Figuring How Much Concrete To Pour A Slab
Pouring a concrete slab is needed when laying a foundation, adding a sidewalk or creating a driveway space in a home.
There are many different kinds of concrete slabs. Each of them has a different application and structural composition that will determine how thick the slab needs to be and how much concrete will be needed to make it.
Lightweight Concrete Pads – Lightweight pads are concrete pads that will not bear a great deal of weight once the concrete is poured and cured.
Examples of lightweight concrete pads include patios, sidewalks and gardening shed floors. For a lightweight concrete pad, the thickness of the slab needs only be 2 to 3 inches.
Moderate Weight Concrete Pads – Moderate concrete pads include slabs that will be used under items that have significant weight and would break a lighter pad.
Examples of moderate weight slabs include pads for hot tub spas, patios or verandas with porches and standard garage floors. The thickness of this type of slab should be a minimum of 3 to 4 inches.
Heavy Weight Concrete Pads – Heavy concrete pads are slabs that will bear a substantial amount of weight.
Many home slab foundations, workshop foundations and pads for RVs, boats and large construction workshops are designed to carry weight without cracking or crumbling under the pressure. Heavy pads should be a minimum of 6 inches or more thick—the heavier the intended weight, the more substantial the thickness of the slab should be.
Calculating Concrete Volume
- Calculate Cubic Feet - Measure the length and width of the area where you intend to lay the slab. Multiply these two numbers together. This will give you the basic area for the slab.
- Determine the thickness of your intended slab. Remember that the more weight the slab will bear, the thicker the slab needs to be.
- Multiply the thickness of the slab by the basic area. This will give you the area of the slab in cubic feet.
- Calculate Cubic Yards - Divide the cubic foot measurement from Step 1 by 27. This will is the volume of cubic yards of concrete needed for the concrete slab.
- Calculate for Overage - Multiply the volume of cubic yards determined in Step 4 by 1.05. This provides 105 percent of the concrete needed for the slab. You will need extra concrete to allow for spillage, waste or any unforeseen events.
If you are planning on mixing your own concrete, or if the slab will be a small one, you will need to purchase enough concrete mix to cover the entire area. Rent a portable concrete mixer and any equipment needed to move the concrete from the mixer to the slab location.
If you order concrete already mixed from a local concrete supplier, they will usually deliver it and pour it directly to the prepared concrete slab forms.