How to Estimate the Cost of a Concrete Slab

How to Estimate the Cost of a Concrete Slab

Know the right formula to figure the concrete material costs of your slab project

By Don Masters

Man smoothing concrete with trowel
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To estimate the cost of a concrete slab, you must first determine the amount of concrete your project will require. You do not have to be a mathematical wizard to accomplish this task; however, you should take the time to measure carefully and check your math. There are few things more irritating than estimating the amount of concrete you need only to find out mid-project that your estimate was short and your project will end up costing more than you figured.

  • What You Need to Know
  • Besides the concrete itself, you're likely looking at other costs for a concrete slab project, including grading the site, laying down a gravel foundation, setting up wooden forms to hold the slab in place while it cures, and mesh or rebar to strengthen the slab.

Step 1:

Mark the ground to indicate the location of the concrete slab. You can use marking paint or use a stick to score lines in the dirt; however, it is critical that you're precise to avoid over- or underestimates.

Step 2:

Determine the surface area of the concrete slab. If the area is square or rectangular, this step is easy. Simply multiply the length of the area by its width. If you have a circle, multiply the radius by itself first and then by 3.1416. If the area you are trying to measure is a triangle, multiply its height by 1/2 of its base. You can also break up the slab area into combinations of circles, triangles and squares for odd-shaped slabs. These measurements need to be in feet, so convert them prior to doing the math. If a measurement is 3 feet by 6 inches, divide the 6 by 12 to arrive at .5 feet. Your measurement will now be 3.5 feet. Do the math for each shape separately, and then add them all together for the final surface area calculation.

Step 3:

Convert the thickness of your slab from inches to feet. For instance, if your plans call for a 4-inch slab, simply divide 4 inches by 12 inches. The result will be .3333 feet of concrete.

Step 4:

Multiply the surface area by the thickness of the slab to obtain the total of volume of concrete you will need for this project. The answer will be in cubic feet. Since you will purchase the concrete by the cubic yard, divide your answer by 27 to give you total cubic yards of concrete needed.

Step 5:

Allow yourself some room for error in your estimate. Multiplying the total cubic yards you need by 1.05 will give you an answer that is 5 percent greater than your estimate. This will allow for any errors you may have made in your layout or calculations.

Step 6:

Multiply your final answer by the current concrete material costs per yard in your area to arrive at the total estimated cost of the project. Concrete material costs will vary by region and are subject to change. Call your local supply yard when you are ready to determine your estimate to get the most recent price per cubic yard.

  • Tips & Warnings
  • Take all measurements twice to ensure their accuracy.
  • Calculate each formula twice to avoid mathematical errors.

About the Author

Donald Masters is a regular contributor to DexKnows.

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