Concrete Calculator

How to Use This Calculator

Choose Slab for patios, driveways, floors, and walkways. Choose Post Hole / Footing for round holes: fence posts, deck piers, sonotubes. For slabs, enter length and width in feet. For post holes and footings, enter the hole diameter in inches. Thickness: 4 inches is common for patios and sidewalks. Most residential driveways use 4 to 6 inches depending on vehicle weight, climate, and soil conditions. Six inches for anything that sees truck traffic or sits in a freeze-thaw climate. Eight inches for garage slabs designed to hold a vehicle lift or heavy equipment. Results include a 10% waste factor for uneven subgrade, spillage, and overage. For L-shaped or irregular pours, use our square footage calculator to measure each section separately and add the cubic yard totals. For multiple identical footings, calculate one and multiply by the footing count.

How to Calculate Concrete

For a rectangular slab: multiply length × width × thickness, all in feet, then divide by 27. There are exactly 27 cubic feet in one cubic yard. That's where the ÷ 27 always comes from.

Formula: cubic yards = (length × width × thickness ÷ 12) ÷ 27

Shortcut for 4-inch slabs: divide total square footage by 81. A 10 × 10 patio is 100 sq ft ÷ 81 = 1.23 cubic yards. This shortcut only works at 4 inches. For any other thickness, use the full formula.

Bag coverage: an 80-lb bag covers 0.60 cubic feet, a 60-lb bag covers 0.45 cubic feet, a 40-lb bag covers 0.30 cubic feet. These are manufacturer-rated yields; real-world yield runs slightly lower depending on water ratio and aggregate. Always round up. That same 10 × 10 slab needs 33.3 cubic feet plus 10% waste = 36.6 cubic feet ÷ 0.60 = 62 bags of 80-lb mix.

Concrete Tips

Match thickness to the load. Four inches is common for patios and sidewalks. Most residential driveways use 4 to 6 inches depending on vehicle weight, soil conditions, and climate. Use 6 inches for anything that sees delivery trucks or sits in a freeze-thaw climate with clay soil. Eight inches for garage floors that will hold a vehicle lift. Two extra inches now is far cheaper than a cracked slab three years in.

Prepare the base before you pour. Concrete cures poorly on loose or wet fill. Compact your gravel base, excavate any soft spots, and replace with compacted crushed stone. A rented plate compactor costs about $100 per day. Skip this step and soft spots telegraph directly to surface cracks within the first year.

Never short-order. Running out mid-pour creates a cold joint: a visible seam where the first section started to set before the second section was placed. Cold joints are structural weak points that crack under load and freeze-thaw cycling. If the calculation shows 5.2 cubic yards, order 6. Extra concrete fills low spots or cures in place for disposal. Running short cannot be fixed cleanly.

Pour early in the morning. Concrete must be placed and finished within 90 minutes of mixing. In summer heat with direct sun, that window shrinks fast. In temperatures below 50°F, request an accelerator admixture from your ready-mix plant, or reschedule. Cold concrete cures slowly, and a frost before it reaches working strength will damage the surface.

What to Buy

Under 1 cubic yard, buy bagged mix. Look for 5000 PSI high-strength mix at any hardware or home improvement store. Eighty-pound bags give the best cost per cubic foot, but plan for two people if you're mixing more than 10 bags. A corded drill with a mixing paddle works for small batches; rent a mixer for anything larger.

Between 1 and 3 cubic yards, call your local ready-mix plant before you commit to bags. A short-load surcharge of $50–100 is common but still often beats the cost and labor of hand-mixing 50–100 bags. Most plants will accommodate a half-load or mini-mix for DIY projects with a day or two of notice.

Over 3 cubic yards, ready-mix is the only practical choice. Specify PSI when you call: 3,500 for driveways, patios, and walkways; 4,000 for structural slabs. In cold climates, request air-entrainment by name; it cuts freeze-thaw surface damage, and every plant carries it. Have enough people on site to place and finish the full pour in one continuous operation before the clock runs out.

Material prices updated June 2026. Ready-mix and bag concrete prices vary by region; call your local plant or supplier for current rates before finalizing a project budget.

Frequently Asked Questions