How Much Concrete for a Patio Slab (10x10 to 20x20)

Order 2 yards for a 12x12 patio at 4 inches: the slab works out to 1.96 cubic yards once the 10 percent waste allowance is on, about $293 of ready-mix at a typical $150 a yard. A 10x10 is 1.36 yards (order 1.5) and a 20x20 is 5.43 yards (order 5.5, about $815). In bags, that 12x12 is 89 sacks of 80 lb mix, which is why patios come on a truck.

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

48.0 cu ft

Ready-mix (incl. waste)

1.96 cu yd

80 lb bags needed

89

Estimated cost

—

Bags needed by bag size

Bag size	Bags needed
40 lb	177
50 lb	141
60 lb	118
80 lb	89

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

The sections below run in the order the job happens: settle the thickness, place the order, set the forms to fall, lay the joints and steel, then pour, finish and cure.

Four inches unless something heavy is going on it

A patio carries people, furniture and a grill. Four inches of 3,000 psi concrete over a compacted base has carried that for decades, and it is what the first table assumes. Three things push a patio to 5 inches with rebar:

• A hot tub. A filled tub puts the weight of the water, the shell and the people in it onto a few square feet. Most makers publish a pad specification; follow it, and put the steel under the tub footprint at minimum.
• An outdoor kitchen. A block and stone island is a line load along one edge of the patio, and a slab that is fine under chairs will settle under it. Reinforce under the island and consider a thickened strip there.
• A vehicle, even occasionally. A slab that a car crosses to reach a side yard or a boat pad is a driveway at that point. The slab thickness guide covers where that line falls.

Five inches adds a quarter to the concrete. On a 12x12 that is 1.96 yards against 2.44, $293 against $367 in material at $150 a yard, which is cheap next to replacing a cracked slab under a hot tub.

Ready-mix for patios at 4 inches, 8x10 to 20x20

Every figure includes a 10 percent waste allowance for over-dig and an uneven base. The order column rounds up to the next quarter yard, the cost column is the concrete alone at $150 a yard, which is about what plants were quoting in 2026, and the bag column is there for comparison rather than as a recommendation.

Patio	Sq ft	Ready-mix order (yd)	Yards with 10% waste	Material at $150/yd	80 lb bags
8x10	80	1.25	1.09	$163	49
10x10	100	1.5	1.36	$204	62
10x12	120	1.75	1.63	$244	74
12x12	144	2	1.96	$293	89
12x14	168	2.5	2.28	$342	103
12x16	192	2.75	2.61	$391	118
14x16	224	3.25	3.04	$456	137
15x15	225	3.25	3.06	$458	138
16x16	256	3.5	3.48	$521	157
16x20	320	4.5	4.35	$652	196
18x18	324	4.5	4.4	$660	199
20x20	400	5.5	5.43	$815	245

Two rows worth a second look. The 16x16 comes to 3.48 yards and the order is 3.5, so there is almost no cushion beyond the waste allowance; if the base is rough, order 3.75. And a 15x15 and a 14x16 take the same 3.25 yard order, a reminder that the shape is yours to choose and the volume only cares about area.

The same patios at 5 inches

Patio	Ready-mix order (yd)	Yards with 10% waste	Material at $150/yd	80 lb bags
8x10	1.5	1.36	$204	62
10x10	1.75	1.7	$255	77
10x12	2.25	2.04	$306	92
12x12	2.5	2.44	$367	110
12x14	3	2.85	$428	129
12x16	3.5	3.26	$489	147
14x16	4	3.8	$570	172
15x15	4	3.82	$573	172
16x16	4.5	4.35	$652	196
16x20	5.5	5.43	$815	245
18x18	5.5	5.5	$825	248
20x20	7	6.79	$1,019	306

The 18x18 at 5 inches lands on exactly 5.5 yards, with no rounding cushion at all. Order 5.75.

