How Much Concrete for a Garage or Shop Slab (12x20 to 40x60)

Give the dispatcher 8 yards for a 24x24 garage floor at 4 inches. The slab itself is 7.82 cubic yards after the 10 percent waste allowance, which is one mixer load and about $1,173 in material at $150 a yard. Poured 6 inches thick, the same floor is 11.73 yards, ordered as 11.75, and takes two trucks. Every size from 12x20 to 40x60 is tabled below at 4, 5 and 6 inches, with loads and cost for each.

Slab / FootingRound Column / Post

Length (ft)

Width (ft)

Thickness (in)

Waste buffer (%)

Bag size

40 lb50 lb60 lb80 lb

Price per cubic yard ($ (optional))

Volume

192.0 cu ft

Ready-mix (incl. waste)

7.82 cu yd

80 lb bags needed

353

Estimated cost

$1173

Bags needed by bag size

Bag size	Bags needed
40 lb	705
50 lb	564
60 lb	470
80 lb	353

Includes a 10% waste buffer. Ready-mix concrete is typically sold in quarter- or half-yard increments. Round up when ordering.

Yards to order for every common garage and shop size

These are the quantities to give the dispatcher. Each one is the slab volume plus 10 percent for over-dig and an uneven base, rounded up to the next quarter yard. Nothing here includes footings, stem walls or a thickened edge, which are covered further down.

Slab	Sq ft	4 in	5 in	6 in
12x20 (one car)	240	3.5	4.25	5
12x24	288	4	5	6
14x22	308	4.25	5.25	6.5
20x20 (two car)	400	5.5	7	8.25
20x22	440	6	7.5	9
20x24	480	6.75	8.25	10
22x22	484	6.75	8.25	10
24x24	576	8	10	11.75
24x30	720	10	12.25	14.75
24x36 (three car)	864	11.75	14.75	17.75
30x30	900	12.25	15.5	18.5
30x40 (pole barn)	1,200	16.5	20.5	24.5
40x40	1,600	21.75	27.25	32.75
40x60	2,400	32.75	40.75	49

For scale, the 12x20 at 4 inches is 147 bags of 80 lb mix, the smallest job on this page and already past the point where bags make sense; the 24x24 at 4 inches is 353 bags. Garage floors are ready-mix jobs from the first row.

Four inches holds a car, five or six holds the truck, the lift and the tractor

The thickness is set by the heaviest thing that will ever sit on the floor, not by the typical thing.

Four inches of 3,500 psi concrete carries passenger cars and light SUVs, and it is what most residential garage floors are, with wire mesh or a light rebar grid. If the garage will only ever hold cars, bikes and a freezer, this is the floor.

Five inches is the floor for a half-ton or three-quarter-ton pickup, a car trailer, a boat on its trailer, or a two-post lift. Lift makers publish a minimum slab thickness and strength for their anchors and will not stand behind a lift bolted into less, so get the sheet for the model you want before the pour, not after.

Six inches with a #4 grid is the shop floor: a compact tractor with a loader, a skid steer, a forklift, a welding table that never moves, or a one-ton truck. Pole barns used for equipment are poured at 6 inches as a matter of course.

Every inch of thickness adds a quarter of the 4 inch volume, so the 20x20 that is 5.5 yards at 4 inches is 7 at 5 and 8.25 at 6. The slab thickness guide covers the same decision for patios, sheds and driveways side by side.

What to tell the dispatcher: strength, air, slump and the first truck time

A ready-mix order is a specification, and the plant will give you its cheapest default for anything you leave out.

• Strength: 3,500 to 4,000 psi. Three thousand is a patio mix. A garage floor gets 3,500 at the least, and 4,000 where it will see de-icing salt, which is any garage in a place where it snows, because the car brings the salt in on its underside and drips it onto the floor for the next four months.
• Air entrainment where it freezes. Specify an air-entrained mix, typically in the range of 5 to 7 percent air, for any floor exposed to freeze-thaw. It cannot be added once the truck has left the plant, and a non-air-entrained floor under salted slush scales within a few winters. A smooth hard-troweled finish on an air-entrained mix needs an experienced finisher, so say which finish you want when you order.
• Slump: 4 inches for a chute pour, up to 5 for a pump, with a water reducer if the crew wants it more workable. Water added at the truck is strength removed from the floor.
• Fiber is a cheap addition against plastic shrinkage cracking and is worth having. It does not replace the rebar.
• Retarder in hot weather, accelerator in cold, both decided by the plant once you tell them the date and the pour size.
• The first truck time and the spacing, which is the next section.

