Concrete Calculator
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How the Volume Is Calculated
Concrete is sold and poured by volume, not area, so every shape here reduces to a standard geometric volume formula converted into cubic yards (the unit ready-mix trucks price by). A slab or rectangular footing uses length × width × thickness. A round column or footing uses the cylinder formula π × r² × height, where r is half the diameter. A tube (hollow cylinder, like a sonotube footing) subtracts the inner cylinder's volume from the outer cylinder's volume before multiplying by height. All dimensions are converted to feet, the result in cubic feet is divided by 27 (the number of cubic feet in a cubic yard) to get cubic yards, and the result is rounded up in practice since ready-mix is typically ordered in quarter- or half-yard increments.
Why the Waste Allowance Matters
Uneven subgrade, formwork seepage, and spillage during placement routinely eat into a "textbook" volume calculation, which is why contractors commonly order 5-10% more concrete than the raw math suggests — this calculator defaults to 10% but treats it as a planning buffer, not a precise physical constant, since actual loss varies by site conditions and crew experience. Ordering short on a ready-mix truck is far costlier than a little extra leftover, since a second short-load delivery carries its own minimum charge.
Bagged Mix vs. Ready-Mix
For small pours, pre-mixed bags are usually more economical than paying a ready-mix minimum; a common rule of thumb is that a 60 lb bag yields about 0.45 cubic feet of cured concrete and an 80 lb bag yields about 0.6 cubic feet, figures used by most bag manufacturers in the US. For larger pours (roughly more than 1 cubic yard) ready-mix delivery is almost always cheaper and less labor-intensive. If you're also framing the area you're pouring into, the square footage calculator can help you nail down length and width first, and the stair calculator is useful if your pour includes concrete steps.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab that's 4 inches thick?
10 ft x 10 ft x (4/12) ft = 33.33 cubic feet, which is 1.23 cubic yards. Most contractors would round up and add a 5-10% waste allowance, bringing the order to roughly 1.35 cubic yards to account for subgrade unevenness and spillage.
How many 60 lb bags of concrete equal one cubic yard?
A 60 lb bag yields about 0.45 cubic feet of cured concrete, and a cubic yard is 27 cubic feet, so it takes about 60 bags (27 / 0.45) to equal one cubic yard. This is why bagged mix is usually only cost-effective for small pours.