How Much Concrete Do You Need? The Complete Calculation Guide
Ordering too little concrete means a second trip and visible pour lines. Ordering too much wastes $150+ per extra yard. Here is how to calculate exactly what you need.
Concrete is measured in cubic yards. One cubic yard equals 27 cubic feet — roughly enough to fill a 3-foot cube. For a standard 4-inch thick patio slab, one cubic yard covers approximately 81 square feet. The formula is straightforward: Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet, then divide by 27 to get cubic yards.
The 10% Rule
Always order 10% more than your calculation shows. Subgrade is never perfectly flat, forms may bow slightly, and some concrete sticks to the chute and tools. On a 3-yard pour, that means ordering 3.3 yards. The $45-50 cost of that extra third of a yard is cheap insurance against coming up short. Coming up short means either a visible cold joint (a weak line where the two pours meet) or paying a $150+ short-load fee for a second delivery of half a yard.
Bags vs Ready-Mix Truck
An 80-lb bag of premixed concrete (Quikrete, Sakrete) makes 0.6 cubic feet. A 4×4-foot, 4-inch-thick pad requires about 9 bags. At $6-7 per bag, that is $54-63 in materials. For anything larger than about 30 bags (half a cubic yard), a ready-mix truck is faster, cheaper per yard, and produces a more consistent result. Most ready-mix companies have a 1-yard minimum and charge $130-160 per yard delivered.