How do you measure for a automotive v-belt to be installed on a pulley?

Answer 1

Inspection
Start by inspecting the existing belt, motor, fan and pulleys. Check to be sure the original belt's groove size fits the pulleys, and make sure the bearings, shafts and set screws are in good condition and secure.

While you're at it, inspect the motor mount, the fan and housings to be certain they will remain in good working order after the new airflow is set and that the equipment will be able to operate long term. We've all serviced equipment, only to have it die an awful death a short time later. Nobody's happy when that happens, so look past the new belt.

Record the old belt number written on the exposed surface of the belt. This is your best reference to find the original size, type, and length of the belt. If you don't have the replacement belt on your truck, these numbers come in handy for the inventory crew to verify your measurements by cross-referencing the belt number with the manufacturer's catalog. A faded page in a manufacturer's catalog is more accurate than field belt and pulley measurements any day.

Do the Math
Now it's time to use the "other tool," a math formula to determine the size of the replacement belt. Work the formula a few times before you try it in the field, just so you're familiar with the math. Practice makes perfect.
Several measurements are required to complete the replacement belt formula. Measure and record the distance between the motor shaft and the pulley shaft, and then measure the diameter of the motor pulley (also called the drive pulley or sheave) and the fan pulley (called the driven pulley).

Apply the following formula to determine a new replacement belt length:

Belt Length = 2C + 1.57 x (D1 + D2) + 1-in.

Where:
C = The distance between the motor and fan shafts
D1 = The diameter of the small pulley
D2 = The diameter of the large pulley

The math is quite easy; it simply includes adding and multiplying. Here is a completed formula to walk you through the steps.

Example:
C = (12-in.) distance between shafts
D1 = (3-in.) diameter of the small pulley
D2 = (8-in.) diameter of the large pulley
New belt length = 2C + 1.57 x (D1 + D2) + 1-in.
2(12-in.) + 1.57 x ( 3"+ 8" ) + 1-in.
24-in. + (1.57 x 11-in.) + 1-in.
24-in. + 17.27-in. + 1-in. = 42.27-in.
So, select a new belt the next length above 43 inches.

Next, determine the thickness of the belt. This is determined by measuring its cross-section. Be careful; old belts may be so badly worn that the dimension may not be accurate. State the new belt size by stating its length and its cross section nomenclature (Click here to view a chart which shows typical HVAC belts and measurements.)

Answer 2

Look here: http://www.gizmology.net/pulleysbelts.htm They have the information you need.