How Much Concrete Do I Need? Slabs & Footings Guide
Concrete is one of the most permanent building materials, which means estimating the correct volume is crucial. Ordering too little results in cold joints—where a new pour meets already dried concrete—ruining structural integrity. Ordering too much leaves you with expensive leftover material and disposal headaches.
The Core Math: Calculating Cubic Yards
- Convert all dimensions to feet (e.g., 4 inches thickness becomes 4/12 = 0.33 feet).
- Multiply: Length (ft) × Width (ft) × Thickness (ft) = Volume in Cubic Feet.
- Divide by 27: Divide the cubic feet result by 27 to convert it to Cubic Yards, which is the standard commercial order unit.
| Project Dimensions | Cubic Feet | Cubic Yards (Raw) | With 10% Waste |
|---|---|---|---|
| 10 × 10 ft @ 4 in | 33.3 | 1.23 | 1.35 |
| 12 × 12 ft @ 4 in | 48.0 | 1.78 | 1.96 |
| 20 × 10 ft @ 4 in | 66.7 | 2.47 | 2.72 |
| 20 × 20 ft @ 4 in | 133.3 | 4.94 | 5.43 |
💡 Pro Tip: For post holes (like fence or deck posts), calculate the cylinder volume: π × radius² × depth. For a standard 10-inch diameter hole dug 24 inches deep, you will need approximately 1.1 cubic feet (or about two 60lb bags) of concrete per post.
Frequently Asked Questions
How many 80lb bags of concrete make a cubic yard?
It takes exactly 45 bags of standard 80lb dry concrete pre-mix to make a single cubic yard (27 cubic feet) of wet concrete.
What is the standard thickness for a concrete patio slab?
A depth of 4 inches is the residential building standard for patios, walkways, and garden sheds. Driveways for heavy vehicles require 6 inches.
Why should I add a waste factor to concrete orders?
Formwork can bow under wet concrete weight, and excavation bottoms are rarely perfectly level. A 10% waste buffer prevents running short.