A metal pulley and a rope.
The size of the pulley is determined by the diameter of the pulley and the easiest way to determine this is to measure the device with a tape measure. In systems that require multiple pulleys, the pulley sizes can be different, and there is usually a ratio that allows the system to operate at optimum efficiency. Pulleys that use belts should also be measured to ensure proper belt width.
Three pulleys hanging from a metal ring on a ship.
The manufacturer lists the size of a pulley as part of its specifications. If a pulley needs to be replaced, the serial number printed on the pulley can be used to verify its size. Websites and catalogs that sell new pulleys will list the size of the pulley along with the product description.
Measuring pulley size manually is relatively easy. A person can measure the diameter of the pulley by turning the pulley so that the circular part is facing forward. A tape measure is placed from one side of the circle to the other, making sure that the measuring edge passes through the midpoint of the pulley.
Pulleys often have a groove around their circumference that guides a chain, cable, belt, or rope.
In some configurations, the proper pulley size will depend on the ratio of the size of a larger pulley to the size of a smaller pulley. Knowing the size of one of the pulleys, as well as the ratio, is all that is needed to figure out what the size of the other pulley in the system is or should be. In this case, the unknown size of the pulley can be determined by dividing the known size by the ratio.
It may also be necessary to measure the height of the cylindrical part of the pulley to determine the size of the pulley. Again, using a tape measure is the easiest way to find your height. If the pulley has a lip on the edges to prevent the belt from slipping, the lip should not be included in the measurement because the usable space on the pulley face does not include the leading edges. This measurement is used to determine the proper size for the belt, which will be slightly narrower than the pulley itself.