The first step in any paving project is knowing how much concrete you'll need. Concrete is comprised of four ingredients--cement, aggregate, sand and water. Premixed concrete bags include all these elements and are the easiest to use, since you know you'll be getting the correct ratio of the four ingredients listed above. These bags come in 60- or 36.3kg. sizes.
Design your patio in advance. For the purpose of this tutorial, we'll use an example patio that is 12 feet long by 15 feet wide, to be poured at a depth of 6 inches.
Convert all the measurements into feet. For this example, the length is 12 feet, the width is 15 feet and the depth is 0.5 feet.
Multiply the three figures from Step 2 to get an accurate measurement of the total volume of concrete needed. In this tutorial, that is: 12 x 15 x 0.5 = 90 cubic feet.
Read the instructions on your bag of premixed cement to see how much coverage it provides. According to Renovation Headquarters, an 36.3kg. bag can generate roughly 0.6 cubic feet of cured (hardened) concrete. A 27.2kg. bag can cover roughly 0.45 cubic feet.
Divide the number you obtained in Step 3 (90 for this example) by the cubic feet of cured concrete of the size bags you plan on using. For example, if you plan on using 36.3kg. bags, divide 90 by 0.6. The resulting number is 150, meaning you'll need 150 of the 36.3kg. bags. If you plan on using the 27.2kg. bags, divide 90 by 0.45. That gives you 200, meaning you'll need 200 of the 27.2kg. bags for your project.
Create your own concrete by combining cement, sand, aggregate and water. The Portland Cement Association says cement comprises 10 to 15 per cent by volume of this mixture. That means an 36.3kg. bag of premixed concrete contains 3.63 to 5.44kg. of pure cement.
Purchase enough bags of cement to create the right amount in your concrete mixture. For this section, we will once again assume you are building a 12-foot-by-15-foot patio at a depth of 6 inches. We will also assume you are buying 2.27kg. bags of cement.
Remember the number of bags of premixed concrete you needed in Section 1. For this section, we will calculate the measurements based on needing 150 of the 36.3kg. bags.
Calculate the total weight of the pre-mixed concrete in the 36.3kg. bags by multiplying 150x80. That equals 12,000, meaning this project requires 5443 Kilogram of raw materials. Remember, 10 to 15 per cent of that is comprised of pure cement.
Multiple the number obtained in Step 5 (12,000) by 10 per cent (0.10) to see how many pounds of cement you'll need. 12,000 x 0.10 = 1,200, meaning you'll need a minimum of 544 Kilogram of cement for this project. Divide 1,200 by 5 (the number of lbs. in a bag of cement) to figure out how many bags you'll need: 1,200 / 5 = 240. So you'll need 240 bags of cement to create enough concrete for this project.
Always buy a few extra bags than your calculations suggest. A small amount of concrete or cement will always dry in the mixing container, resulting in a small loss of material.