A static water column exists at the bottom of your well pipe. Active pumping removes water from this column, and water from the surrounding aquifer replaces the loss in volume. Calculating the actual amount of water in your well pipe requires simple equipment and a little math.
Things You'll Need
 Measuring tape
 Cotton string
 Weight
 Calculator

Measure the diameter of the pipe at the wellhead. Use a measuring tape to measure the inner diameter of the pipe that forms your well casing.

Tie a weight to one end of a roll of cotton string. Lower the weight to the bottom of the well. When the weight no longer pulls the string down, it has hit the bottom of the well.

Pull the string back out of the well. Tie a knot in the string at the point where the string becomes wet. This will indicate the top of the waterline in your well.

Stretch the string out on the ground and measure from the bottom of the weight to the knot. This will give you the height of the water column.

Determine the amount of water in the well pipe by squaring the radius (half the width) of the pipe, multiplying that number by pi (3.1415). Multiply the product of this operation by the height of the water column in inches. For a 6 inch well casing with a 10 foot water column, the equation would look like this: 3 x 3 = 9 (radius of the pipe squared). 9 x 3.1415 = 28.27, then 28.27 x 120 = 3392.4 cubic inches.

Convert the water volume from cubic inches to gallons by multiplying the volume by 0.00432, the fraction of gallons in a cubic inch. In this case the equation would be: 0.00432 x 3392.4 = 14.655 gallons.
Tips & Warnings
 This measurement should be done on a well before it is used the first time. If the pump is in place, turn the power off prior to dropping the string or anything else down the well casing.
 Do not use a fishing sinker or anything else made of lead for the weight on your string. Reduce the risk of water contamination by using only clean cotton string and a nonreactive material like stainless steel for your weight.
References
 Photo Credit well image by Tomasz Plawski from Fotolia.com