What the bag column is telling you

Nothing stops you pouring an 8x10 from 49 bags. A rented mixer, two people and a morning will do it, and the bags per cubic yard guide covers the yields. The trouble starts a size or two up, and it is not the lifting.

A patio surface has to be worked as one piece. The whole slab needs to be screeded, floated, edged, jointed and broomed while all of it is at the same stage of set, because the finish has to match from corner to corner and because you cannot broom concrete that has already gone hard. Mixing 89 bags two at a time is more than forty batches, and at the rate two people can place them the first corner is losing its bleed water while the last batch is still in the drum. The surface finishes in bands, the bands do not match, and where a batch went down against concrete that had already stiffened there is a cold joint, which is a crack you built in on purpose. A truck puts 2 yards on the ground in the time it takes to mix the fourth batch. Above about a yard, order it. The concrete calculator will still give you the bag count, and the point of looking at it is to decide not to.

Short loads: why the 10x10 costs more per yard than the 20x20

Ready-mix is priced per yard with a minimum order, and below a threshold that is typically 3 or 4 yards the plant adds a short-load fee, commonly $60 to $150 in 2026, because the truck and driver cost the same to send out with 1.5 yards as with 9. On a 10x10 that fee can approach the price of the concrete itself. It does not make bags cheaper, for the reasons above, but it changes two things:

• Bundle small pours. If a walkway, a step or a pad for the air conditioner is on the list, pour it the same day from the same truck. The fee is per delivery, not per slab.
• Ask about a volumetric truck. Some suppliers run mixers that batch on site and bill for the yards dispensed, which suits a 1.5 yard patio well and removes the short-load argument entirely. Availability varies by area.

Standby time is the other charge: once the truck has waited past its allowance, usually quoted per yard, the meter runs. That is why the checklist at the end of this page exists. The concrete slab cost guide puts delivery, pumping and finishing labor next to the material figures above.

Fall: an eighth of an inch per foot, away from the house

Water has to leave a patio, and it leaves across the surface. The standard fall is 1/8 inch per foot away from the house, which is 1.5 inches across a 12 foot patio, 2 inches across 16 feet and 2.5 inches across 20. Next to a basement, a door or a crawl-space vent, 1/4 inch per foot is safer, and building codes generally require the finished grade to fall away from a foundation, so check what your building department wants to see at the house wall.

The fall is set in the forms, before anyone has ordered anything. Fix the house-side form at the finished height, which needs to sit below the door threshold and below the bottom of the siding by a couple of inches, then set the far form lower by the full drop and string a line between them. Do not try to create the fall in the finishing by floating the far side down; the concrete will tell you where the form was. If the yard rises away from the house, the patio still has to fall away from it, and the water gets caught at the low edge by a channel drain piped to daylight.

Joints: one at the house, a grid across the slab

Two kinds, doing two jobs.

The isolation joint, which most people call the expansion joint, goes where the patio meets anything that does not move with it: the house foundation, a porch stoop, deck posts, a column footing. It is a strip of 1/2 inch asphalt-impregnated fiber board set against the foundation, full depth, before the pour, and it means the slab can shrink, heave and settle without prying at the house. Leave it out and the slab is cast tight against the foundation, and the first freeze finds the weak point.

The control joints go across the patio in a grid and decide where the shrinkage cracks happen. Space them at 2 to 3 times the thickness in inches, read as feet: 8 to 12 feet apart on a 4 inch slab, 10 to 15 on a 5 inch one. Cut them a quarter of the slab depth, 1 inch on a 4 inch patio, either with a grooving tool while the surface is still workable or with a saw the next morning before the slab has cracked on its own. Keep the panels close to square, with no panel longer than 1.5 times its width.

Laid out on the sizes above: a 10x10 or 12x12 gets one joint each way, four panels. A 16x20 gets a joint at 8 feet one way and at 10 feet the other, four panels of 8x10. A 20x20 at 4 inches gets two joints each way, nine panels a little under 7 feet square; one joint each way would give 10 foot panels, which the rule allows but which leaves little margin on a 4 inch slab. An L-shaped patio cracks from the inside corner first, so run a joint out from that corner.

