How to Use This Calculator
Enter your slab length and width in feet. Set the thickness in inches — 4 inches for a standard patio or sidewalk, 5 to 6 inches for a garage floor or driveway. The calculator returns cubic yards, bag counts for 80lb, 60lb, and 40lb mixes, and a cost range based on current bagged concrete prices. A 10% waste factor is built into every result to account for uneven subgrade, form overflow, and mixing loss. For L-shaped slabs or irregular pours, run the calculator separately for each rectangular section and add the cubic yard totals. Use the project presets to pre-fill common slab sizes and get an instant result.
How to Calculate Concrete for a Slab
The formula is: cubic yards = (length × width × thickness ÷ 12) ÷ 27. Convert thickness from inches to feet first by dividing by 12. Then multiply all three dimensions to get cubic feet. Divide by 27 because there are exactly 27 cubic feet in one cubic yard.
Example: a 12 × 16 garage floor at 5 inches thick. Convert 5 inches to feet: 5 ÷ 12 = 0.417 ft. Multiply: 12 × 16 × 0.417 = 80 cubic feet. Divide by 27: 80 ÷ 27 = 2.96 cubic yards. Add 10% waste: 2.96 × 1.10 = 3.26 cubic yards. Order 3.5 cubic yards from your ready-mix plant.
For bag count, divide total cubic feet by bag yield. One 80lb bag covers 0.60 cubic feet, one 60lb bag covers 0.45 cubic feet. Using the example above: 80 cubic feet × 1.10 ÷ 0.60 = 147 bags of 80lb mix. Always use Math.ceil — never round down. Running short on a pour creates a cold joint that weakens the slab permanently.
Concrete Slab Tips
Prepare the subgrade before you pour. Compact the base to a minimum depth of 4 inches of compacted gravel or crushed stone. Remove any soft spots and organic material. Concrete placed over loose or wet fill settles unevenly, causing slab cracking that no reinforcement can prevent. A rented plate compactor costs about $100 per day and takes an hour on a standard patio — money that pays for itself many times over in avoided repairs.
Cut control joints before the concrete fully hardens. For a 4-inch slab, space them every 6 to 8 feet in both directions. Use a concrete saw or a groover tool while the mix is still workable. Control joints are intentional weak points — they guide where the slab cracks as it shrinks during curing, turning random surface cracks into clean, manageable lines.
Reinforce based on load. Wire mesh handles residential patio loads on stable soil. For garage floors, pool decks, or any slab that will see vehicle or heavy equipment loads, use #3 rebar on 18-inch centers in both directions. Elevate rebar on chairs to center it in the slab depth — rebar resting on the subgrade provides almost no tensile benefit.
What to Buy
For slabs under 1 cubic yard: QUIKRETE 5000 or Sakrete High Strength 80lb bags. Both cure to 5,000 PSI in 28 days, stronger than standard mix and appropriate for structural applications. Two people can mix 20 bags per hour with a corded drill and mixing paddle.
For slabs over 1 cubic yard: call your local ready-mix plant and specify 3,500 PSI for patios and sidewalks, 4,000 PSI air-entrained for garage floors and driveways in freeze-thaw climates. Have all forms set, reinforcement placed, and your finishing crew ready before the truck arrives. Most plants allow a 90-minute window from batch to placement. Go over that window and the concrete is a waste.