General
Concrete Calculator
Calculate cubic yards and bags of concrete needed for slabs, footings, and pours.
40 cubic feet ÷ 27 = 1.48 cubic yards. With 10% overage: 1.63 cubic yards. Order ready-mix concrete by the yard. Bag concrete is practical up to ~1 cubic yard - above that, ready-mix is more economical.
How to calculate concrete in cubic yards
Concrete is sold by the cubic yard. To figure out how much you need, multiply length times width times depth in feet, then divide by 27 to convert cubic feet to cubic yards. A 10x10 slab that is 4 inches thick is about 1.23 cubic yards. This calculator handles the conversion automatically and accounts for different shapes and waste.
How thick should a concrete slab be?
For a standard patio or walkway, 4 inches is the minimum. Driveways should be at least 4 inches thick, but 5 to 6 inches is better if you are parking heavy trucks or equipment. Garage floors are typically 4 to 6 inches. Footings and foundations have their own requirements based on local code. Thicker slabs use more concrete but last longer and handle more weight.
Ordering concrete from a ready mix plant
Ready mix trucks deliver concrete in full or partial loads measured in cubic yards. Most plants have a minimum order, usually around 1 yard. Anything under that and you are better off mixing bags. Always order 5 to 10 percent more than your calculation shows. Running short in the middle of a pour is a nightmare you want to avoid. It is cheaper to send a little back than to need another truck.
Bags vs ready mix
An 80 pound bag of concrete makes about 0.6 cubic feet. You need about 45 bags to make one cubic yard. For small jobs like a few fence posts or a small pad, bags are fine. For anything over 1 cubic yard, order a truck. Mixing 45 bags by hand is brutal and the quality will not be as consistent as ready mix.
Common Questions
How long does concrete take to cure?
Concrete is walkable in 24 to 48 hours but takes 28 days to reach full strength. You can drive on a driveway after 7 days. Keep it damp for the first week for the best cure.
Can you pour concrete in the rain?
Light drizzle is usually fine if you can cover the fresh pour with plastic. Heavy rain will wash out the surface and ruin the finish. Check the forecast and do not take the chance if storms are coming.
Do I need rebar in a concrete slab?
For a 4 inch patio or walkway, wire mesh is usually enough. Driveways and garage floors should have rebar on 24 inch centers. Anything bearing structural weight absolutely needs rebar per your engineer's specs.
What PSI concrete do I need for a driveway?
Use 4,000 PSI concrete for driveways. Standard 3,000 PSI works for patios and sidewalks. If you are in a cold climate with freeze and thaw cycles, stick with 4,000 PSI or higher and add air entrainment.
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