How to Calculate Concrete for Your Slab
Planning a patio, driveway, or shed foundation requires accurate measurements to ensure you order enough concrete without overspending. This Concrete Slab Calculator helps you determine exactly how much material you need in cubic yards or pre-mix bags.
The Concrete Formula
To calculate the volume of concrete required for a slab, the basic formula is:
Volume (cu. ft.) = Length (ft) × Width (ft) × (Thickness (in) ÷ 12)
Since concrete is typically sold by the cubic yard, you must divide the total cubic feet by 27 (since there are 27 cubic feet in one cubic yard).
Why Include a Waste Factor?
It is industry standard to add a "waste margin" or spill factor to your calculation. We typically recommend adding 5% to 10% extra material. This accounts for:
- Spillage during the pour.
- Uneven subgrade (the ground isn't perfectly flat).
- Settling of the formwork.
Our calculator automatically applies the waste percentage you enter to ensure you don't run short in the middle of a job.
Thickness Guidelines
Not sure how thick your slab should be? Here are some common standards:
- 4 Inches: Standard for walkways, patios, and residential shed floors.
- 5-6 Inches: Recommended for driveways that hold passenger vehicles.
- 6+ Inches: Heavy equipment pads or RV parking.
Pre-Mix Bags vs. Ready-Mix Truck
If your project requires less than 1 cubic yard (about 45 bags of 80lb mix), it is often cheaper and easier to mix it yourself using bags. For anything over 2 cubic yards, ordering a ready-mix truck is usually more cost-effective and saves significant labor.