How much concrete for a 12 x 20 slab?

Short answer

A 12 x 20 ft slab is 240 square feet. At 4 inches thick it needs 2.96 cubic yards (80 cubic feet) of concrete.
With 10% extra that's 3.26 cubic yards, so you'd order 3.5 cubic yards of ready-mix (plants sell in quarter-yard steps). By the bag it's 147 bags of 80 lb.
It's typically a patio or a single parking pad.

Concrete for a 12 x 20 slab at 4, 5 and 6 inches

Bag counts include 10% extra for uneven ground and spillage. Cubic yards are shown without extra so you can compare quotes.

Thickness	Cubic yards	80 lb bags	60 lb bags	40 lb bags	Weight with extra
4 in	2.96	147	196	294	6.2 to 6.6 tons
5 in	3.7	184	245	367	7.7 to 8.3 tons
6 in	4.44	220	294	440	9.2 to 9.9 tons

Open this slab in the concrete calculator to change the size, thickness, waste or bag size.

How the math works

12 ft × 20 ft × 4 in ÷ 12 = 80 cubic feet. Divide by 27 to get 2.96 cubic yards. Add 10% and you have 88 cubic feet. An 80 lb bag of premixed concrete yields about 0.60 cubic feet, so 88 ÷ 0.60 rounds up to 147 bags.

Bags or ready-mix?

At 2.96 cubic yards, a 4 inch 12 x 20 slab means mixing 147 bags by hand. Price a ready-mix load too, and ask about the minimum order and short-load fees before you decide.

Forms and gravel base

The slab's edge is 64 feet, so that's how much form board you need around it, plus stakes. Under it, Quikrete's patio steps call for 5 inches of loose gravel tamped down to 4. For a 12 x 20 slab that's 100 cubic feet of loose gravel, about 3.7 cubic yards or 5.3 tons at 105 lb per cubic foot. The slab planner adds rebar or mesh and totals the cost with your local prices.

Other slab sizes

Sources

See how we calculate.