How much concrete for a 12x12 slab?

How the number is worked out

Volume is length × width × thickness, with the thickness converted from inches to feet first. A 10 ft × 10 ft slab at 4 in is 10 × 10 × 0.333 = 33.3 cubic feet, which is 1.23 cubic yards. Add 10% for waste and you are ordering 1.36 cubic yards.

Concrete is sold by the cubic yard, and there are 27 cubic feet in a cubic yard — this is where most hand calculations go wrong, because people divide by 3 instead of 27.

Bagged mix or ready-mix?

The crossover is sharper than most sites admit. Our 10×10 slab at 4 in works out at 62 × 80 lb bags once waste is included. That is about 5,000 lb of material to carry, open and mix, and at typical big-box pricing it costs more than a short load of ready-mix.

A useful rule: under about 0.5 cubic yards, bag it. Over about 1 cubic yard, call a truck. In between, decide on whether you can get a mixer and a helper.

Two things to check before you order ready-mix: most plants have a minimum load (often 1 cubic yard) and charge a short-load fee below 3–4 yards, and once the truck arrives you have a fixed window to place it. Bagged mix lets you work at your own pace, one section at a time.

What this does not include

• The gravel base. A slab normally sits on 4 in of compacted gravel. That is a separate order, and gravel compacts, so you need more than the raw volume.
• Rebar or mesh. Any slab that will carry weight needs reinforcement.
• Over-excavation. If the subgrade dips, the slab gets thicker there and the volume climbs quickly. On rough ground, use 15% waste rather than 10%.