Some jurisdictions set minimum strength and air content for garage floors in severe weathering regions. Check with your local building department before the order goes in.

Mixer loads at a typical 9 to 10 yards per truck

A full ready-mix truck carries about 10 cubic yards, though plants run 9 yard trucks in some areas and smaller ones for tight access, so confirm the capacity when you book. The loads below assume 10 and use the order quantities from the first table.

Slab	Loads at 4 in	Loads at 5 in	Loads at 6 in	Material at 4 in	Material at 6 in
12x20	1	1	1	$489	$733
12x24	1	1	1	$587	$880
14x22	1	1	1	$627	$941
20x20	1	1	1	$815	$1,222
20x22	1	1	1	$896	$1,344
20x24	1	1	1	$978	$1,467
22x22	1	1	1	$986	$1,479
24x24	1	1	2	$1,173	$1,760
24x30	1	2	2	$1,467	$2,200
24x36	2	2	2	$1,760	$2,640
30x30	2	2	2	$1,833	$2,750
30x40	2	3	3	$2,444	$3,667
40x40	3	3	4	$3,259	$4,889
40x60	4	5	5	$4,889	$7,333

Material cost is the waste-included volume at $150 a yard, a typical 2026 figure, before delivery, pumping, short-load fees or labor. The 5 inch costs sit between the two columns; the 24x24 at 5 inches, for instance, is $1,467.

Four orders from the first table land exactly on 10 yards: the 24x30 at 4 inches, the 24x24 at 5, and the 20x24 and 22x22 at 6. On a 10 yard truck each is one load with nothing to spare; on a 9 yard truck each becomes two. Three more pick up a truck on 9 yard mixers without landing on a round number: the 30x30 at 6 inches (18.5 yards) goes from two loads to three, the 40x40 at 5 (27.25 yards) from three to four, and the 40x60 at 6 inches, 49 yards, from five to six. Ask.

On any pour over one truck, the rhythm matters more than the total. Book the trucks 30 to 45 minutes apart, a typical spacing, so the crew finishes placing one load as the next backs in; closer and trucks sit on standby, which is billed, and farther apart and the edge of the last load has stiffened before the next one hits it. Order every truck but the last at full capacity. After the second-to-last truck is placed, measure what is left to fill, and phone the balance in. Plants expect that call and would rather batch an accurate last load than take back a yard of concrete you paid for.

Why the order comes back rounded to the quarter yard

Plants batch in quarter-yard steps, some in half yards, so the quantity you give them is rounded up from the waste-included volume. A 24x24 at 4 inches is 7.82 yards and the order is 8. A 14x22 at 4 inches is 4.18 and the order is 4.25. A 40x60 at 6 inches is 48.89 and the order is 49. The rounding is a small extra cushion on top of the 10 percent, and the material waste factors guide goes into why the allowance on concrete is bigger than on most materials: you cannot stop a slab pour, go to the store and come back.

A floor that runs short gets a cold joint, a seam of weak concrete across the middle of the garage that cracks, lets water through and stays visible for the life of the building. A floor that runs long leaves you with a wheelbarrow or two of concrete to use in the next twenty minutes. Have somewhere ready for it: a pad for the air-conditioning unit, a step, the holes for a fence line sized with the fence posts and footings guide, or a mower pad behind the garage. The plant charges for what it loads, not what you use.

Rebar: a #4 grid at 18 to 24 inches is common, the plans decide

The usual residential garage floor spec is #4 bar (1/2 inch) at 18 to 24 inches each way, sitting at mid-depth on chairs, with the laps tied. On a 4 inch floor under passenger cars, welded wire mesh on chairs is still common and adequate. On anything at 5 or 6 inches, with a lift, a truck or equipment, the bar grid is the floor’s reinforcement and the thickness is only half the story.

Where there is a permit, the engineer or the building department sets the bar size and spacing, and the number here is what to expect rather than what to assume. Expansive clay, which heaves and shrinks with the seasons, can need an engineered slab, sometimes post-tensioned, and no table replaces that. The rebar calculator turns a slab size and spacing into bar count, laps and sticks to buy, and the rebar size chart has diameters, weights and the spacing that goes with each slab thickness.

One rule from the pour itself: the grid has to stay at mid-depth while the crew walks on it. That means chairs or dobies every few feet, not a hook and a pull during the pour. Steel lying on the vapor barrier is money buried for no reason.

