Measure the diameter of the empty core of the belt roll using the ruler. The core is the part of the roll unoccupied by belt material, and may be empty space or a cylindrical object around which the belt is wrapped. Let this inner diameter measurement be represented by the lowercase letter d.
Measure the outer diameter of the belt roll. Place the one end of the ruler at the tip of the belt on the outermost wrap. Line the ruler up with the center point of the core and measure from the belt tip to the opposite point on the outer edge of the roll. Represent this measurement with the uppercase letter D.
Measure the thickness of the belt using the ruler. Represent this measurement with the letter T.
Find the difference (M) between the square of the outer belt roll diameter and the square of the inner belt roll (or core) diameter. For example, if the belt roll has an outer diameter of D = 2 inches, and a core diameter of d = 0.5 inches, then M = (D x D) -- (d x d) = (2 x 2) -- (0.5 x 0.5) = 3.75.
Divide the value of M by the belt thickness, T, and multiply the quotient by the factor 0.785 to obtain the length (L) of the belt. For example, if the belt has a thickness T = 0.125 inches, and the belt roll has an outer diameter of D = 2 inches and a core diameter of d = 0.5 inches, then the length = 0.785 x (M/T) = 0.785 x (3.75/0.125) = 23.55 inches.