How to Calculate Concrete for a 20x20 Patio (Free AI Tool)

Concrete is ordered by volume, so every patio estimate starts with three measurements in the same unit: length, width and slab thickness. Thickness is usually quoted in inches, so convert it to feet first by dividing by 12. A 4 in slab becomes 0.333 ft; a 6 in slab becomes 0.5 ft.

Multiply the three numbers to get cubic feet: 20 ft × 20 ft × 0.333 ft = 133.3 ft³. Ready-mix is sold by the cubic yard, and one cubic yard holds 27 cubic feet, so divide: 133.3 ÷ 27 = 4.94 cubic yards. At 6 inches the same patio works out to 20 × 20 × 0.5 = 200 ft³, or 7.41 cubic yards.

Now add waste. Sub-grade is never perfectly flat, forms bow slightly under the weight of wet concrete, and a little is always left in the chute. Most contractors add 5% on a well-prepared base and 10% on rough ground. Applying 10% to the 4 in patio gives 5.43 yards, which you would order as 5.5 yards because most suppliers sell in quarter- or half-yard increments.

If you are mixing from bags instead of ordering ready-mix, work in cubic feet. An 80 lb bag of concrete mix yields about 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 40 lb bag about 0.30 ft³. The 4 in patio needs 133.3 ÷ 0.60 = 223 eighty-pound bags — well past the point where a ready-mix truck is cheaper and far easier on your back. As a rule of thumb, mix from bags below about 1 cubic yard (45 bags) and order ready-mix above it.

Two last checks before you order: confirm the excavation depth includes any compacted gravel base (typically 4 in of base under a 4 in slab), and decide on reinforcement. Most residential patios use 6×6 welded wire mesh or #3 rebar at 18 in centers, which does not change the concrete volume but does change the total material cost.