Ready Mix concrete is sold by the cubic yard. One cubic yard of concrete will fill a space that is one yard wide by one yard deep, by one yard high. Unfortunately, construction requirements are usually measured in feet or in inches, or often in a combination of the two. Of course, one yard is 3 feet, and one foot is 12 inches, so a reasonably clever person can convert the measured feet and inches into cubic yards. It helps to have some examples.
- First, let's return to that one cubic yard box. It is also 3 feet by 3 feet by 3 feet. Remembering how to calculate volume, we multiply the length times the width times the height. To measure the cubic feet of this box, we take 3 times 3 times 3 (3*3*3=27 cubic feet). So, one cubic yard of space is the same as 27 cubic feet of space. So, 27 cubic feet of space needed to be filled, will require one cubic yard of material.
- Now, if we measure everything in feet, we'll be able to simply add up all the cubic feet of space, divide by 27 and know the required number of yards. However, often we know the length and width in feet, but the depth is measured in inches. Remember, there are 12 inches in a foot, so to convert inches into feet, we simply divide the number of inches by 12. So, six inches divided by 12 is .5 feet. 8"/12= .667 feet. 4" / 12 = .333 feet.
- So, for example, a slab that is 16' x 20' x 4" deep, will need 16 x 20 x (4 / 12) = 106.667 cubic feet. Divide this by 27, and you need 3.95 yards. We sell concrete in quarter yard increments, so four yards are needed. Generally, savvy contractors add 5-10% for waste and spillage - so a prudent order here would be for 4.25 yards.
- Another way to estimate slabs is to calculate the square footage and then divide by a conversion factor for the depth of the slab. This is commonly done, and is the same thing mathematically, but is easier for many people to remember. To use this method, calculate the square footage by measuring the length by the width, and divide by the appropriate conversion factor (see information in the accordion at the bottom titled "Thickness Conversion Factor".