In landscaping, home construction, and remodeling projects, most basic measurements are made in linear feet, and so the two-dimensional area measurements are made in square feet. However, when it comes to buying materials to fill three-dimensional spaces, the measurements are usually in cubic yards. Concrete for driveways, sand or gravel for a patio base, or soil to fill a retaining wall or raised bed planter, for example, are sold by the cubic yard, not the cubic foot.
There are a number of ways you can calculate the volume of a space in cubic yards, but one easy way is to begin by calculating the volume in cubic feet, them making an easy translation of that number into cubic yards. People sometimes describe this process as "converting square feet to cubic yards," but in reality, what you are really doing is adding a third measurement to a square foot calculation. This produces a volume measurement in cubic feet, which can then be easily translated to cubic yards.
For our example, we'll imagine that we're calculating how many cubic yards of concrete we're going to need for a driveway pad. But the same type of calculation could be used for almost any type of material used to fill a three-dimensional space.
Here's how it's done:
Step 1: Find the Surface Area in Square Feet
Any two-dimensional space can have its surface area measured simply by multiplying the length by the width. So, for example, if a planned driveway pad is 30 ft. long by 24 ft. wide, the area is 720 square ft. (30 x 24).
If you have an irregularly shaped area, you can divide it into smaller rectangles, find the area measurement of those rectangles, then add those separate measurements together to come up with the total square footage.
Step 2: Add the Third Dimension to Calculate the Volume
Two-dimensional measurements are used to calculate area, but volume is a three-dimensional measurement, so you will now need to factor in a third measurement—the depth or height of the space. If, for example, we imagine that a driveway pad will have a 6-inch layer of concrete, that means it will be .5 ft. thick (6 inches is one-half foot).
So, we now simply need to multiply the area measurement we calculated in step 1 (720 square feet) by the third measurement (.5 ft), to calculate that our driveway pad will use 360 cubic feet of concrete (720 x .5 = 360 cubic feet).
But as we mentioned before, concrete as ordered from a readi-mix company is sold by the cubic yard, not cubic feet. So, on to the final step:
Step 3: Translate Cubic Feet Into Cubic Yards
This is the trickiest part the whole project, but it's easy to understand the math if you realize that a single cubic yard is a volume of space that measures 3 ft. long on each side. So, a single cubic yard consists of a volume that is 3 ft. x 3 ft. x 3 ft—or 27 cubic feet. In other words, one cubic yard = 27 cubic feet.
Since there are 27 cubic feet in a cubic yard, an overall volume measurement that is made in cubic feet can be translated into cubic yards simply by dividing the cubic ft. measurement by 27.
So in our example, a 360 cubic-foot driveway pad will require 13.333 yards of concrete. (360 divided by 27 equals 13.333). That's how much concrete you will need for your driveway, assuming your measurements were correct.
Professional contractors alway increase their materials estimates by a small percentage to allow for errors or waste during installation. Adding 5 percent or so to the overall estimate is usually a good idea. A good concrete contractor, for example, might make sure that a full 14 cubic yards of concrete is delivered for a job that he expects to require 13.333 cubic yards.