A more complex calculation is needed to find the diameter of the dish "measured along its surface". This is sometimes called the "linear diameter", and equals the diameter of a flat, circular sheet of material, usually metal, which is the right size to be cut and bent to make the dish. Two intermediate results are useful in the calculation: (or the equivalent: ) and , where , , and are defined as above. The diameter of the dish, measured along the surface, is then given by