How to Estimate Concrete for a Patio (Without Running Short)
Running out of concrete in the middle of a pour is a contractor's worst nightmare. You can't just pause and run to the hardware store—the slab will dry, creating a weak "cold joint" that ruins the structural integrity and look of the patio.
The Cubic Yard Formula
Whether you are buying 80lb bags from Home Depot or calling a ready-mix truck, concrete is sold by volume. The industry standard measurement is the Cubic Yard (which is 27 cubic feet).
To find your cubic yardage, you need to calculate length, width, and depth. The tricky part? Depth is usually measured in inches, while length and width are measured in feet.
Formula: [Length (ft) × Width (ft) × (Depth in inches ÷ 12)] ÷ 27 = Cubic Yards
The 10% Rule of Subgrade
You must always order 10% more concrete than your math dictates. Why? Because the ground is never perfectly flat. A 4-inch deep form board might sit over a slight dip in the dirt that is actually 4.5 inches deep. That tiny half-inch deviation across a 20-foot patio will consume an extra half-yard of concrete instantly.
Bags vs. Ready-Mix Truck: The Tipping Point
A standard 80-pound bag of concrete yields exactly 0.60 cubic feet. It takes 45 bags to make a single cubic yard. Mixing 45 bags by hand or in a small drum mixer is brutal, back-breaking work.
- Under 1 Cubic Yard: Use 80lb bags. It's cost-effective and manageable for a DIYer.
- 1 to 1.5 Cubic Yards: You can use bags if you have a mechanical mixer and a couple of friends to help, but you are entering the danger zone for a single pour.
- Over 1.5 Cubic Yards: Call a ready-mix truck. The delivery fee is worth saving your back and ensuring a perfectly mixed, uniform slab.
Skip the manual math.
Calculate exactly how many bags or cubic yards of ready-mix you need, including your waste factor and custom pricing, using our free tool.
Open Concrete CalculatorDon't Forget the Footings
If your patio is going to support a heavy roof, a pergola, or a hot tub, a standard 4-inch slab isn't enough. You will need to dig thickened edges or concrete footings around the perimeter. Footings use a different formula entirely since they are usually cylindrical (like Sonotubes) or deep trenches.