Concrete is priced and sold by the cubic yard. A cubic yard is the volume of an area that is 3 feet by 3 feet by 3 feet. In other words, 27 cubic feet of concrete equals one cubic yard. Figuring the number of cubic yards of concrete you need for a project is generally a lot simpler than you may think, even if there are several odd-shaped areas to be included. So get yourself a good tape measure, a piece of paper, a pencil and a simple pocket calculator and let's get to measuring.
Measure the length, the width and the depth of any square or rectangular form into which you plan to pour concrete. Make measurements in feet and fractions of feet. Write the numbers down for future reference. Using your pocket calculator simply multiply the length by the width and then multiply that number by the depth. The answer you get will be the number of cubic feet of concrete you need.
Using your pocket calculator, divide the answer you got in step 1 by 27. This tells you how many cubic yards of concrete you need in order to fill this form.
Divide odd-shaped forms into smaller geometric shapes, then find the square footage of each individual shape and add all cubic footage together. Then divide your answer by 27 to find the number of cubic yards you need.
Determine the cubic feet of a circular area using the following formula: Pie X radius squared. Pie is a mathematical constant defined as 3.14159. The radius is a measurement from the centre of a circle to any point on the edge of the circle. Square the radius (i.e. multiply the radius by itself), multiply that number by 3.14159 and then multiply that answer by the depth of your circle and you will have the total number of cubic feet in your circular form. Divide the number of cubic feet by 27 to get the number of cubic yards.
Determine the number of cubic feet of any other shaped area by using the fun tool you will find in the Additional Resources section of this article. Remember to add together the cubic feet of all areas of concrete that you need to pour and then divide that answer by 27 to obtain the number of cubic yards you need.
Do not worry if the number of cubic yards do not come out to an even number. Most places will allow you to purchase fractional cubic yards of concrete. It is a good idea to order at least one half cubic yard more material that you think you will need on a large job to account for waste and any errors you may have made. It is better to have slightly too much concrete for a job than not enough.
Wear protective clothing, heavy shoes and gloves whenever working with concrete. If mixing dry concrete wear a breathing mask and eye protection.