Concrete Slab Calculator
Enter your slab dimensions and thickness to get cubic yards for ready-mix delivery or bag count for smaller projects — with a 10% waste factor included.
1Slab Dimensions
Standard thicknesses: 4 inches for patios and walkways. 6 inches for driveways and areas supporting vehicle weight. 4-6 inches for shed floors. Always check local building codes for your specific project.
Enter the additional area. Same thickness as above is assumed.
2Concrete Settings
Ready-mix is more economical above roughly 1 cubic yard
Your Concrete Estimate
Buying Recommendation
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Ready-Mix Note
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Tip: Always add 10% to your concrete order. Running short mid-pour forces a cold joint — a visible seam where fresh concrete meets partially set concrete — which weakens the slab. It is already included in this estimate.
How to Use This Calculator
Using this concrete slab calculator ensures you buy the exact right amount of material to pour a solid, crack-free surface. Follow these five steps:
- 1Measure slab dimensions. Measure the planned length and width of your concrete pour in feet and inches.
- 2Select slab thickness. Use the presets to select thickness. According to American Concrete Institute (ACI) guidelines, 4 inches is the standard thickness for patios and walkways, while 6 inches is required for driveways that must support vehicle weight.
- 3Add a second area (optional). If you are pouring an L-shaped patio, simply toggle this option on and enter the dimensions of the second rectangular section into the concrete slab calculator.
- 4Select buying method. Choose whether you want to see the estimate in bags or cubic yards. As a general rule, paying for a ready-mix truck delivery becomes much more economical than buying and mixing individual bags when your project exceeds roughly 1 cubic yard.
- 5Select bag size. If you are mixing by hand, select your bag size. The heavy 80 lb bags are the most efficient and cost-effective choice for larger pours.
The Formula
This concrete slab calculator uses standard volume geometry. Here is the math in plain English:
Volume in Cubic Feet = Length × Width × (Thickness in inches / 12)
Cubic Yards = Cubic Feet / 27
Bags Needed = Total Cubic Feet / Bag Yield (rounded up)
First, the volume in cubic feet is calculated by multiplying the length by the width, and then multiplying that by the thickness in feet (which is the thickness in inches divided by 12). To convert cubic feet into cubic yards for a ready-mix truck order, divide the cubic feet by 27.
Crucially, the calculator automatically adds 10% to the total volume. This 10% waste buffer accounts for spillage, material left in the mixer, and inevitable subgrade irregularities (because the bottom of your excavated hole is never perfectly flat). Finally, the total cubic feet is divided by the specific yield of your chosen bag size. According to Quikrete product data, an 80 lb bag yields 0.60 cubic feet, a 60 lb bag yields 0.45 cubic feet, and a 50 lb bag yields 0.375 cubic feet.
Frequently Asked Questions
How thick does a concrete slab need to be?
For foot traffic applications like patios and standard walkways, 4 inches is the industry standard thickness. For residential driveways, you must pour a 6-inch slab to safely support the heavy, concentrated weight of a vehicle. For a standard backyard storage shed, 4 to 6 inches is sufficient depending on the size of the shed.
Should I use rebar or wire mesh in a concrete slab?
Yes, you should always use reinforcement. Welded wire mesh helps prevent the concrete from cracking as it shrinks during the curing process. Rebar (steel reinforcing bar) adds massive tensile strength to the slab, allowing it to bridge minor soil settling without snapping. For driveways, a grid of half-inch rebar is highly recommended.
How long does concrete take to cure before I can walk or drive on it?
You can typically walk gently on a new concrete slab after 24 to 48 hours. However, you should not drive a vehicle onto a new driveway for at least 7 to 10 days. Concrete takes a full 28 days to reach its maximum compressive strength.
What is the difference between ready-mix and bagged concrete?
Bagged concrete is a dry powder you buy at a hardware store and mix with water yourself; it is ideal for small walkways or fence posts. Ready-mix concrete is pre-mixed with water at a commercial plant and delivered wet in a spinning mixer truck. Using this concrete slab calculator, you will quickly see that ready-mix is the only viable option for large patios or driveways, as mixing 100 individual bags by hand is too slow and physically exhausting.
How do I prevent cracks in a concrete slab?
All concrete cracks eventually. The key is controlling where it cracks. You must cut or tool "control joints" into the wet concrete every 8 to 10 feet. These straight grooves create a deliberate weak point, forcing the concrete to crack neatly down the invisible bottom of the groove rather than spiderwebbing across the surface of your patio.
Do I need a permit for a concrete patio or driveway?
Most local municipalities require a building permit for any structural driveway or any patio that sits higher than 30 inches off the ground. Small, ground-level backyard patios often do not require a permit, but you should always call your local building department to verify before pouring.