Concrete Calculator

About the Concrete Calculator

Concrete is sold and poured by the cubic yard, but slabs are measured in feet and inches, so the mental math trips up even experienced crews. This calculator converts a slab's length, width, and thickness into cubic yards, adds a waste allowance for spillage and uneven subgrade, and tells you how many 60-pound and 80-pound bags an equivalent hand-mix would take, plus the cost of ordering ready-mix by the yard.

Ordering short is the expensive mistake. A ready-mix truck that has to make a second trip for a fraction of a yard can cost you a short-load fee plus lost time while the first pour starts to set, risking a cold joint. Ordering slightly long is cheap insurance. The waste percentage in this tool exists precisely so you round up in a controlled way rather than guessing at the plant.

How the Concrete Calculator works

Volume in cubic yards is length times width times thickness, with thickness converted from inches to feet by dividing by 12, and the whole figure divided by 27 because there are 27 cubic feet in a cubic yard. The waste percentage then multiplies that base volume: at 10 percent waste the tool multiplies by 1.10.

For bag counts, the calculator converts the waste-adjusted cubic yards back to cubic feet and divides by the yield per bag. A 60-pound bag yields about 0.45 cubic feet and an 80-pound bag about 0.60 cubic feet, and both counts are rounded up to whole bags. Total ready-mix cost is the waste-adjusted cubic yards times your price per yard, so the dollar figure already includes the overage you will actually pay for.

Worked example

Consider a 20 by 12 foot slab poured 4 inches thick, the default. Base volume is 20 times 12 times 0.333 feet, divided by 27, which is 2.96 cubic yards. Adding 10 percent waste gives 3.26 cubic yards, or about 88 cubic feet.

Hand-mixing that pour would take 88 divided by 0.45, rounded up, which is 196 bags of 60-pound mix, or 147 bags of 80-pound mix. At 160 dollars per yard for ready-mix, the material cost is 3.26 times 160, about 521 dollars. The bag counts make it obvious why anything over roughly half a yard is worth ordering by the truck rather than mixing by hand.

Tips & common mistakes

• Standard slab thicknesses are 4 inches for patios and walkways and 5 to 6 inches for driveways that carry vehicles; entering the wrong thickness is the most common source of a bad order.
• Use 10 percent waste on formed slabs with a compacted base and bump it to 15 percent on rough grade, footings, or trench work where the excavation is irregular.
• For a driveway or garage floor, spec at least a 6-inch base of compacted gravel under the slab so your concrete thickness stays uniform and your yardage estimate holds.
• Bagged mix is only economical for small jobs like post footings or a single step; past roughly half a cubic yard the labor and bag cost exceed a short-load ready-mix delivery.
• Order fiber mesh or plan for rebar and wire on any slab carrying loads; the calculator sizes volume, not reinforcement, which you specify separately.

Concrete Calculator: frequently asked questions