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Concrete Calculator

Enter the size of your slab and the thickness you want. The calculator returns the volume in cubic yards plus the number of 40, 60 and 80 lb bags to buy — so you order once and don’t run short mid-pour.

How the concrete calculation works

  1. Measure the slab length and width in feet.
  2. Decide the thickness — 4 in for patios and walkways, 5–6 in for driveways that take vehicles.
  3. Convert thickness from inches to feet by dividing by 12.
  4. Multiply length × width × thickness to get cubic feet.
  5. Divide cubic feet by 27 to get cubic yards — that is the number you give a ready-mix supplier.
  6. For bags, divide cubic feet by the bag yield: 0.60 for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb, then round up.

The formula

Volume (ft³) = Length (ft) × Width (ft) × Thickness (in) ÷ 12. Then divide by 27 to get cubic yards.

Buying materials?

Compare prices before you order:

Common questions

How many 80 lb bags of concrete are in a cubic yard?

About 45. One 80 lb bag yields roughly 0.60 ft³ and a cubic yard is 27 ft³, so 27 ÷ 0.60 = 45 bags. For a 60 lb bag it is about 60 bags, and about 90 bags for 40 lb.

How thick should a concrete slab be?

Four inches is standard for patios, walkways and shed floors. Use 5 inches for driveways carrying cars and 6 inches for heavy vehicles, RVs or where the slab sits on weak soil.

How much does a yard of concrete weigh?

Roughly 4,000 lb, or about 2 tons. That is 145 lb per cubic foot. This matters if you are mixing by hand rather than ordering a truck.

Should I add extra for waste and spillage?

Yes — add 5 to 10 percent. Sub-grade is never perfectly level and some concrete is always lost. Running out halfway through a pour leaves a cold joint that never fully bonds.

Is bagged concrete cheaper than ready-mix?

For anything over about 1 cubic yard, ready-mix is usually cheaper and far less work. Bagged concrete makes sense for fence posts, small pads and repairs where a truck cannot reach.

Sizes people search for

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