In order to do any work involving concrete, you must determine how much you will need. This way you will know if you should buy prepackaged materials, mix your own concrete by hand or with a machine, or have ready-mix concrete delivered. When you go to estimate the amount of concrete required, you base this on the dimensions of a rough layout of the project. When you go to make your measurements, allow for slight irregularities in concrete thickness and for some waste by rounding up to the next whole or 1/2 cubic yard measure. A minimum of 5 to 10  percent extra is the norm.

To find the volume of a square or rectangle shape, multiply the width in feet times the length in feet times the height in inches (WxLxH). Then divide the total by 12 to get cubic feet and round up to the nearest whole number. For example you have a rectangle shaped area, that is 3 feet wide and 14.5 feet long and 8 inches high. So the formula would be (3 x 14.5 x 8) = 348 divided by 12 = 29 cubic feet. Then you divide this number by 27 to covert to cubic yards which in this case would be 1.074 cubic yards, then add in your 10 % and you would have in this example 1.18 cubic yards.

To find the volume of a cylinder like a circular driveway or patio, or for post settings. Multiply the square of the radius in feet times pi (3.14) times the height in inches, then divide the total by 12 to get cubic feet. Then round the result up to the next whole number and divide by 27 to achieve cubic yards. As an example to are building a circular pad, you measure from the center to the edge and you get lets say 16 feet and it is 7 inches high then using the formula R² x 3.14 x h. The result would be 16² x 3.14 x 7 =5626.88. then round off to 5627 divide by 12 = 468.91 cubic feet round it up to 469 then divide by 27 = 39.08 cubic yards + your 10 % = 42.98 or 43 cubic yards.

To find the volume of an irregular shaped area, you will need to break down the shape into sections of rectangles or squares and circles. Then by using the above formulas you will determine the volume of each section in cubic feet. After this add up all the volumes and divide by 27 to get the total number of cubic yards. For example you are building a patio out the back of your house that is a rectangle shape and turns to a circular section around the side of your house. So measure the rectangle and find it is 20 feet wide x 15 feet long x 6 inches high, using the formula (WxLxH) divided by 12, you get 150 cubic feet. Next you measure the circular section, which is lets say 15 feet from center and 6 inches high using the formula  (R² x 3.14 x h), you get 15² x 3.14 x 6 = 4239 divide this by 12 which = 353.25 cubic feet. Round this number up to 354 add in your 10 % =389.4 or 390 cubic feet. Now take the 150 cubic feet from the rectangle add the 390 cubic feet from the circular pad divided this by 27 and you get 20 cubic yards.

If you want to convert to a metric volume just multiply the total cubic yards by 0.765 to get the total cubic meters.