The serpentine belt is like a large rubber band located inside the hood of your car. It loops around several components of your car such as the A/C compressor, idler pulley, power steering pump, crankshaft, and a few other components depending on the make/model of your vehicle. Check under your hood or in your owner's manual for a diagram about where to find your serpentine belt and how it is routed. Serpentine belts break or wear out for a variety of reasons. Air compressors can seize, causing friction and heat, or belts can break with age and use. Proper prevention maintenance helps determine when a belt needs to be replaced. Look for signs that the belt is cracked, weakened, dried out, or gives off a burnt rubber smell. Most of the time, the owner's manual or your local auto store will be able to tell you what size belt you will need. However, you can figure out what size of belt you need on your own without any fancy equipment.
Pop open the bonnet of your car. Make sure you have engaged the parking brake and that the vehicle is off.
Locate the path of your serpentine belt. If you are unsure the of the path, look under your hood for a diagram or in your owner's manual. Follow the path with the piece of string.
With the string tightly wrapped around the correct path of the serpentine belt according to the diagram in your owner's manual, mark the area on the string with a marker where it completely wraps around.
Measure the length of string it took to wrap around the path of the serpentine belt. Purchase a belt one size smaller than the size you measured. The actual belt will have tension on it, whereas the string doesn't. For example, if you measured 80 inches, purchase a belt that is 79 1/2 inches.
If you have the old serpentine belt, simply measure the old belt and buy the same replacement size.