Concrete for a 12x12 Slab

A 12x12 ft concrete slab poured 4 inches thick needs about 1.78 cubic yards (48 ft³) of concrete — roughly 80 bags of 80 lb mix. Adjust the thickness below for footings, patios, or driveways.

LengthWidth

Thickness4"

Waste %%

You need approximately

1.78 cubic yards

48 ft³ of concrete

With 5% waste: 1.87 cubic yards (recommended when ordering)

40 lb bags: 160 bags
50 lb bags: 128 bags
60 lb bags: 107 bags
80 lb bags: 80 bags

Enter your exact thickness — the default 4 inches suits patios and shed floors, while driveways often use 5–6 inches. The tool returns the volume plus bag counts for 40, 50, 60, and 80 lb mixes.

Why order extra? The headline number is the exact material your project needs. When you actually order, add about 5–10% on top — the “with 5% waste” figure in the result is a safe starting point. That margin covers what you lose to compaction, cuts around edges and corners, and ordinary spillage during handling. It is cheaper to have a little left over than to stop mid-job and wait on a second delivery.

Field notes

Past about 50 bags, stop bagging and order ready-mix — hand-mixing that volume risks cold joints between batches and eats a whole day. Order 5–10% over; you can't pause a pour to run for more, and a short load means a visible seam. Dampen the subgrade and set your forms dead level first, because concrete finds every low spot and won't forgive a sloppy base.

How the concrete for a 12x12 slab works

The math behind the tool:

cubic yards = (length ft × width ft × thickness ft) ÷ 27
80 lb bags = cubic feet ÷ 0.6

Material density

Material	Density	Per cubic yard
Cured concrete	150 lb/ft³	2.02 tons

Sources: Portland Cement Association (concrete density and mix guidance).

Frequently asked questions

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

A 12x12 ft slab at 4 inches thick needs about 1.78 cubic yards of concrete, or roughly 80 bags of 80 lb mix. At this size, ready-mix delivery is usually cheaper than bags.

How many cubic yards of concrete do I need?

Multiply length × width × thickness in feet and divide by 27. A 10 × 10 ft slab at 4 inches thick needs about 1.23 cubic yards of concrete.

How many 80 lb bags of concrete are in a cubic yard?

It takes about 45 bags of 80 lb concrete mix to make one cubic yard, since each 80 lb bag yields roughly 0.6 cubic feet.

How much concrete for a 10x10 slab?

A 10×10 ft slab poured 4 inches thick needs about 1.23 cubic yards (33.3 cubic feet) of concrete — roughly 56 bags of 80 lb mix, though ready-mix delivery is usually cheaper at this size.