Weigh the load being moved on your scale. As an example, assume a weight of 10 lbs.
Measure the outer radius of the cylinder being used to move the load with your tape measure. The radius will be equal to one-half of its diameter. Be sure that you measure in inches. For the above example, assume a radius of 5 inches.
Square the value of the radius of the cylinder. For the above example, 5 * 5 = 25.
Multiply your result from Step 3 by the weight measured in Step 1. For the above example, 25 * 10 = 250.
Divide your result from Step 4 by the gravitational constant of 386 inches/second-squared to get the load inertia of your system. For the above example, 250/386 = a load inertia of 0.648 lb-in.-seconds^2.