Multiply length, width, and thickness in feet, divide by 27, and add 5 to 10 percent. A 10 by 10 foot slab at 4 inches needs about 1.23 cubic yards before the extra.
The formula
Convert the thickness from inches to feet by dividing by 12. A 4 inch slab is 0.333 feet thick. Multiply the length by the width by the thickness to get cubic feet, then divide by 27 to get cubic yards.
Pick a thickness
Four inches suits walks, patios, and shed floors. Five to 6 inches suits driveways and garage floors that carry vehicles. Footings and walls follow your plans.
Bags or ready mix
An 80 pound bag yields about 0.6 cubic feet, so one cubic yard needs about 45 bags. Below about 1 cubic yard, bags make sense. Above that, mixing by hand is slow and uneven, and ready mix is worth the minimum delivery fee.
Add extra
Order 5 to 10 percent more than the exact volume. A subgrade that is a half inch low across a 10 by 10 foot slab adds about 0.15 cubic yards.
Frequently asked questions
How many bags of concrete are in a yard?
About 45 bags of 80 pounds, 60 bags of 60 pounds, or 90 bags of 40 pounds.
How much does a yard of concrete weigh?
About 4,050 pounds, or roughly 2 tons.
Is it better to order too much?
Yes. A slight surplus costs less than a second delivery with a cold joint.