How much concrete do I need for a 12x12 slab?

1.78 cubic yards at 4 inches thick and 2.67 at 6 inches, before waste. Ready-mix is ordered in quarter or half yards, so ask for 2 or 2.9 yards respectively. Twelve by twelve means feet here; a 12 x 12 m slab is a different job entirely.

How many bags of concrete do I need for a 12x12 slab?

88 bags of 80 lb or 118 bags of 60 lb at 4 inches, including 10% waste; 132 bags of 80 lb at 6 inches. That is well over a ton of dry mix, which is why most people order a truck for this size.

How thick should a 12x12 concrete slab be?

Four inches (100 mm) on a compacted gravel base is the usual thickness for a patio, shed base or walkway. A slab that will carry a car, a hot tub or a workshop with heavy machines is normally 5 to 6 inches with reinforcement, often with thickened edges. Your local building code and, for anything structural, an engineer set the final figure.

Should I order bags or a truck for a 12x12 slab?

A truck. 88 bags of 80 lb is a full day of mixing for two people, and the slab must be poured in one go. Ready-mix at 2 yards is a small order, so expect a short-load fee; it is still less work and a more consistent mix.

How much rebar or mesh do I need for a 12x12 slab?

Welded wire mesh comes in sheets and rolls; a 12 x 12 slab needs about 144 square feet plus overlap, so roughly 160 square feet of mesh. Rebar on an 18 inch grid takes about 9 bars each way, each 12 ft long, so around 18 bars of #3 or #4. Neither is included in the concrete volume.

How much concrete for a 12x12 slab? (plus 16x16, 20x20, 24x24)

A 12 x 12 ft slab (144 square feet) poured 4 inches thick holds 48 cubic feet, or 1.78 cubic yards, of concrete: order 2 cubic yards of ready-mix with 10% waste, or buy 88 bags of 80 lb. At 6 inches it is 2.67 cubic yards before waste. The tables take the same slab up to 24 x 24 ft and give the 4 x 4 m metric version.

Open the concrete calculator

Concrete for a 12x12 slab

The tables use a 10% waste allowance and the bag yields printed on typical bags: 0.60 cubic feet for 80 lb and 0.45 for 60 lb. Ready-mix volume is the figure with waste, rounded up to the next quarter yard by the plant.

Square slabs at 4 inches thick, 10% waste
Slab	Area (sq ft)	Volume (cu ft)	Volume (cu yd)	Order (cu yd)	80 lb bags	60 lb bags
12 x 12 ft	144	48	1.78	1.96	88	118
16 x 16 ft	256	85	3.16	3.48	157	209
20 x 20 ft	400	133	4.94	5.43	245	326
24 x 24 ft	576	192	7.11	7.82	352	470

Bigger slabs: 16x16, 20x20, 24x24 at 6 inches

Doubling the side of a slab quadruples the concrete. A 20 x 20 ft slab at 4 inches is already 5.4 cubic yards with waste, and at 6 inches for a garage floor or a driveway pad it is half as much again.

Square slabs at 6 inches thick, 10% waste
Slab	Area (sq ft)	Volume (cu ft)	Volume (cu yd)	Order (cu yd)	80 lb bags	60 lb bags
12 x 12 ft	144	72	2.67	2.93	132	176
16 x 16 ft	256	128	4.74	5.21	235	313
20 x 20 ft	400	200	7.41	8.15	367	489
24 x 24 ft	576	288	10.67	11.73	528	704

How the numbers are worked out

Volume is length x width x thickness in the same unit: 12 x 12 x 0.333 ft = 48 cubic feet. Divide by 27 for cubic yards, multiply by 1.10 for waste, then either round up to the quarter yard for a truck or divide by the bag yield and round up to whole bags. The concrete calculator does this for any rectangle, and the bags per cubic yard guide has every bag size side by side.

Bags or a ready-mix truck?

Below about one cubic yard, bags are the practical choice; above it, a truck. Every slab on this page is over a yard. Bag totals are shown because people ask, but 88 bags of 80 lb is more than 3.5 US tons of dry mix to carry, mix and place in a single session. A ready-mix order of 2 yards costs a short-load fee at most plants, but the mix arrives at a specified strength and the slab cures as one piece.

