Goal:  Create a tape measure that will automatically (no calculation needed) calculate the diameter of a pipe by wrapping it around the outside (circumference) of the pipe.  The tape measure should be able to measure diameters up to 8 inches.  The tape measure should be accurate the the nearest 1/2 inch diameter.

Explain/show the process you used to solve the problem.  Show the math used while explaining the process in words to create the measuring tape tool.