Concrete Calculator
Calculate cubic yards and bag counts for slabs, driveways, and round footings.
Your Ready-Mix Price (optional)
Enter your supplier's quote for an exact cost estimate.
Cubic Yards to Order
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cubic yards · includes 10% waste
Cubic Feet
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cubic feet · no waste added
Bags Needed (includes 10% waste)
80-lb Bags
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60-lb Bags
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40-lb Bags
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Estimated Ready-Mix Cost
Typical range · $125–185/yd³ · excludes labour
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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, calculate each rectangle 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 - 6 inches depending on vehicle weight, soil conditions, and climate. where passenger vehicles park. 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. QUIKRETE 5000 PSI and Sakrete High Strength are reliable for residential slabs and footings. 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 significantly improves freeze-thaw resistance and is a standard option at every plant. Have enough people on site to place and finish the full pour in one continuous operation before the clock runs out.
Frequently Asked Questions
How many cubic yards of concrete do I need for a 10×10 patio at 4 inches thick?
A 10 × 10 patio at 4 inches thick needs about 1.23 cubic yards before waste, or roughly 1.35 cubic yards including a 10% waste factor. Most suppliers will round delivery to the nearest quarter or half yard. For bagged concrete, that is 62 bags of 80lb, 82 bags of 60lb, or 123 bags of 40lb mix.
How many 80lb bags of concrete make one cubic yard?
One 80lb bag of concrete mix covers 0.60 cubic feet. There are 27 cubic feet in a cubic yard, so you need 45 bags of 80lb for exactly one cubic yard, before waste. With a 10% waste factor, plan for 50 bags per cubic yard ordered.
What is the right concrete thickness for a driveway?
Use a minimum of 4 inches for a residential driveway where only passenger vehicles park. Increase to 6 inches for driveways that receive delivery trucks or in areas with heavy freeze-thaw cycles. Thinner pours crack under vehicle weight within a few seasons.
What is the difference between 40lb, 60lb, and 80lb bags of concrete?
They are all the same concrete mix; only the bag weight differs. An 80lb bag covers 0.60 cubic feet, a 60lb bag covers 0.45 cubic feet, and a 40lb bag covers 0.30 cubic feet. Eighty-pound bags cost less per cubic foot but are heavier to carry. Use 40lb or 60lb bags for overhead pours or when working alone.
When should I use ready-mix instead of bagged concrete?
Use ready-mix for pours of 3 cubic yards or more. At that volume, mixing bags by hand or with a small mixer takes too long. Concrete must be placed within 90 minutes of mixing, and you risk a cold joint if early sections begin to set before the pour is complete. Ready-mix is also far cheaper per cubic yard at volume.
How do I calculate concrete for round footings or fence posts?
Enter the hole diameter in inches and the depth in inches using the Post Hole / Footing mode on this calculator. The formula is π × (radius in feet)² × depth in feet, then divide by 27 to get cubic yards. A 10-inch diameter hole at 24 inches deep uses about 1.09 cubic feet of concrete, roughly 2 bags of 80-lb mix per footing.
What PSI concrete do I need for a driveway or patio?
Use 3,500 PSI for residential driveways, patios, and walkways. In climates with hard freeze-thaw winters, specify air-entrained 4,000 PSI. The air entrainment is what prevents surface scaling and spalling, not just the higher strength number. Ask for it by name when you call the plant; it's a standard option at every ready-mix facility.
How long does concrete take to cure?
Concrete reaches roughly 70% of its design strength in 7 days and full strength at 28 days. Stay off new slabs for at least 7 days and keep heavy loads off for the full 28-day cure. Keep the surface moist for the first 3–7 days. Cover with plastic sheeting or mist twice daily. Concrete that dries out too fast in the first week will surface-crack regardless of the mix strength.