 Welcome to Calculating Belt Length. To find the length of a mechanical drive belt, such as a V-belt or timing belt, you could use the following formula. But this process is not as simple as it could be. Here is another way to determine belt length if the pulleys or sheaves are not at an extreme diameter ratio, such as 12 to 1. Step 1. Measure the center to center distance. Multiply the center to center distance by 2, because there are two of these distances. Step 2. Find the circumference of each pulley by multiplying pi times the diameter. Since the belt only touches the pulley half the way around, or half of the circumference, you will need to do some division. You will need both of these lengths for the next step. Add the pulley's half circumferences to the center to center length. With center to center distances and idler pulley adjustment that are built into the drive, the belt length calculation does not need to be perfect. Choose a belt length of the next larger size than calculated. In the above example, choose a 42 inch belt. Now you try it. Use the steps to find the answers. Round total belt length calculated to the nearest hundredth. Pause the video as you figure them out. Congratulations! You've completed calculating belt length.