How to Calculate Concrete for Slabs
Whether you are pouring a patio, a driveway, or a shed foundation, calculating the correct amount of concrete is the first step to a successful project. Ordering too little results in expensive second deliveries and cold joints, while ordering too much is a waste of money.
This Concrete Slab Calculator helps you determine exactly how much premix cement (in 80lb or 60lb bags) or ready-mix concrete (in cubic yards) you need for your project.
The Concrete Formula
The math behind calculating concrete volume is based on measuring the Length, Width, and Thickness of your slab. Since concrete is sold by volume, we must convert all measurements to a standard unit (usually cubic feet or cubic yards).
The basic formula is:
- Volume (cu ft) = Length (ft) × Width (ft) × (Thickness (in) ÷ 12)
- Volume (cu yds) = Volume (cu ft) ÷ 27
Understanding Premix Bag Yields
If you are mixing the concrete yourself using bags from a hardware store, you need to know the "yield" of each bag. The yield is the volume of wet concrete produced after mixing with water.
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
Our calculator uses these standard yield rates to estimate the number of bags required.
Why Add a Waste Margin?
It is standard industry practice to order 10% more concrete than your mathematical calculation. This accounts for:
- Spillage during transport and pouring.
- Uneven subgrade (dips in the ground) that increase the slab depth.
- Settling of the formwork.
Selecting "10% Waste" in the calculator ensures you won't run out of material mid-pour.
Frequently Asked Questions
How thick should my concrete slab be?
For a standard concrete patio or walkway, a thickness of 4 inches is standard. For a driveway that needs to support heavy vehicles, a thickness of 5 to 6 inches is recommended. Always check local building codes for specific requirements.
How many 80lb bags of concrete make a yard?
One cubic yard equals 27 cubic feet. Since an 80lb bag yields about 0.60 cubic feet, you need approximately 45 bags of 80lb concrete to make one cubic yard.
Can I pour a slab in sections?
Yes, but you must use expansion joints or control joints. If you stop pouring and let the concrete harden before continuing, you create a "cold joint," which is a weak point in the slab. It is best to calculate correctly and pour the entire section at once.