How to Calculate Concrete Needed To Pour a Slab

Determining with some precision the amount of concrete you'll need to pour a slab for any kind of home improvement project will save you time, money and effort. After all, if your estimate is incorrect, you could wind up with a concrete mixer truck delivering less than you need, resulting in an additional expense for a second delivery. By the same token, an inaccurate guess could wind up with you over-purchasing bags of concrete that then go to waste. Fortunately, it's relatively easy to measure your slab and convert the resulting measurements into the appropriate units, which will make ordering your concrete much simpler and more precise.

Concrete pouring during commercial concreting floors of building
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How to Calculate Concrete Needed To Pour a Slab

Measuring Slab Volume

The method of measuring your slab's volume is fairly straightforward. First, you'll need to measure the surface area of the slab. To do this, simply measure the length by the width, assuming you're dealing with sides of equal measure.

If your slab has sides of unequal measurement or has cutout areas that will not be poured with concrete, you'll need to calculate the area of the space to be excluded and then subtract it from the total.

Next, multiply the surface area by the thickness of the slab. Convert the thickness into feet so that you'll get the volume product in cubic feet. This, in turn, makes it simpler to calculate the number of yards you'll need for your concrete order.

Converting to Cubic Yards and Cubic Feet

In most cases, you'll need to provide the amount of concrete you need in cubic yards in order to place the correct order. If you're working with a smaller project, such as the floor of a garden shed, cubic feet will suffice.

And if you're purchasing bagged concrete, you'll need yet a third number: The number of require bags. To make it even more complicated, the answer to the question "How many bags do you need?" will depend on whether you're talking about 40-pound bags, 60-pound bags or 80-pound bags.

Hence, a little conversion table is a useful thing to have on hand. Remember: 1 cubic yard equals 27 cubic feet, which equals 46,656 cubic inches. That means a 80-pound bag of concrete mix will yield 0.6 cubic feet of concrete, a 60-pound bag will yield 0.45 cubic feet and a 40-pound bag yields 0.3 cubic feet.

Ready-Mix vs. Bagged Concrete

For home projects, such as pathways, driveways, sheds and more, you can choose one of two major sources for your concrete needs: Ready-mix concrete that is delivered in a mixer truck or bags of concrete that you'll need to mix yourself on-site.

When choosing between these two sources, first estimate your concrete needs, and then get estimates for each type. Ready-mix via truck delivery seems much more expensive, but if your project will need dozens of bags, each of which must be mixed on-site, the expense may be well worth saving the hours of time and physical labor.

Reinforcement

Another issue you'll need to consider is whether your slab will require reinforcement.

Steel reinforcement – either a rebar grid or rebar grid with interlocking steel mesh – is designed to strengthen a concrete slab, helping to keep it uniform and prevent cracks from appearing in the concrete.

Any concrete slab that will bear significant weight loads will require steel reinforcement. This includes slabs that will bear the weight of multiple passenger vehicles, foundation walls and footings.

Generally, if the project is big enough to require reinforcement, it may also require a building permit. If that is the case, you'll note the specifications for your required reinforcement in the project plans.

Examples of projects that will generally not require extensive steel reinforcement include shed floors, sidewalks and pathways. However, if the underlying soil is spongy or less than firm, it's beneficial to add reinforcement regardless of how much weight the slab will bear.

To calculate the amount of reinforcement steel you'll need for your project, you can take advantage of one of the many calculators available on the internet that are designed to perform this specific calculation. You'll need to have in hand the following measurements: The length of your slab, the width of your slab, the amount of spacing between each bar you intend to leave and required splice – otherwise known as overlap – of your rebars.

The steel used to reinforce the slab won't rust. That's because oxygen must be present before rust can form. Since the reinforcement steel is completely encased in concrete, no oxygen is available to it.