How Much Concrete Do I Need?

Concrete is ordered by volume, and the math is simple: length × width × thickness. The mistakes happen in the units and in the allowance for waste.

Convert everything to feet first

Thickness is usually given in inches. Divide by 12 before multiplying: a 4 in slab is 0.333 ft thick. A 10 × 10 ft slab at 4 in is 10 × 10 × 0.333 = 33.3 cubic feet.

Ready-mix is sold by the cubic yard, which is 27 cubic feet. 33.3 ÷ 27 = 1.23 cubic yards.

Add a waste allowance

Subgrade is never perfectly flat, forms bow slightly, and some concrete stays in the truck or wheelbarrow. Most contractors add 5–10%. Running short in the middle of a pour creates a cold joint — a weak seam — so erring slightly high is cheaper than the alternative.

Bags or ready-mix?

An 80 lb bag of pre-mixed concrete yields about 0.6 cubic feet, so a single cubic yard needs about 45 bags. Beyond roughly one cubic yard, mixing by hand becomes slow and the first batches start to set before the last are placed. Short-load ready-mix or a rental mixer is usually the better choice.

What this does not decide

Thickness, reinforcement, base preparation and control joints depend on the load and local practice. Driveways generally need more than patios. Check with your building department for anything structural.