Concrete Slab Calculator
Comprehensive Guide to Calculating Concrete for Slabs
Whether you are pouring a new patio, a driveway, or a shed foundation, calculating the exact amount of concrete required is the most critical step in the planning process. Ordering too little results in expensive "short load" fees or cold joints, while ordering too much leads to waste disposal issues. This guide will help you understand the math behind the concrete slab calculator.
How to Calculate Concrete Volume
Concrete is sold by volume, typically in Cubic Yards (often just called "yards"). To find this volume manually, you must calculate the cubic footage of your slab and convert it.
The Formula
The standard formula for a rectangular slab is:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Note: Since thickness is usually measured in inches, you must divide the inches by 12 to get feet.
Example: For a 10′ x 10′ patio that is 4 inches thick:
- Convert thickness: 4″ ÷ 12 = 0.33 feet
- Calculate Volume: 10 × 10 × 0.33 = 33 Cubic Feet
- Convert to Yards: 33 ÷ 27 = 1.22 Cubic Yards
Estimating Pre-Mix Bags (60lb vs 80lb)
For smaller DIY projects, you won't order a ready-mix truck; you will buy bags of pre-mix concrete (like Quikrete or Sakrete) from a home improvement store. To calculate the number of bags needed:
- Determine Yield: An 80lb bag typically yields about 0.6 cubic feet of cured concrete. A 60lb bag yields about 0.45 cubic feet.
- Divide Total Volume: Divide your total required cubic footage by the bag yield.
Our calculator automates this using standard density assumptions of approximately 133-150 lbs per cubic foot for cured concrete.
Recommended Thickness Guide
- 4 Inches: Standard for sidewalks, patios, and non-load-bearing shed bases.
- 5-6 Inches: Recommended for residential driveways hosting passenger vehicles.
- 6+ Inches: Heavy equipment pads or commercial aprons.
Why Add a Waste Margin?
We typically recommend adding a 5% to 10% safety margin (waste factor) to your calculation. This accounts for:
- Spillage during the pour.
- Uneven subgrade (dips in the ground) that require more concrete to fill.
- Concrete remaining inside the mixer or wheelbarrow.