The slab is one pour; the footings and stem walls are another

The tables count the flat floor and nothing else. A garage with walls has a foundation under those walls, and in most of the country that foundation is built first, in a separate pour, with the floor placed inside it afterwards.

In frost regions that means a footing trench dug below frost depth, a poured footing, and a stem wall of poured concrete or concrete block brought up to floor height, with the garage framed on top. The floor is then poured inside the stem walls on its own base, either floating free of them or keyed into a ledge. A block stem wall is counted with the concrete block calculator, and its grouted cores are a different material again, covered in the mortar, grout and core fill guide. The footings themselves go into the concrete calculator as a long narrow slab, with the perimeter entered as the length and the width and depth taken from the plans.

In warm climates and for many pole barns, the floor and its edge go in together as a monolithic slab with a thickened edge, the perimeter turned down into a footing typically around 12 inches wide and 12 to 20 inches deep. That edge is not in the tables either. Run it as a second entry in the calculator with the perimeter as its length: a 24x24 has 96 feet of edge, a 40x60 has 200. On a pole barn, the posts carry the building and the slab is poured around them later, which is why pole-barn floors are often the last thing built and the easiest to size: the table figure is the whole order.

Frost depth, footing size and whether a monolithic slab is permitted at all are local questions. Check with your local building department.

Base, vapor barrier and the slope to the door

Under the floor goes 4 inches of compacted crushed stone over a stripped and compacted subgrade, sized as loose volume with the gravel calculator. More base is cheap insurance over wet or soft ground, and on a 40x60 floor the grading of that base is most of a day’s work on its own; a laser level pays for its rental here.

A vapor barrier, 10 mil polyethylene or thicker with lapped and taped seams, goes directly under the concrete. Garages store things that rust and are increasingly finished as living space, and a floor without a barrier is a floor that sweats.

Garage floors are set to drain to the door at roughly 1/8 inch per foot, so snowmelt and a washed car run out rather than pooling under the workbench. Across a 24 foot deep garage the back wall sits 3 inches higher than the threshold, which the framing crew needs to know before the bottom plates go down. Some jurisdictions require that slope, or a drain, in any garage that parks a vehicle, and a floor drain brings the plumbing code with it. The building department will say which.

Pole barns at 30x40 and 40x60: pumps, crews and cutting the floor into panels

A 30x40 at 6 inches is 24.5 yards, three trucks. A 40x60 at 6 inches is 49 yards, five trucks and about $7,333 in concrete. These are not weekend pours with three friends; they are a morning with a crew of five or six, a power screed, and a plan for where every truck parks.

Access decides the pump. A mixer chute reaches a dozen feet or so and the truck cannot drive onto the vapor barrier, so once the far side of a 40 foot bay is out of reach the choices are a line of wheelbarrows or a line pump, which the plant can usually book for you. A pump adds a daily charge that in 2026 typically runs from several hundred dollars upward depending on the size and the hours, and it is almost always cheaper than the labor it replaces on a floor this size. A pump also wants a slightly higher slump, which goes back into the order.

Pour in lanes. A 40x60 is placed as strips the width the crew can screed, working from one end, with the trucks staged so each lane is finished before the next starts. Then the floor is cut into panels: control joints at 2 to 3 times the thickness read as feet, so 12 to 18 feet on a 6 inch slab by the rule, though most crews stop at 15, sawn the next morning a quarter of the depth. A 40x60 cut at 13 feet 4 inches across and 15 feet along gives twelve panels; a 30x40 cut at 15 feet across and 13 feet 4 inches along gives six. Where the slab wraps a post, run a joint through the post, since the re-entrant corner is where the crack will start otherwise.

Reading the cost column: material only

Every dollar figure on this page is material only: the waste-included volume at $150 a yard, which is a typical 2026 national figure. Delivered prices in 2026 typically range from about $125 to $185 a yard depending on region, strength and air entrainment, so a 24x24 at 4 inches that shows $1,173 here can land a few hundred dollars either side of that before anything else is added.

On top of material: delivery or a short-load fee on orders under about 3 or 4 yards, which catches the one-car sizes; the pump, if used; and finishing labor, which on a garage floor is usually quoted per square foot and on the smaller slabs commonly exceeds the cost of the concrete. The concrete slab cost guide puts those pieces together size by size, and the widget at the top of this page accepts your own per-yard quote in place of the $150.