Volume, yards, and bags

Concrete is sold by volume, not weight or area. Every estimate — every bag, every truck — comes back to one question: how many cubic feet of space are you filling?

The one formula

A slab is a box. Its volume is just:

length × width × thickness

The only trap is units. Length and width are usually in feet, but thickness is in inches. Convert the thickness to feet first (divide by 12), and the answer comes out in cubic feet.

Why yards, then?

Ready-mix concrete is ordered in cubic yards. One cubic yard is a block 3 ft on every side — that's 3 × 3 × 3 = 27 cubic feet. So to go from cubic feet to cubic yards, you divide by 27.

A 10 × 10 ft slab, 4 inches thick:

• 10 × 10 × (4 ÷ 12) = 33.3 cubic feet
• 33.3 ÷ 27 = 1.23 cubic yards

Where bag counts come from

Bagged concrete lists a yield — the cubic feet one bag fills. An 80 lb bag yields about 0.60 ft³. Divide your total cubic feet by the yield and round up. Our 33.3 ft³ slab needs ⌈33.3 ÷ 0.60⌉ = 56 bags.

That's the entire chain behind the number a calculator hands you. Now you can check it — and explain it to a client.