Concrete Calculator
Concrete is the rare material where the arithmetic is easy and the logistics decide the project. The volume is length ร width ร thickness โ the part everyone gets right. The decisions that cost money sit around it: the waste allowance for uneven subgrade and spillage (10% is the working default; skimping it means a cold joint in the middle of a pour), the bag count at 40, 50, 60 or 80 pounds, and the point where bagged concrete stops making sense and a ready-mix truck becomes the cheaper, faster, structurally better answer. This calculator runs all three: cubic yards for the truck order, bag counts for the small pour, and an explicit recommendation flag the moment the volume crosses about a cubic yard. Columns and post holes are handled as cylinders โ enter the diameter in inches and the count, and the per-hole volume scales automatically.
The short answer
A 20ร10 ft slab at 4 inches is 66.7 ftยณ = 2.47 cubic yards before waste โ 123 eighty-pound bags with a 10% allowance. Past about one cubic yard, order ready-mix instead: bags cost more, arrive slower, and a truck pour avoids the cold joints that weaken a big slab.
Uneven subgrade, spillage and form irregularities โ 10% is the working default.
Concrete needed
2.72 ydยณ
66.7 ftยณ in the forms + waste allowance; 1 ydยณ = 27 ftยณ.
- 80 lb bags
- 123
- Order ready-mix instead
- 2.7 ydยณ
Each 80 lb bag yields 0.60 ftยณ โ 163 bags at 60 lb, 245 at 40 lb.
Past about one cubic yard, bagged mixes cost more, arrive slower and finish weaker than a truck pour.
How to use Concrete Calculator
- 1
Pick the pour shape
Slab or footing for rectangles (length ร width ร thickness), column for cylinders (diameter ร height ร count).
- 2
Enter the dimensions
Length and width in feet, thickness in inches, column diameter in inches. The calculator converts everything to cubic feet, then to yards.
- 3
Read yards, bags and the recommendation
The result shows volume with waste, the bag count per size, and โ above about a cubic yard โ a ready-mix flag worth heeding.
Why use this tool
- Cubic yards with the waste allowance built in, ready for a truck order
- Bag counts at 80, 60, 50 and 40 lb from published yields
- Slab, footing and column shapes โ cylinders scale by hole count
- Explicit ready-mix flag once the volume passes about one yard
- Runs in your browser โ site measurements never leave the page
Frequently asked questions
- How many 80 lb bags of concrete do I need?
- Divide the volume in cubic feet by 0.60 โ the published yield of an 80 lb bag โ and round up. A 20ร10 ft slab at 4 inches is 66.7 ftยณ, which is 112 bags before waste; with a 10% allowance, 123. At 45 bags per cubic yard, any pour over a yard or two is usually cheaper as a ready-mix truck.
- How much concrete is in a cubic yard?
- One cubic yard is 27 cubic feet, which is 45 eighty-pound bags, 60 sixty-pound bags, or 90 forty-pound bags. Concrete is ordered by the yard because that is what the truck delivers in โ 0.25-yard increments on most ready-mix loads.
- How thick should a concrete slab be?
- 4 inches is the standard for a patio or walkway; 4-5 inches with reinforcement for a residential driveway carrying cars; 6 inches or more for trucks or heavy equipment. Thickness drives volume linearly โ doubling it doubles the concrete, which is why the 4-inch default matters on every quote.
- How much extra concrete should I order for waste?
- 10% is the working default for formed slabs on reasonably even ground; 15-20% on rough or sloped excavation where the subgrade is uneven. Running short mid-pour is worse than the extra bags cost โ a pour that stops and restarts leaves a cold joint that permanently weakens the slab.
- When should I order ready-mix instead of bags?
- Past roughly one cubic yard. A single 10ร10 slab at 4 inches is 1.23 yards โ about 66 eighty-pound bags, or two hours of mixing by hand for a pour that a truck finishes in ten minutes with better consolidation. Bags are for post holes, footings and small repairs; trucks are for slabs.
- How do I calculate concrete for post holes?
- Treat each hole as a cylinder: volume = ฯ ร (diameter รท 2)ยฒ ร depth. A 12-inch hole 4 feet deep takes 3.14 ftยณ โ about 6 eighty-pound bags each. Enter the diameter, height and hole count in the column mode and the calculator scales it across the run.
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