Concrete piers can support a deck, porch, house or barn in all but the softest soils. You must do some calculating to make sure the piers support the weight without sinking or heaving when the ground freezes. Calculations also help you plan the amount of concrete. Your county extension agent or building code officials can help determine your soil type and its freezing depth, as well as provide typical weights per square foot for different types of buildings.
Divide the total weight that the piers must bear by the pounds per square foot that your soil will support. Compact gravel may support as much as 5443 Kilogram per square foot, while soft clay may only support 907 Kilogram.
Divide the result by the number of piers you plan to install, to calculate how many square feet the bottom of each pier must be.
Take the square root of the result in Step 2 to calculate the actual dimensions of the piers in feet. For example, if each pier must cover 3 square feet, the square root of 3 is 1.73, which means each pier must be 1.73 feet by 1.73 feet. To convert to inches, multiply the result by 12, so 1.73 feet would equal approximately 20 3/4 inches. If you plan to install round piers, divide the result from Step 2 by 3.14, take the square root, then multiply by 2. The result will be the diameter of a round pier in feet.
Add the depth of each pier in feet to the height above the ground that you want your building or deck to be. The piers should extend below the point where the ground freezes in winter. This will give you the total height of the pier.
Multiply the length and width of each pier in feet by the height from Step 4 to calculate the cubic feet of concrete needed for each pier, if the piers are square. If the piers are round, divide the diameter from Step 3 by 2, square the result and multiply by 3.14, then multiply by the result from Step 4 to calculate the cubic feet for each pier. Multiply the cubic feet of each pier by the number of piers, to calculate the total cubic feet for the job. Divide by 27 to calculate the number of cubic yards of concrete necessary.
For a small job in which you plan to mix concrete from bags rather than order it by the yard, divide the number of cubic feet by .45 if you're using 60-pound bags or .6 if you're using 80-pound bags. The result will be the number of bags you need. Allow 5 to 10 per cent extra concrete to compensate for waste. Use an online concrete calculator to simplify calculations (see the Resource section below).