How to Calculate Concrete for a Patio Slab
Pouring a concrete patio, shed pad, or walkway is one of the most rewarding DIY home improvement projects. However, ordering concrete requires calculating volume in cubic yards. Underestimating leaves you short mid-pour with setting concrete, while overestimating wastes money on extra ready-mix delivery fees.
Calculating concrete volume requires multiplying length, width, and depth in feet, then converting cubic feet into cubic yards.
Open Concrete Calculator
The Core Concrete Formula
Convert slab thickness from inches to feet by dividing by 12. Multiply length by width by depth (in feet) to find total cubic feet. Finally, divide cubic feet by 27 (since 1 cubic yard = 27 cubic feet).
Worked Example of a 12x16 Foot Patio Slab
Suppose you are pouring a 12 ft by 16 ft patio slab at a standard 4-inch depth (0.333 feet):
- Volume in Cubic Feet: 12 ft x 16 ft x 0.333 ft = 64 cubic feet
- Volume in Cubic Yards: 64 / 27 = 2.37 cubic yards
- Adding 10% Waste Buffer: 2.37 x 1.10 = 2.61 cubic yards
How Many 80lb Bags of Concrete Do You Need?
If mixing pre-bagged concrete, one standard 80lb bag yields roughly 0.60 cubic feet of wet concrete. For 64 cubic feet plus 10% waste (70.4 cu ft total): 70.4 / 0.60 = 118 bags.
Calculate bag counts and ready-mix costs for any project with our free Concrete Calculator.