Wire mesh, rebar and the chairs that make either one work

Welded wire mesh, the 6x6 grid sold as 6x6-10/10, is the usual patio reinforcement. It does not prevent cracks; it holds the cracks tight so they do not open or offset. It only does that if it is sitting in the middle of the slab, which means on chairs or dobie blocks at about half the slab depth, set before the pour. The habit of laying the mesh on the gravel and hooking it up during the pour reliably leaves most of it on the bottom, where it does nothing.

Rebar takes over where there is a load to carry: under the hot tub, the kitchen island, or the strip a car crosses. The common patterns on a residential patio are #3 bars at 18 inches or #4 bars at 24 inches each way, on chairs, with the laps and bar counts worked out by the rebar calculator. The rebar size chart has the diameters and weights per foot. Fiber in the mix, which the plant can add, reduces the fine plastic shrinkage cracking that shows up in the first day and is cheap; it is not a replacement for steel where the load is real.

Placing, floating, brooming and the week after

The truck backs up, the chute swings out, and from here on nothing waits. Place from the far corner toward the truck so nobody is walking through finished work. Pull the concrete to the top of the forms with a straight 2x4 screed in a sawing motion, keeping a small surplus ahead of the board, and fill the low spots as you go rather than coming back. If the mix is stiff, ask for a plasticizer in the next truck; do not have the driver add water, which weakens the whole slab for a few minutes of easier pushing.

Bull float immediately after screeding to push the stone down and bring paste up, then stop. Bleed water rises to the surface and sits there as a sheen; working the surface while it is there traps water under the paste and the top flakes off in a year. Wait until the sheen has gone and a thumb pressed on the slab leaves a quarter inch dent. Then run the edger around the forms, cut the control joints with the groover, smooth with a magnesium float, and finally pull a broom across the surface in one direction, perpendicular to the fall, for traction. A broom finish hides more than a trowel finish does, which on a first patio is a point in its favor.

Curing is the week after. Concrete gains strength by keeping its water, so the slab is either kept wet (a hose and a sheet of poly, or wet burlap) or sealed with a curing compound sprayed on as soon as the broom finish will take it, for about 7 days. Stay off it for 24 to 48 hours, keep furniture off for a week, and give it the full 28 days before the hot tub is filled. In hot wind the surface dries faster than it sets, so pour early; below about 40 degrees Fahrenheit it needs blankets, and the concrete calculator page covers the cold-weather limits.

Ready before the truck turns in

The driver’s clock starts on arrival, and a crew standing around looking for the groover pays for it. On the morning of the pour:

• Forms staked, braced at the joints, oiled, and checked once more with the string line for the fall.
• Base in place: 4 inches of compacted crushed stone, ordered through the gravel calculator as loose volume, with the over-dig that the 10 percent waste allowance in the tables is there to cover. The material waste factors guide is the reason that allowance sits on the concrete and not the gravel.
• Isolation board fixed to the house wall and around any posts.
• Mesh or rebar on chairs, tied, with nothing lying on the gravel.
• A route for the truck or a plywood path for wheelbarrows across the lawn. Chute reach is in the low to mid teens of feet and varies by truck; if the far corner is beyond it, you need wheelbarrows and a third person, or a pump booked in advance.
• Tools laid out: screed board, bull float with handles, edger, groover, magnesium float, broom, and a tarp in case of rain.
• A washout spot agreed with the driver, away from drains and planting.
• Three people, minimum, for anything 10x10 and up.

If any of that is not ready, the better call is to move the delivery, not to pour anyway. And if the whole sequence sounds like more than the patio is worth, pavers over the same compacted base are the alternative that can be laid over several weekends: the paver calculator sizes that job and pavers for a patio tables the counts by size.