How Much Concrete Do I Need? Slabs, Footings and Piers

Cubic feet to cubic yards

Concrete is sold by the cubic yard, and a cubic yard is 27 cubic feet (3 × 3 × 3 ft). Every concrete estimate is the same three moves: find the volume of the form or hole in cubic feet, divide by 27, then add some for waste. The place people go wrong is mixing units, so convert the thickness to feet before you multiply. Four inches is 4 ÷ 12 = 0.333 ft; six inches is 0.5 ft. The concrete calculator does the conversion, the waste allowance and the bag count for slabs, footings and round piers.

Slabs, footings and round piers

A slab or footing is length × width × thickness, all in feet. A 10 × 12 ft patio at 4 in.: 10 × 12 × 0.333 = 40 ft³, which is 40 ÷ 27 = 1.48 yd³. A continuous footing is the same math with the trench width and depth: a 20 in. wide, 8 in. deep footing 60 ft long is 1.667 × 0.667 × 60 = 66.7 ft³, or 2.47 yd³.

A round pier is π × r² × depth, with the radius in feet. An 8 in. form tube has a 4 in. radius, 0.333 ft: 3.1416 × 0.333² × 4 ft deep = 1.4 ft³. A 12 in. tube 4 ft deep is 3.14 ft³. Add the bell or footing pad under the pier if the plans call for one. Measure the hole you dug, not the one on the drawing; an auger wanders and a hand-dug hole is never a clean cylinder.

Bags or a truck

Bagged concrete mix yields about 0.60 ft³ per 80 lb bag, about 0.45 ft³ per 60 lb bag and about 0.30 ft³ per 40 lb bag. Check the bag, because yields vary a little by product. That works out to roughly 45 bags of 80 lb mix per cubic yard, 60 of the 60 lb or 90 of the 40 lb. The 8 in. pier above takes 1.4 ÷ 0.60 = 2.3 bags, so buy three. The 10 × 12 patio takes 40 ÷ 0.60 = 67 bags before waste, over 5,000 lb of material to carry, mix and place before the first batch starts to set.

That is why the usual break point is about one cubic yard. Below it, bags win on convenience, and you can mix as you go for piers, post holes or a small landing. Above it, ready-mix is cheaper per yard, arrives at a consistent mix, and lets you place the whole slab in one go with no cold joints. Most bagged mix is rated 4,000 psi at 28 days if you keep the water close to the instructions, about 3 quarts per 80 lb bag. Soupy mix is weak mix.

Waste, minimum loads and ordering

Add 5 to 10% for waste: spillage, an uneven subgrade that eats an extra half inch across the slab, forms that bow, and what stays in the chute. Use the high end on a rough gravel base or hand-dug footings. Running short with the truck gone is far worse than an extra quarter yard, so round up. The 40 ft³ patio with 10% waste is 44 ft³, or 1.63 yd³, so you order 1.75 yards, or 2 if the plant sells in half yards.

Ready-mix plants have a minimum order, often 1 yd³, and most add a short-load fee for orders under about 3 or 4 yards; the fee and the threshold vary by supplier, so ask when you call. Have the pour ready before the truck arrives: forms braced, reinforcement in place, subgrade damp, a path for the chute or the wheelbarrows, and enough hands. The truck has to discharge within 90 minutes of batching (ASTM C94), and the driver's time on site is often metered after a short unloading window. Order the slump you want, typically 4 in. for a slab, and do not add water at the truck beyond what the ticket allows.

How thick and how strong

For residential flatwork, 4 in. is the common thickness for patios, sidewalks and shed slabs, and 5 to 6 in. is common for driveways and anywhere a vehicle parks; garage floors are usually 4 to 6 in. Thicken the edge where a slab carries a wall. Footings follow the 2021 IRC: Table R403.1(1) sets the minimum width by soil bearing capacity and the number of stories, the minimum thickness is 6 in., and R403.1.4 puts the bottom of the footing below the local frost line, which the building department will tell you.

Strength matters as much as thickness outdoors. 2021 IRC Table R402.2 calls for 3,500 psi air-entrained concrete for porches, carport slabs, steps and garage floors exposed to the weather in severe-weathering areas, which covers most of the northern states, and 2,500 psi for interior slabs and footings. Air entrainment is what keeps a slab from spalling through freeze-thaw cycles and road salt, and you have to ask for it on the order. The local authority having jurisdiction and its amendments govern.

Subgrade, reinforcement and curing

Concrete only performs as well as what is under it. Strip the topsoil, compact the subgrade, and lay a compacted gravel base, commonly 4 in., so the slab bears evenly and drains. The 2021 IRC (R506.2) requires a vapor retarder under slabs that will be inside heated space. Wet the base down before the pour so it does not pull water out of the mix. Welded wire mesh or #3 and #4 rebar on chairs, held in the middle of the slab rather than lying on the base, controls crack width; fiber in the mix is another option for flatwork. Cut or tool control joints about a quarter of the slab depth, spaced in feet at two to three times the thickness in inches, so a 4 in. slab gets joints every 8 to 12 ft.

Then cure it. Concrete gains strength by staying wet, not by drying out. Keep the slab damp or covered for seven days, keep foot traffic off for a day or two and vehicles off for at least a week, and never pour on frozen ground or let fresh concrete freeze. It reaches its rated strength at 28 days. BuildSmith runs the concrete, rebar and slab math on the job site without a signal, and the roofing squares and paint coverage calculators cover the next orders if the slab is going under a shed or garage.