Calculating the right amount of concrete for a slab prevents costly overorders and frustrating shortfalls mid-pour. The formula is straightforward but easy to get wrong if you skip the unit conversions.
The Concrete Slab Formula
Volume (cubic feet) = Length × Width × (Thickness ÷ 12). Then divide by 27 to convert to cubic yards, since concrete is sold by the yard.
Example: A 20×20 ft patio at 4" thick = 20 × 20 × (4/12) = 133.3 cu ft ÷ 27 = 4.94 cubic yards.
When to Add a Waste Factor
Always add 5–10% overage to account for spillage, subgrade irregularities, and the concrete that remains in the chute. Running short mid-pour is a serious problem — cold joints are structural weaknesses.
Ready-Mix vs. Bags
Ready-mix trucks are cost-effective for pours over 1 cubic yard
An 80 lb bag yields approximately 0.60 cubic feet of concrete
A 60 lb bag yields approximately 0.45 cubic feet of concrete
For small jobs under 1 yard, bags are usually more practical
Most trucks have a minimum delivery of 1 yard with short-load fees