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Concrete Slab Calculator — How Much Concrete for a Slab

Enter your slab dimensions to instantly calculate cubic yards, cubic feet, and the number of bags needed. Free, no signup required.

Concrete Slab Calculator

Enter your slab dimensions to calculate cubic yards and bags needed

Enter dimensions above to see your estimate

How to Calculate Concrete for a Slab

Calculating the volume of concrete for a slab uses a straightforward formula: multiply the length by the width by the thickness, all in feet. The result is the volume in cubic feet. Divide by 27 to convert to cubic yards, which is how ready-mix concrete is sold and priced. For example, a 12-foot by 12-foot slab at 4 inches (0.333 feet) thick works out to 48 cubic feet, or 1.78 cubic yards.

The calculator above handles all unit conversions automatically. You can enter each dimension in feet or inches independently, so you do not need to manually convert thickness from inches to feet before calculating. Enter your measurements, read your results, and order with confidence.

Slab Thickness Recommendations

Selecting the correct slab thickness is one of the most important decisions in any concrete project. Too thin and the slab will crack under load; too thick and you waste material and money. Here are the standard thickness guidelines for common applications:

  • Sidewalks and walkways: 4 inches is standard for pedestrian foot traffic.
  • Residential driveways: 4 to 6 inches depending on vehicle type and frequency of use.
  • Garage floors: 4 to 6 inches; use 6 inches when heavy vehicles or trucks will be parked.
  • Basement floor slabs: 3.5 to 4 inches is typical for non-structural basement slabs.
  • Heavy equipment pads: 6 to 8 inches or more depending on the load rating required.

Standard Slab Sizes and Concrete Estimates

To give you a sense of concrete volumes at common slab sizes, here are estimates for a 4-inch-thick slab:

  • 10 x 10 ft slab: approximately 1.23 cubic yards (56 bags of 80 lb concrete)
  • 12 x 12 ft slab: approximately 1.78 cubic yards (81 bags of 80 lb concrete)
  • 20 x 20 ft slab: approximately 4.94 cubic yards (order ready-mix)
  • 10 x 20 ft slab: approximately 2.47 cubic yards (order ready-mix)

Any pour over one cubic yard is best handled with ready-mix concrete delivered by truck. Mixing that volume by hand or with a small mixer is extremely labor-intensive and increases the risk of inconsistent mix ratios and cold joints.

When to Use Ready-Mix vs. Bags

The one-cubic-yard mark is the practical break-even point between bagged concrete and ready-mix. Below one cubic yard, bags offer flexibility: you can mix only what you need, work at your own pace, and avoid minimum-order charges from ready-mix plants. Above one cubic yard, ready-mix is almost always the better choice in terms of cost, quality, and labor.

Ready-mix arrives at a consistent, tested mix design. The driver will discharge the concrete directly into your forms if access allows, saving hours of mixing time. Bagging a 10 x 10 foot slab at 4 inches, for example, would require mixing more than 56 bags of 80 lb concrete by hand. That is over 4,400 pounds of material to open, mix, pour, and screed in a single session before the concrete begins to set.

How to Prepare Your Subgrade

A well-prepared subgrade is just as important as the concrete itself. Start by removing all organic material, topsoil, and soft spots from the pour area. The subgrade should be firm and uniformly compacted. For slabs poured directly on grade, a 4-inch layer of compacted gravel or crushed stone provides excellent drainage and a stable base.

In areas with high moisture or where the slab will be used for a living space, install a 6-mil polyethylene vapor barrier directly beneath the slab. This prevents ground moisture from migrating up through the concrete. Place rebar or wire mesh on top of the vapor barrier before pouring, supporting it on chairs or dobies so it sits in the lower third of the slab cross-section where it will be most effective at controlling tension cracks.

Adding a Waste Factor

Always add at least 10 percent to your calculated concrete volume before placing your order. Subgrades are rarely perfectly flat, which means your forms will consume more concrete than the idealized volume suggests. Spillage during pouring, slight variations in form dimensions, and the inevitable need to slightly overfill before screeding all contribute to real-world material usage exceeding the calculated number.

Running out of concrete mid-pour is one of the most costly mistakes you can make. When fresh concrete is poured against concrete that has begun to set, it creates a cold joint, a structural weakness that can allow water infiltration and lead to cracking and delamination over time. Ordering 10 percent extra is cheap insurance against this outcome. The calculator above shows bag quantities with and without the 10% waste buffer so you can plan accordingly.

Frequently Asked Questions

How thick should a concrete slab be?
Most residential concrete slabs are poured at 4 inches thick. Sidewalks and patios intended for foot traffic only are fine at 4 inches. Driveways that will support passenger vehicles should be 4 to 6 inches. Garage floors are typically 4 to 6 inches, with 6 inches recommended if heavy trucks or SUVs will be parked. Basement floor slabs are usually 3.5 to 4 inches. Always consult local building codes for the specific requirements in your area.
How many bags of concrete do I need for a 10x10 slab?
A 10 x 10 foot slab poured at 4 inches thick requires approximately 1.23 cubic yards of concrete. That translates to roughly 56 bags of 80 lb concrete, 74 bags of 60 lb concrete, or 112 bags of 40 lb concrete. We recommend adding 10% to these totals for waste, so plan on about 62 bags of 80 lb, 82 bags of 60 lb, or 123 bags of 40 lb. For a pour of this size, ordering ready-mix concrete is more practical and cost-effective.
How much does a concrete slab cost?
The cost of a concrete slab depends on size, thickness, location, and whether you use ready-mix or bags. Ready-mix concrete typically costs $125 to $175 per cubic yard delivered. Labor for a professionally poured slab adds $3 to $10 per square foot depending on your region. A 10 x 10 slab done by a contractor averages $500 to $1,500 all-in. DIY with bags is cheaper on labor but significantly more work for anything over half a cubic yard.
Do I need rebar in my concrete slab?
For most residential flatwork like patios, sidewalks, and driveways, rebar or wire mesh reinforcement is recommended to control cracking. Rebar (typically #3 or #4 bar) is placed in a grid pattern, usually 18 inches on center, and positioned in the lower third of the slab thickness. Structural slabs, foundation walls, and any slab supporting significant loads should always have rebar per an engineered design. Wire mesh is a lower-cost alternative for non-structural slabs but provides less tensile strength.
What is the correct concrete mix for a slab?
A standard 4,000 PSI concrete mix (often called a 4-sack or 4-bag mix) is suitable for most residential slab applications including driveways, garage floors, and patios. In cold climates, specify an air-entrained mix with 5 to 7 percent air content to resist freeze-thaw cycling. The water-to-cement ratio should be kept low (around 0.45 to 0.50) for strength and durability. Avoid adding excess water to the mix, as this weakens the finished slab and increases shrinkage cracking.
How long does a concrete slab take to cure?
Concrete reaches about 70 percent of its design strength in 7 days under normal curing conditions and achieves full design strength at 28 days. You can typically walk on a slab after 24 to 48 hours and drive on a residential driveway after 7 days. To maximize strength, keep the slab moist during the first 7 days by covering it with plastic sheeting or wet burlap, or apply a curing compound. Avoid heavy loads on the slab until the 28-day cure is complete.

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