Concrete Calculator: Yardage, Bags & Ready-Mix Cost

Concrete is sold by the cubic yard, but slabs are measured in feet and inches — so the math is a unit conversion. Take length × width × thickness, convert thickness from inches to feet (divide by 12), and that's your volume in cubic feet. Divide by 27 (the number of cubic feet in a cubic yard) and you have the cubic yards of concrete you need before any waste factor.

A 20 ft × 10 ft patio at 4 inches thick is 20 × 10 × (4/12) = 66.7 cubic feet, or 66.7 ÷ 27 ≈ 2.47 cubic yards. With a 10% waste factor that becomes about 2.72 cubic yards. Ready-mix is typically sold in quarter-yard increments, so you would order 2.75 yards.

For irregular shapes, break the project into rectangles and add the volumes. Circular slabs use π × radius² × thickness. Stem walls and footings are usually calculated separately from the slab they support, then added to the order.

A 60 lb bag of premix yields about 0.45 cubic feet (≈ 1/60 of a cubic yard). An 80 lb bag yields about 0.6 cubic feet (≈ 1/45 of a cubic yard). For a 2.5-cubic-yard pour, that is roughly 150 sixty-pound bags or 113 eighty-pound bags — and many hours of mixing.

The practical break-even is about one cubic yard. Below that, bagged premix from a hardware store is usually faster and cheaper than the short-load fee a ready-mix supplier charges. Above one cubic yard, ready-mix wins on price, consistency, and finish quality. Above three cubic yards, ready-mix is the only sensible choice.

Ready-mix prices in mid-2026 typically run $150–$200 per cubic yard delivered, plus a short-load fee of $50–$150 per yard short of a full truck (usually 8–10 yards). Pumping adds another $1.50–$4.00 per cubic foot depending on reach.

Sidewalks and patios: 4 inches over a compacted 4-inch gravel base. Residential driveways: 4 inches for cars, 5–6 inches if a delivery truck or RV will park on it. Garage floors: 4 inches with #4 rebar at 18-inch centers. Footings: code-required depth (often 12 inches × twice the wall width), check local code.

Waste factor accounts for over-excavation, uneven subgrade, spillage at the chute, and the small amount left in the truck. 10% is a safe default for most homeowner pours. Add 5% on top of that for pump pours or complex form work, and an extra bag or two if you are mixing by hand and want a comfortable cushion.

Reinforcement (rebar, mesh, fiber) is a separate calculation and is not included in the yardage above. For slabs over 4 inches, plan on either #3 rebar in a 16-inch grid or 6×6 W2.9×W2.9 welded wire mesh.

Forgetting to subtract for haunches, thickened edges, or stem walls when ordering separately — and then running short.

Ordering exactly to the calculated volume with no waste factor. The truck driver will not wait while you scramble for more concrete, and a cold joint formed by a second pour is a visible long-term defect.

Mixing slabs and footings in the same order without telling the dispatcher. Footings often want a stiffer mix (lower slump) than a finished slab.

Skipping the gravel base. Concrete needs a uniform, compacted, free-draining base or it will crack along the weak spot regardless of how thick the slab is.