Determine the drive speed of the drive shaft. How you do this will vary greatly depending on the equipment you're working with. Some engines, motors or devices may have a tachometer. Alternatively, you may find information on the manufacturer's website or in the documentation.
Measure the drive pulley's pitch diameter. The pitch diameter is the point where little or no slippage occurs between the belt and the pulley. Although it's very difficult to determine accurately, in most cases it will be somewhere in between the base and the top of the pulley's groove. Inspect the belt while it's off of the pulley by bending it. The outer surface will stretch, and the inner will compress. The pitch diameter coincides with the point where neither compression nor stretching occurs.
Calculate the pulley ratio by dividing the pulley's pitch diameter by the pitch diameter of the drive pulley. For example, suppose the drive-shaft has a diameter of 2.2 inches, and two pulleys driven by the belt have diameters of 4.4 inches and 2.8 inches. The first ratio = 4.4/2.2 = 2, and the second ratio = 2.8/2.2 = 1.27.
Calculate the speed of each pulley by dividing the drive speed by the pulley ratio. For example, given a drive speed of 750 RPM, the speed of the first pulley = 750/2 = 375 RPM, and the speed of the second pulley = 750/1.27 = 591 RPM.