How to calculate footer size

Written by jennifer lyons | 13/05/2017
How to calculate footer size
A frost line is the depth that frost penetrates the soil. (frosted tree image by Fantasista from

The footing of a building is what the weight of the exterior walls sits upon. Footings evenly distribute the weight of the building so that the soil is able to support the load. Footings keep houses from settling due to an uneven distribution of weight. Choosing the right kind of footing and the size of footing depends on the type of soil the house is being built on as well as the frost line of the area.

Get your soil tested. Continuous spread footing is good for homes being built on weak soil. It supplies a stable base around the total perimeter of the structure. It normally runs from 16 to 24 inches wide and 6 to 16 inches deep.

Calculate the dimensions of the building. The length is the perimeter of the structure, the width is the same number as the shorter side of the building in inches, and the depth, in this case, is half of the longer side of the building in inches. So if you have a building that is 22 feet by 30 feet, l =104 feet, w=22 inches and d=15 inches.

Convert your width and depth to feet. Divide your numbers by 12. Width is 22/12 = 1.83 and depth is 15/12 = 1.25.

Multiply length, width, and depth to determine the cubic feet of concrete needed. 104 x 1.83 x 1.25 = 237.9 cubic feet.

Get your soil tested. Spot footing is generally used on heavier, stronger soil. Spot footing uses single points of contact such as a post or pad to support the building. The pads are placed in a grid. Pads are typically 2 feet squares that range from 10 to 12 inches thick.

Calculate the weight of the building. For heavier buildings, you will want pads that are closely spaced together, for lighter buildings, such as one story homes, the pads can be placed farther apart. Pads are normally placed between 2 and 3 feet apart from each other, depending on the strength of the soil.

Determine the cubic feet of concrete needed for the spot footing. If you are using 12 inch (one foot) thick pads and need 20 to support the weight of the building (four rows of five pads), multiply 2' x 2' x1' x 20. You will need 80 cubic feet of concrete. Spot footing uses reinforced concrete rated from 3,000 to 5,000 pounds per cubic square inch in compression.

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