Concrete Slab Calculator
Standard slab is 4 inches
0% (Exact)
5% (Tight)
10% (Recommended)
15% (Safe)
Avg price is $115 – $150 per yard delivered
Total Volume Needed:
0 cu. yards
Volume in Cubic Feet:
0 cu. ft
Estimated Cost (Truck):
$0.00
Pre-Mix Bag Alternatives:
80lb Bags (Quikrete/Sakrete):
0 bags
60lb Bags:
0 bags
How to Calculate Concrete for a Slab
Whether you are pouring a patio, a driveway, or a foundation for a shed, determining the exact amount of concrete required is crucial. Ordering too little can result in a "cold joint" and structural weakness, while ordering too much is a waste of money. This calculator helps you determine the volume in cubic yards and estimated costs, including a safety margin for spillage and uneven subgrades.
The Concrete Calculation Formula
Concrete is sold by volume, typically in cubic yards. To calculate the volume of a rectangular slab manually, you need to find the cubic footage and divide by 27 (since there are 27 cubic feet in one cubic yard).
Step 1: Convert dimensions to feet
Ensure your length and width are in feet. If your depth is in inches, divide it by 12 to convert it to feet.
Step 2: Calculate Volume (Cubic Feet)
$$ \text{Volume (ft}^3) = \text{Length (ft)} \times \text{Width (ft)} \times \text{Depth (ft)} $$
Step 3: Convert to Cubic Yards
$$ \text{Cubic Yards} = \frac{\text{Volume (ft}^3)}{27} $$
Why Include a Waste Margin?
Professional contractors rarely order the exact mathematical amount. It is industry standard to add 5% to 10% extra material. This accounts for:
- Spillage during the pour.
- Uneven excavation (a hole that is 4.5″ deep instead of 4″ consumes significantly more concrete).
- Concrete remaining in the truck or mixer.
- Settling of the subbase.
Bags vs. Ready-Mix Truck
Should you buy bags from the hardware store or order a truck?
- Pre-Mix Bags (60lb or 80lb): Best for small jobs under 1 cubic yard (approx 45 bags). Ideal for setting posts, small pads, or repairs. An 80lb bag typically yields about 0.6 cubic feet of cured concrete.
- Ready-Mix Truck: Best for jobs over 1 cubic yard. It guarantees a consistent mix and saves immense physical labor. However, ready-mix companies often charge "short load fees" for orders under 3-5 yards.
Common Slab Thicknesses
- 4 Inches: Standard for sidewalks, patios, and residential driveways (light vehicle traffic).
- 5-6 Inches: Recommended for heavy duty driveways (RVs, trucks) or hot tub pads.
- 6+ Inches: Heavy industrial use or commercial foundations.