Metric: 4 x 4, 5 x 5 and 6 x 6 m slabs

A 12 x 12 ft slab is a little under 3.7 x 3.7 m, so 4 x 4 m is the nearest common metric size. Bags of 25 kg yield about 12 litres of concrete; for slabs this size a delivery is the usual answer and the volume with waste is what you order.

Square metric slabs, 10% waste
Slab	Thickness	Volume (m³)	Order (m³)	25 kg bags
4 x 4 m	100 mm	1.6	1.76	147
5 x 5 m	100 mm	2.5	2.75	230
6 x 6 m	100 mm	3.6	3.96	330
4 x 4 m	150 mm	2.4	2.64	220
5 x 5 m	150 mm	3.75	4.13	344
6 x 6 m	150 mm	5.4	5.94	495

Before you pour

The volume is concrete only. Allow separately for a 4 inch (100 mm) compacted gravel base, which the gravel calculator works out, plus a vapour barrier under indoor slabs and mesh or rebar.
Thickened edges (a deeper footing around the perimeter) add real volume: an edge 12 inches wide and 12 inches deep around a 12 x 12 slab is another 8 inches below the 4 inch slab along 48 ft of perimeter, about 32 cubic feet or 1.2 cubic yards on its own. Add it as a second slab in the calculator.
Measure the thickness in the middle of the formwork, not only at the edges; a base that dips in the centre swallows concrete.
Cut or form control joints at about 8 to 12 ft spacing on a 4 inch slab so it cracks where you want it to.
Smaller job? The 10x10 slab guide covers the sizes where bags still make sense.

Frequently asked questions

How much concrete do I need for a 12x12 slab?: 1.78 cubic yards at 4 inches thick and 2.67 at 6 inches, before waste. Ready-mix is ordered in quarter or half yards, so ask for 2 or 2.9 yards respectively. Twelve by twelve means feet here; a 12 x 12 m slab is a different job entirely.
How many bags of concrete do I need for a 12x12 slab?: 88 bags of 80 lb or 118 bags of 60 lb at 4 inches, including 10% waste; 132 bags of 80 lb at 6 inches. That is well over a ton of dry mix, which is why most people order a truck for this size.
How thick should a 12x12 concrete slab be?: Four inches (100 mm) on a compacted gravel base is the usual thickness for a patio, shed base or walkway. A slab that will carry a car, a hot tub or a workshop with heavy machines is normally 5 to 6 inches with reinforcement, often with thickened edges. Your local building code and, for anything structural, an engineer set the final figure.
Should I order bags or a truck for a 12x12 slab?: A truck. 88 bags of 80 lb is a full day of mixing for two people, and the slab must be poured in one go. Ready-mix at 2 yards is a small order, so expect a short-load fee; it is still less work and a more consistent mix.
How much rebar or mesh do I need for a 12x12 slab?: Welded wire mesh comes in sheets and rolls; a 12 x 12 slab needs about 144 square feet plus overlap, so roughly 160 square feet of mesh. Rebar on an 18 inch grid takes about 9 bars each way, each 12 ft long, so around 18 bars of #3 or #4. Neither is included in the concrete volume.

How many bags of concrete for a 10x10 slab?
Bags of 80, 60 and 40 lb concrete for a 10 x 10 ft slab at 4, 5 and 6 inches with 10% waste, plus the metric 3 x 3 m version in 25 kg and 20 kg bags.
How many bags of concrete per fence post?
Bags of 80, 60 and 50 lb concrete per 4x4 or 6x6 fence post for 8, 10 and 12 inch holes at 24, 30 and 36 inches deep, plus the metric 100 mm post in 25 kg bags.
How many bags of concrete in a cubic yard?
80, 60, 50 and 40 lb bags per cubic yard and per cubic foot, 25 kg and 20 kg bags per cubic metre, what a yard of bags weighs, and when a ready-mix truck makes sense.
How many bags of concrete for a sonotube? 8, 10, 12 and 16 inch
Bags of 80, 60 and 50 lb concrete for 8, 10, 12 and 16 inch sonotubes at 2, 3 and 4 ft, concrete per foot of tube, and 200 to 300 mm forms in 25 kg bags.

Last updated 2026-09-12. Figures are planning estimates from typical product data: check the yield, size or coverage printed on what you buy and round up.