Learn how to accurately determine the amount of concrete you’ll need for a residential driveway.By Anne Balogh, ConcreteNetwork.com
Decorative Concrete Institute in Temple, GA
Of all the exterior concrete flatwork you can install around your home, a driveway slab is likely to take up the most square footage and require the greatest investment in labor and materials, including the concrete itself. But just how much concrete you might need to order can vary widely, depending on whether your driveway is short and narrow, long and winding, or poured thicker than usual to support heavy traffic.
While determining how much concrete you’ll need for a rectangular slab is fairly straightforward, figuring out the amount required for an irregular or circular driveway shape is trickier. To avoid miscalculations, either on the plus or minus side, here are some basic rules of thumb for determining the quantity of concrete you’ll need for driveway slabs of various shapes and sizes.
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Determine the size of driveway you need
Although the length of your driveway will often be determined by the distance from your garage to the street, the width will depend on how many vehicles you plan to accommodate and the size of your property. For example, a two-car driveway should have a minimum width of 20 feet to allow enough room for car doors to open. Extending the slab to a width of 24 feet (if space permits) is even better because it will provide ample room for people to easily and safely walk between parked cars.
Instead of a rectangular driveway, you may want to consider installing a circular or semi-circular driveway. Circular driveways offer the advantages of multiple entry points and easy turnaround access. However, the cost is often higher than that for a rectangular slab because more labor is required to build and install curved forms (see Concrete Driveway Cost).
Learn more: Driveway Width Standards and Planning Tips.
Measure the driveway dimensions
Once you come up with a layout for your concrete driveway, you’ll need to take precise measurements of the area to accurately figure out how much concrete will be required. Here are general guidelines for measuring driveways of different configurations:
- For a rectangular slab, simply measure the driveway length and width.
- To figure out the dimensions of irregularly shaped slabs, create a scaled drawing of the project on graph paper then divide the irregular shape into a series of rectangles to determine the area for each in the overall layout. Add the results of all your shapes to get the total area.
- For a circular slab, measure the diameter of the driveway by taking its radius (the distance from the center of the circle to its edge) and doubling it.
When taking your measurements, be sure to include the apron of your driveway. Although some municipalities may have restrictions on which materials are acceptable under their building code requirements, concrete is often the preferred material for an apron and may even be required by some community regulations (see Make Your Home Stand Out with a Decorative Concrete Driveway Apron).
Determine the driveway thickness
The thickness, or depth, of a concrete driveway also factors into how much concrete the project will require.
In general, a concrete driveway should be poured at a minimum thickness of 4 inches (see Concrete Driveway Thickness). However, if your driveway slab will be supporting heavy vehicle traffic, you may want to increase the thickness to 5 or 6 inches, which will increase it’s load-carrying capacity nearly 50%. But first, you should check the local building codes in your city or town, which may have its own minimum thickness requirements for a concrete driveway.
Calculate the cubic yards of concrete needed
Once you know the length, width, and thickness of your driveway, you can plug those measurements into a basic formula (length x width x thickness = volume in cubic feet) to get the precise quantity of concrete needed. But first, you must convert the driveway thickness into feet so all the measurements are in common units. Because ready-mix concrete is sold by volume, measured in cubic yards, you’ll then need to divide the total by 27 to convert cubic feet to cubic yards. Our online concrete calculator can help you do the math.
For example, a 4-inch-thick one-car concrete driveway measuring 10 feet wide by 20 feet long will require approximately 2.47 cubic yards of ready-mix concrete. Since concrete is usually ordered to the nearest ¼ to ½ cubic yard, you would round the 2.47 figure to 2.5 cubic yards.
Leave room for error
Nobody wants to suffer the consequences of not ordering enough concrete to finish a job and being forced to put the project on hold until another delivery of concrete arrives. To allow for possible errors, and to compensate for concrete spillage and an uneven subgrade, always plan to order 10% to 15% more concrete than the computed volume.
In our driveway example above, 10% x 2.5 cubic yards of concrete = 0.25, for a total yardage of 2.75. Even if you have a small amount of concrete left over, this margin of safety will give you the peace of mind that you can complete your project without coming up short. If you have any doubts about your driveway measurements, your ready-mix supplier can often help you with your calculations, especially if you’re dealing with an oddly shaped slab.
Learn more about ordering ready-mixed concrete.
What about rebar?
Another essential component of a concrete driveway is reinforcement (either wire mesh or steel rebar) to provide additional structural capacity for vehicle traffic and to help minimize cracking. Because rebar is usually installed in a grid pattern with a spacing between bars of approximately 12 to 18 inches, you can come up with a rough estimate of the number and length of rebar needed for a standard rectangular driveway.
The basic formula is to convert the length and width of your slab to inches and then subtract 6 inches from each result to allow about 3 inches of clearance from the edges of the slab. Next, divide each dimension by the planned grid spacing, and round up to whole numbers.
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