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: 1.35 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. Foundations and structural elements: 6–12+ inches (requires engineering specifications). Thicker slabs resist cracking better and carry more load.
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 18-inch centers is standard for driveways. 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.
Tips for a Good Pour
Compact the subbase before pouring. Use forms (2×4 or 2×6 lumber) to contain the pour. Order slightly more concrete than calculated — you can always finish with a little extra. Have a plan for the truck — ready-mix typically needs to be placed within 90 minutes of batching. During hot weather, wet the subbase before pouring to reduce rapid drying.
Frequently Asked Questions
10 × 10 × (4/12) = 33.33 cu ft ÷ 27 = 1.23 cubic yards. Add 10% overage = 1.35 cubic yards to order. In bags, that's about 56 bags of 80-lb premix.
One 80-lb bag yields 0.6 cubic feet. One cubic yard = 27 cubic feet. So 27 ÷ 0.6 = 45 bags per cubic yard. Add 10% for waste = ~50 bags per yard to be safe.
4 inches for standard residential use. 5–6 inches for heavier vehicles or frequent heavy use. Always pour on a properly compacted base of 4–6 inches of gravel for best results.
Wait at least 7 days before light vehicle traffic and 28 days before heavy vehicles. Concrete reaches full strength (design strength) at 28 days. Keep it moist and avoid heavy loads during the curing period.
Ready-mix is delivered by a truck, freshly batched, in exact quantities. Best for pours over 1 cubic yard. Bags (premixed dry concrete) are mixed with water on-site — convenient for small projects but labor-intensive and more expensive per cubic foot at large volumes.