, defined
Volume (SHIFT+ALT+V): Calculates the volume of an area with a defined depth.
Count (SHIFT+ALT+C): Places a markup for each mouse click, associating a running total of counts with each markup as it is placed.
Cutout menu: Contains tools for cutting out or subtracting a polygonal or elliptical area from an existing Area or Volume measurement.