How to Calculate Piers for a Barn

Establish the bearing capacity of the soil at the barn site. Rocky soil can support up to 12,000 lbs. per square foot, while rotten blue clay may support less than 2,000 lbs. per square foot. For instance, stable soil with a high mix of organic content and gravel should be fairly reliable for configuring small-end pier sizes.

Add up the total weight of the barn materials. (The lumber yard where the materials are purchased can help with this, or a truck weigh station or area sanitation station, as available, can provide the total weight of the materials.)

Divide the total materials' weight by the pounds-per-square-foot the soil will support. For instance, if the barn materials weigh 6 tons and the soil supports 5,000 lbs. per square foot, you would divide 12,000 by 5,000. The quotient is 2.4.

Divide the number of piers called for in the barn plans by the quotient from Step 3. For instance, if the plans call for eight posts, divide 8 by 2.4.The quotient is 3.3. This number (3.3) represents the square feet per pier.

Take the square root of the quotient from Step 4. For instance, if the square foot calculation from Step 4 was 3.3, find the square root of 3.3. The square root of 3.3 is 1.8. This means that a square pier must measure 1.8 feet by 1.8 feet.

Locate the ground-depth winter freezing point. (The local county extension office or building permit-issuance agency will have information on this.) For instance, if the ground regularly freezes in the winter to a depth of 2 feet, plan on a 3-foot-deep pier.

Add the below-freezing point measurement of the pier to the above-ground measurement of the pier design. For instance, if the barn will have a dirt floor, then the concrete pier could be flush with ground level; it would be a total of 3 feet vertically. However, for a wood floor, the pier may extend 6 inches above ground level, for a total of 3 1/2 feet of pier height.

Multiply the length and width of the pier; multiply this product by the height of the pier. This product represents the cubic feet in concrete needed for each pier. For instance, a 1.8-foot-by-1.8-foot, 3 1/2-foot-high pier will require 11.34 cubic feet of concrete per pier.