🏗️ Concrete Slab Calculator
Calculate concrete volume in cubic yards and number of bags needed.
Concrete Needed (with waste)
How to Use This Calculator
Enter your slab length and width in feet, then set the thickness in inches. The calculator converts everything to cubic yards (the unit ready-mix concrete is sold in) and also tells you how many 60 lb or 80 lb bags you need if you are mixing it yourself.
Measure the slab area in feet. Length times width gives you the square footage.
Choose your thickness. Patios and walkways need 4 inches. Driveways should be 4 to 6 inches. Garage floors work well at 4 inches with a 6-inch apron at the door.
Set the waste percentage to at least 10%. Uneven subgrade, overpours at forms, and spillage eat into your order fast.
Pick your bag size. An 80 lb bag yields 0.60 cubic feet, so for anything over 1 cubic yard order ready-mix instead of bags.
Concrete Slab Formula
One cubic yard equals 27 cubic feet. A 10x10 ft slab at 4 inches thick is 33.3 cubic feet, or about 1.23 cubic yards. At that size, you are right at the line between bagging it yourself (55 bags of 60 lb) and ordering a small ready-mix delivery.
Example Projects
Tips for Your Project
For any slab over 1 cubic yard, ordering ready-mix is almost always faster and less expensive per cubic foot than bagging it yourself. Call at least two days ahead, and have your forms, rebar, and base gravel ready before the truck arrives. A standard ready-mix truck carries 9 to 10 cubic yards and will charge a short-load fee for anything under 3 yards.
Never add extra water to make the mix easier to work with. Watery concrete is weak concrete. If you need more working time, ask your supplier for a mix with a higher slump rating or a set retarder additive. Control joints every 8 to 10 feet will prevent random cracking as the slab cures and shrinks.
Frequently Asked Questions
How do I calculate concrete for a slab?
Multiply length × width × thickness (all in feet) to get cubic feet. Divide by 27 to get cubic yards. Add 10% for waste.
How many bags of concrete do I need?
A 60 lb bag yields ~0.45 cubic feet; an 80 lb bag yields ~0.60 cubic feet. Divide your total cubic footage by the yield per bag.
What thickness should a concrete slab be?
Patios and walkways: 4 inches. Driveways: 4–6 inches. Garage floors: 4 inches. Structural slabs: consult an engineer.
Should I add extra concrete?
Always add 5–10% for waste from spillage, uneven subgrade, and overpours. Our calculator includes a 10% waste factor option.
How much does concrete weigh?
Concrete weighs approximately 145 lbs per cubic foot (2,300 kg/m³). A cubic yard weighs about 3,915 lbs (1.96 tons).