Concrete is sold by volume: cubic yards from a ready-mix truck, or bags of premix that each yield a fixed number of cubic feet. Getting the number right matters because running short mid-pour leaves a cold joint and ordering a yard too much wastes $150 or more. This calculator works out the volume for the common shapes, adds waste, and converts to yards, bags and weight.
How it is calculated
- Slab: length × width × thickness, with the thickness converted from inches to feet (÷ 12). A 20 × 10 ft slab 4 in thick is 20 × 10 × 0.333 = 66.7 ft³.
- Footing: length × width × depth, with width and depth in inches converted to feet.
- Round column: π × (diameter ÷ 2)² × height.
- Multiply by the count, add the waste allowance, then convert: 27 ft³ per cubic yard, 35.3 ft³ per cubic meter.
- Bags: an 80 lb bag yields 0.60 ft³, 60 lb 0.45, 50 lb 0.375, 40 lb 0.30. Round up.
Example
A 20 × 10 ft patio, 4 in thick, with 10% waste: 66.7 ft³ × 1.1 = 73.3 ft³ = 2.72 yd³ (2.08 m³). That is 123 bags of 80 lb premix weighing nearly five tons, or a ready-mix order of 2.75 yards, about $410 at $150 a yard plus any short-load fee.
Bags or a truck
Bagged mix makes sense up to about half a yard: post holes, small pads, a few steps. Beyond that the arithmetic favors delivery: an 80 lb bag costs $5–7 and yields 0.6 ft³, so a yard in bags is $250–300 plus a day of mixing, against $130–180 delivered. Ready-mix trucks carry about 10 yards; suppliers charge extra for small loads, usually under 3 yards, and for waiting time beyond the first hour.
Thickness guide
Walkways and patios: 4 inches. Driveways and garage floors: 5–6 inches, thicker for heavy vehicles. Footings: at least 8 inches deep and twice the wall width, deeper below the frost line in cold climates. Slabs on grade want a compacted gravel base and, for anything larger than a small pad, wire mesh or rebar and control joints every 8–12 feet.
Frequently asked questions
How many bags of concrete do I need?
Divide the volume in cubic feet by the yield per bag: 0.6 ft³ for an 80 lb bag, 0.45 for 60 lb. A 10 × 10 ft slab 4 in thick is 33.3 ft³, about 56 bags of 80 lb, or 61 with 10% waste.
How much does a yard of concrete cover?
A cubic yard is 27 ft³: 81 square feet at 4 inches thick, 65 sq ft at 5 inches, 54 sq ft at 6 inches.
How much waste should I add?
5–10% for slabs on a prepared base, more for footings dug in rough soil where the trench is wider than planned. Running short costs more than a little extra.
How much does concrete weigh?
About 4,000 lb per cubic yard (2,400 kg per cubic meter), or 150 lb per cubic foot. A 20 × 10 ft, 4 in slab weighs about five tons.
How much does a yard of concrete cost?
Roughly $125–200 per cubic yard delivered in 2025–2026, depending on region and mix, plus short-load fees for orders under about 3 yards.
How many cubic feet will a 50 lb bag of concrete cover?
About 0.375 cubic feet, three eighths of a foot. The other common sizes: an 80 lb bag yields 0.6, a 60 lb bag 0.45, a 40 lb bag 0.30. A cubic yard is 27 cubic feet, so filling one with 50 lb bags takes 72 of them — which is why anything much over half a yard is usually cheaper and easier delivered ready-mixed.
Sources
By Calcelate Team. Formula from the sources above.
- 2026-09-14 · Formula and texts checked