How to Calculate Concrete for a Footing
A footing is what the exterior walls of the building sits on. The footing disperses the weight of the building over a wider area and helps prevent settling.
There are two main types of footings; spot footings, which the posts of a building or a pier would sit on, and continuous spread footing, which the frame of the building would sit on. The first thing you need to do is determine what type of footing you are going to pour; spot footing or continuous spread footing. This is determined by the structure being built and the type of soil on which it is being built. (See Tips.)
Things You Will Need
 Building dimensions
 Pencil
 Paper
 Calculator
Tip
If you are building a post and beam structure (like a pole barn) or a pier you can use spot footing. Most buildings, including residential homes, are built on a continuous spread footing. Refer to the Minimum Width of Concrete or Masonry Footings reference to determine the width of the footing required. If you are building on sand, you will need to use spot footing, as the sand will not allow the continuous spread footing to disperse the weight of the building across it.
Warning
Some municipalities have specific dimensional requirements that must be met for construction. Please contact your county office for these requirements. Some municipalities require permits. Contact your county permit office for information on these permits.
How to Calculate Concrete for a FootingSpot Footing

Spot footing is generally a 2’ by 2’ square which is 1012” deep. l=2’ w=2’ d=12”

First you need to convert all of your measurements to feet. Do this by dividing the depth by 12”.
12” / 12” = 1’. 
Second you multiply all dimensions to determine the cubic feet of concrete required.
2’ x 2’ x 1’= 4 cubic feet. 
Multiply this number by the number of posts you will be supporting with the spot footing. For example, a pier supported by 8 posts would be 4 cubic feet x 8 = 32 cubic feet.

Concrete is generally sold in cubic yards. Since the measurement is currently in cubic feet, you need to convert that to cubic yards. Do this by multiplying the cubic feet by .037037037.
32 cubic feet x . 037037037 = 1.185185184 cubic yards. 
Concrete used for this type of footing must be between 3,000 and 5,000 psi in compression.
How to Calculate Concrete for a FootingContinuous Spread Footing

Continuous spread footing is generally 16” to 24” wide, running the length of the exterior walls and 6” to 16” deep. Building dimensions: 24’ by 32’ l=24’ + 32’ + 24’ + 32’ = 112’ w=24” d=16”

First you need to convert all of your measurements to feet. Do this by dividing the width and depth by 12”.
24” / 12” = 2’ and 16” / 12 = 1.33’. 
Second you multiply all dimensions to determine the cubic feet of concrete required.
112’ x 2’ x 1.33’= 297.92 cubic feet. 
Concrete is generally sold in cubic yards. Since the measurement is currently in cubic feet, you need to convert that to cubic yards. Do this by multiplying the cubic feet by .037037037.
297.92 cubic feet x .037037037 = 11.03407406 cubic yards. Concrete used for this type of footing must be between 2,000 and 5,000 psi in compression. 
A third and less frequently used footing is the grade beam footing. It is essentially a continuous spread footing with spot footing where the soil is not satisfactory to disperse the load of the building over. To calculate grade beam footing first calculate the continuous spread footing required, then calculate the required spot footings and then add together the cubic yards of concrete to determine the amount of concrete required for this type of footing.
The Drip Cap
 A footing is what the exterior walls of the building sits on.
 There are two main types of footings; spot footings, which the posts of a building or a pier would sit on, and continuous spread footing, which the frame of the building would sit on.
 Second you multiply all dimensions to determine the cubic feet of concrete required.
2’ x 2’ x 1’= 4 cubic feet.  Concrete is generally sold in cubic yards.
 Do this by multiplying the cubic feet by .037037037.
32 cubic feet x .  Concrete used for this type of footing must be between 3,000 and 5,000 psi in compression.
References
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