Concrete Calculator

Tip: running short in the middle of a pour leaves a cold joint that can crack. It is always cheaper to have a little left over than to stop and wait for more.

Worked example

A 10 ft by 10 ft patio poured 4 inches thick covers 100 square feet. Four inches is 4 / 12 = 0.333 ft, so the slab holds 33.3 cubic feet. Adding 10% for uneven ground and spillage brings it to 36.7 cubic feet.

Dividing by 27 gives 1.36 cu yd, which is what you would order from a ready-mix supplier.

In bags, an 80 lb bag yields about 0.6 cubic feet, so you would need 62 bags. With 60 lb bags you would need 82.

How to calculate concrete for a slab

Multiply length by width by thickness to get volume. Thickness is usually measured in inches, so divide it by 12 first to keep every number in feet. The result is cubic feet. Divide by 27 to get cubic yards, the unit ready-mix trucks sell by.

Then add an allowance. Excavated ground is never perfectly flat, forms bow a little, and some concrete is always lost in the wheelbarrow and chute. 5% to 10% is typical for slabs.

Bags or a truck?

Premixed bags are measured by yield, the volume one bag makes once mixed:

• 80 lb bag: about 0.6 cubic feet.
• 60 lb bag: about 0.45 cubic feet.
• 40 lb bag: about 0.3 cubic feet.

A cubic yard takes 45 bags of 80 lb. For a small pad or a few fence posts, bags are convenient. Beyond about one cubic yard, hand mixing gets slow, and concrete in one section may start to set before you finish the next. Many suppliers deliver short loads for a fee, and rental mixers help in between. For anything over two or three yards, a ready-mix truck is usually the better choice.

Common slab thicknesses

• Walkways and patios: 4 inches.
• Driveways for cars: 4 to 5 inches.
• Driveways for trucks, RVs and heavy equipment: 6 inches.
• Shed, hot tub and AC pads: 4 to 6 inches, depending on the load.

Thicker slabs, rebar or wire mesh and a well compacted gravel base all add strength. The base is usually 4 inches of compacted gravel, which you can size with the gravel calculator.

Footings, posts and round shapes

This calculator is for rectangular slabs. For round footings and post holes, the volume is 3.14 x radius squared x depth, with all measurements in feet. A 12 inch round footing 3 ft deep holds 3.14 x 0.5 x 0.5 x 3 = 2.36 cubic feet, about four 80 lb bags. For fence posts, subtract the volume of the post itself. The fence calculator includes a quick bag estimate per post.

For steps, calculate each step as its own slab and add them. For a slab with a thickened edge, add the volume of the edge separately: the length of the perimeter times the extra width and depth.

Ordering ready-mix

Ready-mix is ordered in cubic yards, often in quarter-yard steps, and many suppliers charge a short-load fee under a minimum such as 3 or 4 yards. Tell them what you are pouring so they can recommend a strength, commonly 3,000 to 4,000 psi for residential flatwork, and whether you need air entrainment for freeze and thaw resistance. Have your forms, base and helpers ready before the truck arrives, since most suppliers allow only a short unloading window before charging extra.

Curing and timing

Concrete gains strength as it cures, not as it dries. Keep a new slab damp for the first week by misting it or covering it with plastic, especially in hot or windy weather. You can usually walk on it after a day or two and drive on a driveway after about a week, though full strength takes around 28 days. Avoid pouring when frost is expected within a few days unless you can protect the slab with insulated blankets.

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Results are estimates, not a contractor quote. Check quantities with your supplier before you order, and follow local building codes.