Length and width
Measure the length and width of the concrete to be poured. If you are pouring a pad, then just multiply the width by the length to figure the area. For footings, combine the entire length of the footings and multiply by the width. For example, if you are pouring a footing for a 12-foot-by-12-foot building, take the length of the sides and multiply by the number of sides: 12 x 4 = 48 and multiply by the width (16 inch footings converts to 1.33 inches), so 48 times 1.33 is the square footage of the footings.
Next, find the cubic feet of concrete to be poured--later you will convert this to cubic yards because concrete is sold by the cubic yard. If your pad is 4 inches thick, that will convert to .33 feet--you must stay in feet. For footings--they are usually 8 inches thick for light construction--convert the 8 inches into .66 feet. So, you have the following formula, for the example footings, which will give you cubic feet: 48 (length of footings) x1.33 (16-inch footings) x.66 (8-inch deep footings) = 42.13 cubic feet of concrete.
Convert the cubic feet
Calculate a slab by multiplying length by width by thickness (4 inches is equivalent to .33 feet). Divide that number by 27 and you will have the cubic yards for the slab. For the footings, take the 42.13 cubic feet calculated earlier and divide by 27 (1 cubic yard of concrete equals 27 cubic feet). The final formula is 48 x 1.33 x .66 = 42.13 divided by 27 = 1.56 cubic yards of concrete. Round it up to the next half-yard. You would need to order 2 yards of concrete to pour the footings. If you are pouring the slab at the same time, be sure to add in the concrete for that.