Concrete calculator
Cubic yards and bags for slabs and footings.
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How it's calculated
Volume is length times width times thickness, with the thickness converted from inches to feet. Divide cubic feet by 27 for cubic yards, which is how ready-mix is ordered. Bagged concrete yields about 0.30 cubic feet per 40 lb bag, 0.45 per 60 lb and 0.60 per 80 lb. Add 5–10% for uneven sub-grade and spillage, and round bags up.
How many 80 lb bags make a yard of concrete?
About 45. A yard is 27 cubic feet and an 80 lb bag yields 0.60 cubic feet. For 60 lb bags it is 60, and for 40 lb bags 90. Past about a yard, a ready-mix truck is usually cheaper and far less work.
How thick should a slab be?
4 inches for patios, walkways and shed floors; 5–6 inches for driveways and anything that carries vehicles; footings depend on your frost line and local code.
What about round footings or posts?
A round hole's area is π × radius². For a 12-inch hole, that is 0.785 square feet: enter length 0.785 and width 1 with the depth as thickness, and set quantity to the number of holes.
Results are estimates for informational purposes only and are not financial, medical or legal advice.