Concrete Slab Calculator
Enter your slab's length, width, and thickness to find the volume in cubic yards (for ready-mix delivery), number of 60-lb bags needed, and cubic feet. Essential for driveways, patios, footings, and any concrete pour.
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Formula
Cubic Yards = (L × W × T_in ÷ 12) ÷ 27
Length × width gives the slab area in square feet. Dividing thickness in inches by 12 converts to feet. Multiplying gives cubic feet. Dividing by 27 (the number of cubic feet in a cubic yard) gives cubic yards, the standard unit for ready-mix concrete orders. Each 60-lb bag yields approximately 0.022 cubic yards (45 bags per cubic yard), so bags = cubic yards × 45, rounded up.
How to use the Concrete Slab Calculator
- 1
Enter your length (ft)
- 2
Enter your width (ft)
- 3
Enter your thickness (inches)
- 4
Read your results instantly
Results update in real time as you type.
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Standard Concrete Thickness Guidelines
4 inches is the standard residential driveway and patio thickness. Footings for load-bearing walls are typically 8-12 inches deep. Sidewalks are usually 3-4 inches. Garage slabs are 4-6 inches. The more load the slab must bear, the thicker it should be. Concrete also requires steel rebar or wire mesh reinforcement for most structural applications — consult a licensed contractor for load-bearing projects.
Ordering Ready-Mix vs. Bagged Concrete
Ready-mix concrete is delivered by the truck, with minimum orders typically around 1 cubic yard. For small projects under 0.5 cubic yards (about 22 bags), bagged concrete from a hardware store is more economical. For projects between 0.5 and 1 cubic yard, the minimum delivery charge for ready-mix may still be competitive. For anything above 1 cubic yard, ready-mix is almost always cheaper and faster. Always order 5-10% extra to account for waste, spillage, and uneven subgrade.
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Preparing the Subgrade
Before pouring, compact the subgrade (the soil beneath the slab) and add a 4-6 inch layer of compacted gravel for drainage. For driveways, install rebar on chairs 2 inches above the gravel. Cure concrete slowly by keeping it moist for at least 7 days. Avoid pouring in freezing temperatures without cold-weather protection measures. The finished slab reaches about 70% strength in 7 days and full design strength in 28 days.
Tips & Insights
Add 10% Extra to Your Order
Always order 5-10% more ready-mix than calculated. Uneven subgrade, spillage, and slight overpouring are inevitable. Running short before finishing is a major problem.
Mixing Bagged Concrete
A standard 60-lb bag requires about 0.5 gallons of water. Follow the bag instructions exactly — too much water weakens the final strength significantly.
Get Multiple Quotes for Ready-Mix
Ready-mix prices vary by region, season, and mix design. Get at least three quotes and confirm the delivery fee — some companies charge per trip regardless of load size.
Worked Examples
10×10 Patio, 4 Inches Thick
1.23 cubic yards, 56 bags, 33.3 cubic feet.
20×30 Driveway, 4 Inches
7.41 cubic yards, 334 bags — order ready-mix.
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Frequently Asked Questions
How many cubic feet are in a cubic yard?
27. A cubic yard is a 3×3×3 foot cube: 3 × 3 × 3 = 27 cubic feet.
How many 60-lb bags per cubic yard?
Approximately 45 bags. Each 60-lb bag yields about 0.45 cubic feet, and 27 ÷ 0.6 ≈ 45.
What thickness should a patio slab be?
4 inches is standard for patios and residential driveways. Areas with heavy vehicle traffic (RVs, trucks) should be 5-6 inches with rebar reinforcement.
Do I need rebar in a patio slab?
For pedestrian patios, wire mesh is usually sufficient. For driveways and any slab subject to vehicle loads, use #4 rebar on 18-inch grid spacing.
Can I pour concrete in cold weather?
You can, but temperatures below 40°F slow curing significantly. Use heated blankets or enclosures to keep fresh concrete above 50°F for at least 7 days after pouring.
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