Concrete Calculator

How slab volume is calculated

Length width thickness gives cubic feet, but thickness is usually measured in inches while length and width are in feet, so this tool first converts thickness to feet (dividing by 12) before multiplying. The result in cubic feet is then divided by 27 (since 27 cubic feet = 1 cubic yard) to get cubic yards, the unit concrete is actually ordered and priced in. For precise area measurements, use our free square footage calculator. A 1010-foot patio at 4 inches thick works out to 10 10 (412) = 33.3 cubic feet, or about 1.23 cubic yards.

How round column and footing volume is calculated

For a cylindrical shape like a fence post footing or a round column, volume is p radius height. This tool takes the diameter you enter, converts it to feet and halves it to get the radius, then applies that formula and multiplies by the number of identical columns you're pouring. A 12-inch-diameter, 4-foot-deep footing works out to about 3.14 cubic feet per post roughly 7 bags of 60lb mix for a single post, which is why multiple fence post footings add up faster than people expect.

Converting cubic yards to bags

Ready-mix trucks price and deliver by the cubic yard, but for small jobs, pre-mixed bags are more practical. Standard manufacturer yields are about 0.011 cubic yards per 40lb bag, 0.017 per 60lb bag, and 0.022 per 80lb bag meaning roughly 90, 60, or 45 bags fill a full cubic yard, respectively. This tool divides your calculated cubic yardage by the selected bag's yield and rounds up, since you can't buy a fraction of a bag.

Why over-ordering by 5-10% is standard practice

The volume this calculator shows is the exact theoretical amount for perfectly flat, perfectly formed dimensions. Real job sites rarely match that exactly subgrade is uneven, wooden forms flex slightly under wet concrete's weight, and some mix is always lost to spillage or overfill at the edges. Contractors routinely add 5-10% on top of the calculated volume for this reason, and running short mid-pour (forcing a second batch or truck) is a much bigger headache than having a small amount left over. For driveway or foundation projects, use our free mortgage calculator to understand long-term housing costs alongside construction budgets.

FAQ

How many bags of concrete do I need per cubic yard?
Roughly 90 bags of 40lb, 60 bags of 60lb, or 45 bags of 80lb mix per cubic yard, though this varies slightly by brand and this calculator's per-bag yields (0.011, 0.017, and 0.022 cubic yards respectively) are standard manufacturer figures.

Should I order extra concrete?
Yes most contractors add 5-10% extra to account for spillage, uneven subgrade, and form deflection. This calculator shows the exact theoretical volume, so add that margin yourself before ordering.

Why does thickness matter so much for a slab?
Volume scales directly with thickness, so going from a 4-inch to a 6-inch slab increases concrete needed by 50%, not a small amount. Thickness is entered in inches here but converted to feet internally to match the length and width, which are in feet.

Should I use bagged concrete or ready-mix delivery?
Bagged mix is usually more practical under about half a cubic yard for a small patio, footing, or fence post job. Beyond that, ready-mix delivery from a truck is typically cheaper per yard and far less labor, since mixing dozens of bags by hand for a full driveway is impractical. For cost comparisons between options, try our free percentage calculator to calculate savings on bulk orders.

How is the volume for a round column or footing calculated?
Using the cylinder volume formula: p radius height, where radius is half the diameter you enter. This calculator converts your diameter from inches to feet, squares the radius, multiplies by height and p, and then by however many identical columns you're pouring.

Does concrete volume shrink or expand as it cures?
Wet concrete volume is what you're ordering and what this calculator estimates; it doesn't meaningfully expand, though a small amount of shrinkage during curing is normal and is one more reason contractors typically over-order by 5-10% rather than ordering the exact calculated volume.