Concrete Calculator
Calculate cubic yards, cubic feet, and bags of concrete for any slab or pour.
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How to Calculate Concrete Volume
The concrete calculator converts your project dimensions into the volume of concrete you need. Whether you're ordering ready-mix by the cubic yard or buying premixed bags at a hardware store, knowing the exact quantity prevents costly over-ordering or frustrating mid-project shortages.
The Formula
Volume (cubic feet) = Length × Width × (Depth ÷ 12). Convert to cubic yards by dividing by 27. For a 10×10 slab at 4 inches thick: 10 × 10 × (4/12) = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. Always add 10% overage: 1.36 cubic yards to order.
Bags vs. Ready-Mix
One 80-lb bag yields 0.6 cubic feet. One 60-lb bag yields 0.45 cubic feet. For the 1.23 cubic yard example above: 1.23 × 27 = 33.2 cubic feet ÷ 0.6 = 56 bags of 80-lb mix. At $7–$9 per bag, small pours cost $400–$500 in bags versus $150–$200 for ready-mix delivery (plus the minimum order charge). Ready-mix becomes cost-effective above roughly 1 cubic yard.
Standard Slab Thicknesses
Sidewalks and patios: 4 inches. Residential driveways: 4–5 inches. Garage floors: 4–6 inches. Heavy-duty driveways (trucks, RVs): 6 inches. Slabs for heavy equipment: 8 inches or more. Foundations and structural elements: 6–12+ inches (requires engineering specifications). Thicker slabs resist cracking better and carry more load. Local building codes may set minimums, so check before you pour.
Reinforcement
Most residential slabs benefit from reinforcement. Wire mesh (6×6 W1.4×W1.4) is commonly used for patios. #3 or #4 rebar on 12- to 18-inch centers is standard for driveways and garage floors that carry vehicles. Either way, the steel belongs in the lower third of the slab, held 1.5–2 inches off the base on plastic chairs; mesh left lying on the ground does nothing. Rebar is not included in the concrete calculation but should be ordered separately — typically it takes about 1 lb of rebar per square foot for a reinforced slab.
Sub-Base and Control Joints
A poorly prepared base is the most common reason driveways and patios crack within a few years. Excavate to the slab depth plus 4–6 inches, fill that extra depth with compactable gravel (road base), and run a plate compactor over it before setting forms. Then plan control joints: shallow grooves, cut or tooled to about one quarter of the slab thickness (1 inch in a 4-inch slab), every 8–10 feet in each direction. Concrete shrinks as it cures and will crack somewhere; joints make it crack in a straight line where you chose instead of randomly across the surface.
Tips for a Good Pour
Use forms (2×4 or 2×6 lumber) to contain the pour. Have a plan for the truck — ready-mix typically needs to be placed within 90 minutes of batching. During hot weather, wet the sub-base before pouring to reduce rapid drying. After finishing, keep the slab covered with plastic or wet burlap for at least 7 days.
Frequently Asked Questions
10 × 10 × (4/12) = 33.33 cu ft ÷ 27 = 1.23 cubic yards. Add 10% overage and order about 1.36 cubic yards of ready-mix. In bags, that is 56 bags of 80-lb premix before waste, or 62 bags with the 10% allowance.
One 80-lb bag yields 0.6 cubic feet and one cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags per cubic yard. A 60-lb bag yields 0.45 cubic feet, so you need 60 of those per yard. Add 10% for waste, which is about 50 bags of 80-lb mix per yard.
4 inches for sidewalks, patios, and interior floors. 4–5 inches for standard residential driveways, 5–6 inches for garage floors and driveways that see trucks or RVs, and 8 inches or more for heavy-equipment or structural slabs, which should be engineered. Always pour on a compacted base of 4–6 inches of gravel and check local building codes.
Cement is one ingredient in concrete: a powder that reacts with water (hydration) and binds everything together. Concrete is the finished material, a mix of roughly 10–15% cement, 60–75% sand and gravel, and 15–20% water. A "cement driveway" is really a concrete driveway.
You can walk on it after 24–48 hours. Wait at least 7 days before light vehicle traffic, when concrete reaches about 70% of its design strength, and 28 days before heavy vehicles. Keep the slab moist (plastic sheeting or wet burlap) for the first week so it cures to full strength.
Ready-mix is delivered by a truck, freshly batched, in exact quantities, and is the better value for pours over about 1 cubic yard. Bags (premixed dry concrete) are mixed with water on-site. They are convenient for footings, fence posts, steps, and small pads, but labor-intensive and more expensive per cubic foot at larger volumes.