Concrete is a mixture of water, sand, gravel and cement. You can hand-mix your own cement for do-it-yourself construction projects at home as long as you keep your batches to under 2 cubic yards total volume. It is impractical to hand-mix larger volumes because of the weights and handling times. In order to determine how to mix a cubic yard of concrete, you need to know how much of each ingredient to put into the mixture.

Measure the size of the largest piece of gravel. If it is 3/8 inch, then record the gravel proportion as one and a half. If the maximum gravel size is 1/2 inch, then the gravel proportion should be set equal to two. If the maximum gravel size is 3/4 inch, then the gravel proportion should be two and a half. If the maximum gravel size is 1 inch, then the gravel proportion should be two and three quarters. If the maximum gravel size is 1 1/2 inches, then the gravel proportion should be three.

Add four to the gravel proportion. Label this number "A." Divide "1" by A. This result is the percentage of cement in the mixture needed to make 1 cubic yard of concrete.

Multiply 40 1/2 feet by the percentage of cement your mixture needs. The result is the number of cubic feet of cement you need to put in your mix. The total volume of concrete is equal to approximately two-thirds of the volume of the separate ingredients. Because 1 cubic yard is equal to 27 cubic feet, you need three times twenty-seven divided two cubic feet, or 40 1/2 cubic feet, of ingredients in your mix.

Multiply the number of cubic feet of cement you need by 94. The result is the number of pounds of cement you need. There are 94 pounds of cement in a cubic foot, and cement is usually sold by weight.

Divide "2 1/2" by A. The result is the percentage of sand you need in your mixture. Multiply 40 1/2 feet by the percentage of sand you need to get the volume of sand you need, in cubic feet. Multiply this result by 100 to determine the weight of sand in pounds that your mixture requires. The density of sand is 100 pounds per cubic foot, and sand is also usually sold by weight.

Divide the gravel proportion by A. The result is the percentage of gravel you need in your mixture. Multiply 40 1/2 feet by the percentage of gravel you need to get the volume of gravel you need, in cubic feet. Multiply the result by 100, which is the approximate weight of 1 cubic foot of gravel. The result is the weight of gravel you need for your mixture.

Divide "1/2" by A. The result is the percentage of water that your mixture needs. Multiply this percentage by 40 1/2 cubic feet to obtain the number of cubic feet of water your mixture needs. Divide this result by 0.13 to get the number of gallons of water your mixture needs.