To determine the total material you need will be the square footage of your project.

The math to calculate the square footage is straight forward:

A x B = C    C / 144 = Square Feet

If you measure your countertops in inches and create a rough drawing, it will be enough for any stone fabricator to create an estimate/quote. 

Below you will find pictures of sample kitchens which show you how to measure your countertops in specific situations. These kitchens need to be measured as illustrated by the diagrams since the materials have unusual shape.