Concrete Slab Calculator
Please enter valid positive numbers for dimensions.
Material Requirements
Total Volume (Cubic Yards):
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Total Volume (Cubic Feet):
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Premix Bag Estimate (Pre-mixed Concrete):
80lb Bags Needed:
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60lb Bags Needed:
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How to Calculate Concrete for a Slab
Planning a patio, driveway, or shed foundation requires precise calculations to ensure you order enough concrete without excessive waste. This Concrete Slab Calculator helps determine the exact volume of concrete needed in cubic yards and cubic feet, as well as the number of premix bags required for smaller DIY projects.
The Concrete Calculation Formula
To calculate the concrete volume for a rectangular slab, you use the standard volume formula: Length × Width × Thickness. However, since concrete is sold by the cubic yard, unit conversion is necessary.
Here is the step-by-step logic used in this calculator:
- Convert Thickness: Convert the thickness from inches to feet by dividing by 12. For example, a 4-inch slab is 0.33 feet thick.
- Calculate Cubic Feet: Multiply Length (ft) × Width (ft) × Thickness (ft).
- Convert to Cubic Yards: Divide total cubic feet by 27 (since there are 27 cubic feet in one cubic yard).
- Add Safety Margin: It is standard industry practice to add 5-10% for spillage, uneven subgrade, and waste.
Example Calculation
Let's say you are pouring a patio that is 12 feet long and 10 feet wide with a standard thickness of 4 inches.
- Area: 12 × 10 = 120 sq. ft.
- Volume: 120 × (4/12) = 40 cubic feet.
- In Yards: 40 / 27 = 1.48 cubic yards.
- With 5% Waste: 1.48 × 1.05 = 1.55 cubic yards.
Standard Slab Thicknesses
Choosing the right thickness is critical for the durability of your project.
| Application | Recommended Thickness |
|---|---|
| Sidewalks & Garden Paths | 4 Inches |
| Residential Driveways (Standard Vehicles) | 4 – 5 Inches |
| Driveways (Heavy Trucks/RVs) | 6 Inches |
| Garage Floors | 4 – 6 Inches |
Premix Bags: 80lb vs 60lb
For smaller projects (typically under 1 cubic yard), buying premixed bags from a hardware store is often more economical than ordering a ready-mix truck. The calculator estimates bags based on standard yields:
- An 80lb bag typically yields approximately 0.60 cubic feet.
- A 60lb bag typically yields approximately 0.45 cubic feet.
Always round up to the nearest whole bag to ensure you don't run out mid-pour.