How to Calculate Concrete Volume
Concrete is sold and billed in the United States by the cubic yard (27 cubic feet). To calculate how much ready-mix concrete you need for a rectangular slab, patio, or sidewalk:
Cubic Feet = Length (ft) × Width (ft) × (Thickness in inches ÷ 12)
Cubic Yards = Cubic Feet ÷ 27
Order Total = Cubic Yards × (1 + Overage Margin)
Cubic Yards = Cubic Feet ÷ 27
Order Total = Cubic Yards × (1 + Overage Margin)
Why Contractors Always Add 10% Overage
Ready-mix dispatchers advise adding at least 10% to your measured volume. Excavations and subgrades are rarely laser-flat; settling of gravel, slight form deflection under the weight of wet concrete, and spillage will quickly leave you short. Running "short by a wheelbarrow" results in expensive short-load delivery fees or an ugly cold joint.
Ready-Mix Truck vs. Bagged Concrete
| Job Size (Volume) | Recommended Source | Bag Equivalent (80-lb) |
|---|---|---|
| Less than 0.5 cu yd (under 13.5 cu ft) | Pre-mix bags (DIY / small mixer) | 1 to 22 bags |
| 0.5 to 1.5 cu yd | Pre-mix bags or U-cart trailer | 23 to 67 bags |
| Greater than 1.5 cu yds | Ready-Mix concrete truck delivery | 68+ bags (order ready-mix truck) |
Frequently Asked Questions
How many square feet does 1 cubic yard of concrete cover?
At 4 inches thick, 1 cubic yard covers 81 square feet. At 6 inches thick, 1 cubic yard covers 54 square feet.
What is the minimum thickness for a concrete driveway?
A minimum of 5 inches is recommended for passenger cars, and 6 inches with #4 rebar grid is required if large pickup trucks, RVs, or delivery vans park on the surface.