Concrete Slab Calculator
Standard residential slabs are 4 inches thick. Driveways are typically 6 inches.
Multiply for multiple identical pours (e.g., 4 identical footings)
5% is standard. Use 10–15% for rough subgrades or complex shapes.
Enter your supplier's price per cubic yard
Enter Your Dimensions
Fill in the length, width, and thickness of your slab to see the concrete volume, bag counts, and cost estimate.
How to Use This Calculator
Choose Your Unit System and Shape
Select Imperial (feet and inches) or Metric (meters and centimeters) based on your project plans. Then select the shape: Slab/Patio for flat rectangular pours, Wall/Footing for vertical or below-grade rectangles, or Column/Pier for round forms.
Enter Your Dimensions
For slabs and walls, enter the length and width in feet (or meters) and the thickness in inches (or centimeters). For columns, enter the diameter in inches (or centimeters) and the height in feet (or meters). Use the quantity multiplier if you have multiple identical pours.
Set Your Waste Factor
Choose a waste overage percentage. Use 5% for smooth subgrades and simple shapes, 10% for uneven ground or outdoor slabs, and 15% for complex shapes or budget estimates. The results show both the exact and adjusted volumes side by side.
Review Volume, Bag Counts, and Cost
Check the volume in all three unit systems. Review the bag comparison table to find the most economical bag size for your project. Optionally enter your supplier's price per cubic yard or cubic meter to get an instant cost estimate. Export the results to CSV for your records.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab at 4 inches thick?
A 10×10 foot slab at 4 inches (0.333 feet) thick has a volume of 10 × 10 × 0.333 = 33.3 cubic feet, or about 1.23 cubic yards. With a recommended 10% waste buffer, you need approximately 1.36 cubic yards or 36.7 cubic feet. Using 80 lb bags (yield ≈ 0.60 ft³ each), that works out to about 62 bags. With 60 lb bags (yield ≈ 0.45 ft³), you would need around 82 bags. For a project this size, comparing the cost of bags against ordering one and a half yards of ready-mix concrete from a local supplier is worthwhile.
What is the standard thickness for a concrete driveway or patio?
Residential driveways intended for passenger cars should be at least 4 inches thick, with 6 inches recommended for heavier vehicles or areas with expansive soils. Standard residential patios are typically 4 inches thick, which is adequate for foot traffic and light furniture. Driveways expected to support trucks, RVs, or heavy equipment should be 6 to 8 inches thick. Foundation slabs for sheds, garages, and workshops are usually 4 inches, while attached garage slabs that support vehicles are commonly 6 inches. Always check local building codes, as minimum thickness requirements vary by jurisdiction.
Should I order more concrete than my calculation shows?
Yes — always order more than the bare calculated volume. The industry standard is to add 5–10% as a waste or overage factor. The extra volume accounts for uneven subgrades (the ground is rarely perfectly level), form flex under concrete pressure, spillage during placement, material left in chutes and mixing equipment, and rounding in bag yields. For a standard smooth subgrade pour, 5% is usually sufficient. For outdoor slabs on rough native soil, uneven sand beds, or complex curved forms, add 10–15%. Running short during a pour creates a cold joint — a structural weak point — so it is always better to have a small surplus.
What is the difference between cubic yards and cubic feet for concrete?
Cubic yards and cubic feet are both volume units used for concrete, but at different scales. One cubic yard equals 27 cubic feet (3 feet × 3 feet × 3 feet = 27). Ready-mix concrete is typically sold and quoted by the cubic yard in the United States. Premix bags list their yield in cubic feet. When comparing costs, always convert to the same unit. This calculator shows both simultaneously so you can work with whichever unit your supplier or bag packaging uses. Metric users work in cubic meters, where 1 cubic meter equals approximately 1.308 cubic yards or 35.31 cubic feet.
When should I use premix bags versus ordering ready-mix concrete?
The decision depends primarily on volume, but also on site access, timeline, and labor availability. For pours under 0.5 cubic yards (about 13 cubic feet), premix bags are almost always more practical and economical — you avoid delivery minimums and can work at your own pace. For 0.5 to 2 cubic yards, either option may work depending on whether you have a mixer, adequate labor, and access to a ready-mix truck. For pours over 2–3 cubic yards, ready-mix concrete is almost always the better choice: hand-mixing that volume is extremely labor-intensive, takes much longer, and produces inferior quality due to batch inconsistencies. Ready-mix also flows and consolidates better for large pours.
What is the magic number shortcut for 4-inch slabs?
A quick mental shortcut for estimating cubic yards of concrete in a 4-inch-thick slab is to divide the total square footage by 81. For example, a 20×20 foot area is 400 square feet, and 400 ÷ 81 ≈ 4.9 cubic yards. This shortcut is derived mathematically: 4 inches equals 1/3 foot, and one cubic yard equals 27 cubic feet. Dividing 27 by (1/3) gives 81. The shortcut only applies to exactly 4-inch slabs. For other thicknesses, use the full formula or simply enter your dimensions into this calculator. Note that this shortcut does not include a waste factor, so you should still add 5–10% to the result.