Concrete Slab & Footing Calculator
0% (Exact)
5% (Recommended)
10% (Heavy Waste)
Volume Needed
0.00
Cubic Yards
(0.00 Cubic Feet)
80lb Bags (Pre-mix)
0
Bags needed
60lb Bags (Pre-mix)
0
Bags needed
How to Calculate Concrete for Your Project
Whether you are pouring a patio, a driveway, or footings for a deck, determining the correct amount of concrete is crucial. Ordering too little can result in a "cold joint" that weakens the structure, while ordering too much is a waste of money.
The Concrete Formula
To calculate the volume of concrete required for a slab, you need to determine the volume in cubic feet and then convert it to cubic yards (for truck orders) or number of bags (for DIY projects).
The basic formula is:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Note that thickness is usually measured in inches, so you must divide the thickness by 12 to convert it to feet before multiplying.
Understanding Concrete Density & Bags
For pre-mixed concrete sold in hardware stores:
- Standard Density: Approximately 133 lbs per cubic foot.
- 80lb Bag Yield: Roughly 0.60 cubic feet.
- 60lb Bag Yield: Roughly 0.45 cubic feet.
Why Add a Safety Margin?
It is industry standard to add a 5% to 10% safety margin (waste factor) to your calculation. This accounts for:
- Spillage during the pour.
- Uneven subgrade (ground) depth.
- Settling of the formwork.