How to Calculate Concrete Needed for a Cylinder
Calculating the volume of a cylinder is an important calculation to make when determining how much material is needed to fill a hole or other similar shape. The basic formula for calculating the volume of a cylinder is pi X r^2 X h. Concrete is typically sold in cubic feet.
Things You Will Need
 Ruler or measuring tape marked in inches
 Calculator
Tip
Always round your volume upward. It is better to have a little too much concrete than not enough. For a very large cylinder, use more digits of Pi to get a more accurate volume.
Converting the calculated volume to cubic feet will help you determine how much concrete to purchase.

Measure the diameter of the cylinder. To measure the diameter, measure one circular end of the cylinder across the widest point. Your ruler or measuring tape should cross the center of the circle. Record the measurement in inches.

Use your calculator to divide the length of diameter by 2. The result is the length of the radius. For example, if the measured diameter is 12 inches, the radius is 6 inches. Record the length of the radius.

Measure the height of the cylinder in inches and record the result.

Multiply the length of the radius by itself. For example, if your radius measures 6 inches, multiply 6 by 6, for a total of 36. Record the result of this calculation.

Use the number 3.142 for the value of pi. Multiply the result of step 4 by 3.142 and record the result. For example, with a radius of 6, the result is 113.112.

Multiply the result of step 5 by the height of the cylinder. This calculation will provide the volume of the cylinder in cubic inches. For example, if a cylinder 12 inches in diameter has a height of 40 inches, the result is a volume of 4523.48 cubic inches.

Concrete is most often measured in cubic feet. To convert from cubic inches to cubic feet, divide the volume by 1728. This is the number of cubic inches on one cubic foot. For example, a volume of 4523.48 cubic inches multiplied by 1728 is 2.6183 cubic feet.

Round your calculated volume up, depending on how the concrete is packaged. For example, if the concrete is packaged in full cubic feet, you need to purchase three bags to fill a cylinder with a volume of 2.6183 cubic feet.