Concrete Calculator

A concrete order is a one-shot decision. Short the truck and you are hand-mixing bags in a rush to fill the last few feet before the pour goes off. Over-order and you are paying for concrete that gets dumped in a low spot behind the shop or short-loaded back onto the truck at a return charge. Every contractor learns to hate both outcomes early, and every calculator in the world is trying to help you avoid them.

The math itself is simple. Concrete is priced and delivered by the cubic yard, and one cubic yard is 27 cubic feet. To find the volume of any rectangular pour, multiply length in feet by width in feet by thickness in feet, then divide the total by 27. The only trick is converting thickness from inches to feet, because pads and slabs are almost always specified in inches. A four inch thick slab is one-third of a foot. A six inch thick pad is one-half of a foot.

A worked example makes it stick. A twenty foot by ten foot slab poured four inches thick has a volume of 20 x 10 x (4/12) = 66.67 cubic feet, or 66.67 divided by 27 = 2.47 cubic yards. Add a ten percent waste factor for form flex, subgrade variation, and truck cleanout and you order 2.72 cubic yards. Most ready-mix producers round to the nearest quarter yard, so you would ask for three yards and finish the job with confidence.

Slabs and pads are the easy case because they are rectangular. Driveways, patios, garage floors, and shop floors all reduce to length by width by thickness. For irregular pours, break the shape into rectangles or triangles, calculate each, then add the volumes together. A rectangular driveway with a flared apron at the street becomes one rectangle plus two triangles, and each triangle is one half of base times height.

Footings look rectangular from above but are usually specified in inches for width and depth. A twelve inch wide by twelve inch deep continuous footing running forty feet is 40 x 1 x 1 = 40 cubic feet, or about 1.48 cubic yards. Turned-down slab edges follow the same logic. A four inch thick garage slab with a twelve inch by eight inch turned-down edge needs the slab volume plus the perimeter footing volume, minus the overlap where the two intersect.

Columns and piers are cylinders. Volume of a cylinder is pi times radius squared times height. A ten inch diameter pier that goes down four feet has a radius of 0.417 feet, a footprint of 0.545 square feet, and a volume of 2.18 cubic feet. Ten of those piers is 21.8 cubic feet, or about 0.81 cubic yards. Stairs are usually estimated as a triangular prism plus the tread rectangles. When in doubt, take the outer dimensions as if the shape were a solid block, calculate that, and subtract any large voids.

Bagged concrete is priced by the bag and yields a known volume. A sixty pound bag of standard mix yields about 0.45 cubic feet, which works out to sixty bags per cubic yard. An eighty pound bag yields about 0.60 cubic feet, or forty-five bags per cubic yard. High-early or fiber-reinforced mixes may yield slightly less because of the additives. Always round up whole bags because you cannot buy a fraction and any waste is worth the cost of not stopping the pour to run to the yard.

Ready-mix trucks solve the bag-count problem but introduce a new one. Most producers have a minimum load charge, typically one to three cubic yards depending on the market, and a short-load surcharge that grows steeply as the order shrinks below a full truckload of about nine to ten yards. If you are between two and four yards, do the math on total delivered cost per yard for the two options. Bagged mix and a mixer trailer can beat a short load once you factor in surcharge and fuel.

One rule that saves money on every job: measure the actual sub-base after excavation, not the drawing. Ground rarely finishes as flat as it looked on paper, and an extra inch of depth across a large slab is real concrete. On a twenty by forty foot slab, one extra inch of depth is 66 cubic feet, or 2.47 additional yards. That is the difference between a full truck and a short-load surcharge.

In North and Central Texas, subgrade behavior drives more slab failures than the mix design. Blackland clay swells when wet and shrinks when dry, which is why you see so many turned-down edges, moisture barriers, and post-tension slabs east of Interstate 35. If you are pouring on native clay without engineered fill, expect the subgrade to move and design the reinforcement to bridge those movements. Fiber mesh is cheap insurance; welded wire mat lifted into the middle of the pour is better; rebar on chairs is best for anything that will carry vehicle traffic.

Weather is the other variable. Summer pours in Central Texas frequently hit ninety-five degrees before ten in the morning, and slab surfaces flash-set before you can finish them. Order the truck for a five or six in the morning arrival, wet the sub-base, use a retarder if the ready-mix producer offers one, and have every tool staged before the first yard hits the ground. Winter pours are the opposite risk: any pour done below forty degrees needs blankets and an accelerator, and any pour done below freezing needs to be delayed.

None of this changes the volume math, but it changes how much waste you should carry. On a fast summer pour, ten percent waste is a floor, not a ceiling. On a controlled interior pour with fresh forms, five percent is plenty.

Concrete contractors in Texas mostly get work three ways: referrals from general contractors, repeat homeowner work, and inbound calls from local search. The first two are relationships you already have. The third is the only channel where new work shows up without you doing anything, and it is the channel that shared-lead brokers try to insert themselves into.

The problem with buying concrete leads from a broker is the same problem every trade has. The lead is sold to three or four other contractors at the same time. You are racing to be the first callback, dropping price to close, and paying per lead whether the job runs or not. If you would rather own the phone in your county outright, that is exactly the model Peak Local Leads is built on: we build and rank the website, you take the calls free for seven days, and only pay a flat monthly if the work is real.