They don't teach the math in quilting classes any more. Back in the days..........that was how we started.
To be simple.....you take a section/piece/square or whatever and calculate square inches
Then multiple those square inches by the number of times unsed in the quilt
Then divide into the square inches in a yd of fabric (42"x36") That is how much you will need.
Now if you want to cut borders without piecing then you need the length.