Concrete Slab Calculator

The number of bags depends on your slab volume, the bag size you buy, and whether you include a waste allowance. One 80 lb bag yields about 0.022 cubic yards, so divide the final order volume, including the 10% overage, by 0.022 to estimate how many 80 lb bags you need. A 12 x 10 ft slab at 4 inches thick needs roughly 1.48 yd3 with overage, or about 67 bags of 80 lb mix.

Slab thickness depends on how the surface will be used and what loads it must carry. Patios and walkways are commonly poured at 3.5 to 4 inches, passenger-car driveways at 4 inches, heavy-vehicle driveways at 5 to 6 inches, garage floors at 4 to 6 inches, and shed bases at 3.5 to 4 inches. Local code, soil quality, reinforcement, and freeze-thaw conditions can all change the final requirement.

Use the standard slab formula: length in feet multiplied by width in feet multiplied by thickness in feet, then divide by 27. The main step people miss is converting thickness from inches to feet before running the math. For example, a 10 x 10 ft slab at 4 inches thick is 10 x 10 x (4 / 12) / 27 = 1.23 yd3. Adding a 10% buffer raises the order quantity to about 1.35 yd3.

Yes. A 10% overage is a common planning allowance for slab work because field conditions are rarely perfect. The extra concrete helps cover spillage during the pour, uneven subgrade that increases depth in spots, small layout changes, and rounding differences in takeoff calculations. Ordering too close to the exact math can leave you short during finishing, which is usually more costly than carrying a modest buffer.

Cubic feet and cubic yards both measure volume, but they are used in different buying contexts. One cubic yard equals 27 cubic feet. Ready-mix suppliers usually quote and deliver concrete by the cubic yard, while bagged products often publish yield in cubic feet. Showing both units helps you compare a truck order with bag equivalents and keeps the calculator useful for both DIY jobs and larger contractor takeoffs.

Material cost depends on local ready-mix pricing, delivery minimums, additives, and total order size, so there is no universal number that fits every project. A practical planning range for ready-mix material is often around $110 to $165 per cubic yard, but your quote may fall outside that range. For a 10 x 12 ft patio at 4 inches thick, material alone is often only part of the budget because finishing, prep, reinforcement, and labor can add significantly more.

Rebar is commonly recommended when a slab carries heavier loads, such as driveways, garage floors, or other structural flatwork. For lighter patios and walkways, welded wire reinforcement or fiber reinforcement may be sufficient depending on the design. Our calculator gives an approximate linear-foot estimate based on a built-in evenly spaced grid assumption, but actual reinforcement layout should follow project engineering, local code, and slab joint design.

Yes. Switch to the round slab option and enter diameter plus thickness instead of length and width. The calculator uses the circular volume formula based on pi times radius squared times thickness, then converts the result into cubic yards, metric volume, bag counts, overage, and rebar guidance. That makes it useful for circular pads, tank bases, fire pit slabs, and similar round pours without doing any manual geometry.