How concrete is worked out
Volume is length times width times thickness, all in feet, so inches are divided by 12 first. A 10 by 10 foot slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet. Concrete is sold by the cubic yard, 27 cubic feet, so that slab is 1.23 cubic yards.
Slab or footing = length × width × (thickness ÷ 12)Round column = 3.1416 × (diameter ÷ 24)² × height × columnsCubic yards = cubic feet ÷ 27For a round column or a post hole, the diameter in inches divided by 24 is the radius in feet. A 12 inch tube 4 feet deep holds 3.14 cubic feet.
Add 5 to 10% for waste: ground that dips, forms that bow, and what stays in the mixer and the wheelbarrow. Running short in the middle of a pour leaves a cold joint, so the order errs high.
Bags or a truck
Premixed concrete in bags yields, by Quikrete's published figures, about 0.6 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag and 0.3 for a 40 lb bag. A cubic yard takes 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb.
Past a yard or so, a ready-mix truck usually beats mixing bags by hand. The supplier's minimum order and its charge for short loads decide where that line falls; a call with the cubic yards from this page gets the price.
How thick to pour
- Sidewalks and patios: 4 inches.
- Driveways: 4 inches for cars, 5 to 6 inches where heavier trucks park.
- Footings: at least 6 inches thick (IRC R403.1.1), as wide as the soil and the load require, with the bottom below the local frost depth (IRC R403.1.4).
Common questions
How many bags of concrete make a yard?
45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb, at Quikrete's yields.
How much concrete does a 10 by 10 slab take?
At 4 inches thick, 1.23 cubic yards, or 1.36 with 10% waste: 62 bags of 80 lb. At 6 inches, 1.85 cubic yards, 2.04 with waste.
How is concrete measured for an odd shape?
Split it into rectangles and circles, work out each one, and add them. The square footage calculator adds the areas; multiply the total by the thickness in feet for the cubic feet.
Does the order include gravel or rebar?
No. The gravel base is ordered by the same length and width at its own depth, often 4 inches compacted. Rebar and wire mesh are counted from the slab's size and the spacing the plans call for.