Sign footings must be calculated before construction begins. Failure to calculate these footings may cost you time and money, especially if you order too much concrete or not enough. The method for estimating sign footings has been simplified by the construction industry to include the average do-it-yourselfer. You can expect to complete this task is 15- to 30 minutes, depending on the size and scope of the project.
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Multiply the length times the width of the footer to determine the square footage. If the footer has more than four sides, then keep the sections separated. For example, if your sign footer is in the shape of a T or an L, then separate the sections on paper by labeling them. This will help you in case you need to make changes to one section. If you have a section that is 4 feet by 6 feet, then multiply to get 24 square feet. Record this on your paper.
Add all square footages together to get total square feet. Record this calculation under total square feet on your paper. For example, if you have a footing that several sections of, say 24, 28, and 48 square feet, then add them together.
Multiply the total square footages times the thickness of the footer. Because the calculations are in feet, convert your footing thickness--if it is less than one foot--into feet. Divide the thickness by 12. For example, if your footer is 8 inches thick, then divide that by 12 inches to get .66. If you have a footer that is 16 inches thick, then that means that your footer is 1'4". Convert the 4 inches into feet to get .33. Now your footer is 1.33 feet thick. Multiply 1.33 times the total square feet of, say, 48 to get 63.84 cubic feet.
Convert cubic feet into yards in order to determine the amount of yards that your sign footer will require. Divide the total cubic footage by 27, which is the equivalent of one cubic yard. Dividing 63.84 cubic feet by 27 equals 2.36 yards. Round up to the nearest yard to get 3 yards of concrete for your footer.