Concrete Slab Calculator

How Much Concrete a Slab Needs

A slab is priced and delivered by volume, so the first job is turning a flat area into cubic yards. Length and width come in feet, thickness comes in inches, and that mismatch is where most estimates go wrong. Convert the thickness to feet first, multiply the three dimensions to get cubic feet, then divide by 27 because a cubic yard is 3 ft x 3 ft x 3 ft.

A worked example

Take a 12 ft by 20 ft patio poured 4 inches thick. The area is 240 square feet. Thickness in feet is 4 divided by 12, which is 0.3333. Volume is 240 x 0.3333, which is 80 cubic feet. Divide by 27 and the slab needs 2.96 cubic yards of concrete before any allowance for waste.

Nobody pours the exact calculated figure. Sub-grade is never perfectly flat, forms bow slightly under the weight of wet concrete, and a little is always lost in the chute and the wheelbarrow. A 10 percent allowance on a small slab is normal, and 5 to 7 percent is enough once a pour passes about 10 cubic yards. On the 12 ft by 20 ft patio, 2.96 plus 10 percent is 3.26 cubic yards, so the order rounds to 3.5 yards. Ready mix plants sell in quarter yard steps, and running short by a quarter yard in the middle of a pour is far more expensive than paying for a little extra.

Slab Thickness by Use

Thickness is not a style choice. It is set by what will sit on the slab and how often, and local building code may set a minimum that overrides any rule of thumb here. Confirm with the building department before ordering.

4 inches: patios, sheds, walkways

Four inches is the standard for foot traffic and light static loads. Patios, garden paths, shed floors, hot tub pads on a well prepared base, and air conditioner pads all sit at 4 inches. It is also the usual thickness for an interior slab on grade in a house, over a vapor barrier. Below 4 inches a slab cracks under its own restraint and offers almost nothing in bending, so 3 inch pours are best kept to decorative work with no load on them.

5 to 6 inches: driveways and parking pads

Cars and light pickups belong on 5 inches at minimum, and 6 inches is the safer standard for a driveway that will see a delivery truck, a trailer, or a heavy SUV. The extra inch adds about 25 percent to the concrete cost on a 4 inch baseline but roughly doubles the load the slab can carry before it cracks, because bending strength rises with the square of the thickness. On a driveway, spending on thickness is almost always better value than spending on reinforcement.

6 inches and more, reinforced, for heavy vehicles

An RV pad, a workshop floor with a lift, a farm equipment apron, or any surface that takes loaded trucks starts at 6 inches with rebar and often goes to 8 inches. At this point the slab is a structural element and the reinforcement layout, the concrete strength and the sub-grade compaction all need to be specified rather than guessed. A 6 inch slab with a proper rebar mat and a compacted base will outlast an 8 inch slab poured on soft ground with nothing in it.

The Gravel Sub-Base

Concrete is strong in compression and weak in bending, which means it only performs if it is evenly supported. The sub-base does that job. Use 4 inches of compacted crushed stone as an absolute minimum under any slab, and 6 to 8 inches on clay, on fill, or anywhere frost heave is a factor.

The right material is angular crushed stone with fines in it, sold as crusher run, road base, or 21A depending on the region. Round pea gravel will not lock together and will not compact. Spread the stone in lifts of no more than 4 inches and run a plate compactor over each lift. A base that has been dumped and raked but not compacted will settle under the slab within a year and leave voids, and a slab bridging a void cracks.

Work out the base volume the same way as the concrete. Four inches of stone under a 240 square foot patio is 240 x 0.3333 divided by 27, which is 2.96 cubic yards, and crushed stone compacts by roughly 20 percent, so order about 3.5 cubic yards to end up with 4 compacted inches.

Vapor Barrier Under Interior Slabs

Any slab under a heated or finished space needs a vapor barrier directly beneath it. Ground moisture moves upward through concrete continuously, and without a barrier it will lift flooring adhesive, cup hardwood, blister epoxy coatings and feed mold under carpet. Use a minimum 10 mil polyethylene sheet, and 15 mil under finished floors. Lap the seams by at least 6 inches, tape them, and turn the sheet up the perimeter forms.

The barrier goes directly under the concrete, not under the gravel. A layer of sand over the barrier used to be common practice and is now generally advised against, because that sand traps water and releases it into the slab later. Exterior slabs such as patios and driveways do not use a vapor barrier, since they need to drain freely.

Wire Mesh, Fiber and Rebar

Reinforcement does not stop concrete cracking. Concrete shrinks as it cures and it will crack somewhere. Reinforcement holds the crack tight so the two sides stay level, keeps aggregate interlocked across the joint, and adds load capacity. There are three common options and they are not interchangeable.

• Welded wire mesh. A 6 x 6 W1.4 x W1.4 sheet is the usual choice for patios, sidewalks and light residential slabs. It only works if it sits in the middle third of the slab depth, so it has to be supported on chairs. Mesh laid on the ground and pulled up with a hook as the concrete goes in usually ends up on the bottom, where it does nothing.
• Synthetic fiber. Polypropylene fibers are batched into the mix at the plant, typically about 1.5 pounds per cubic yard. They control the fine plastic shrinkage cracks that form in the first hours, which makes them useful on any slab, and they save the labor of placing mesh. They do not add structural capacity and they do not replace rebar under vehicle loads.
• Rebar. Number 3 bar (3/8 in) or number 4 bar (1/2 in) tied into a grid at 12 to 18 inch centers is the choice for driveways, garage floors, RV pads and anything reinforced for heavy vehicles. Support it on chairs so it sits at mid depth or slightly below, keep at least 3 inches of cover from the earth face and 1.5 inches from formed faces, and lap splices by at least 40 bar diameters.

A practical combination on a driveway is fiber in the mix for early crack control plus a number 4 rebar grid for load. Mesh and rebar together on the same slab is rarely worth the cost.

Control Joints

Control joints are deliberate weak lines that decide where the slab cracks. Cut them, or the slab picks its own line. Two rules matter.

The first is timing. Saw cut within 6 to 12 hours of the pour, as soon as the surface is firm enough that the blade does not ravel the edges. In hot weather that window closes early, in cold weather it stretches out. Cutting later than about 12 hours often means the slab has already cracked. Tooled joints struck with a groover during finishing are an alternative on small pours.

The second is spacing. In feet, the joint spacing should be no more than 2 to 3 times the slab thickness in inches. A 4 inch slab therefore gets joints every 8 to 12 feet, a 5 inch slab every 10 to 15 feet, and a 6 inch slab every 12 to 18 feet. Keep panels roughly square, with a length to width ratio no worse than 1.5 to 1, and cut the joint at least one quarter of the slab depth, so 1 inch deep in a 4 inch slab.

Also isolate the slab from anything rigid it touches. A half inch expansion joint material against a house foundation, a column, a step or an existing slab lets the new concrete move independently instead of cracking against a fixed edge.

Curing

Concrete does not dry, it hydrates, and that reaction needs water. A slab that loses its surface moisture in the first week ends up with a weak, dusty top layer that scales and wears. Keep the surface damp for a full 7 days, using wet burlap, plastic sheeting, a soaker hose on a timer, or a spray applied curing compound. Curing compound is the practical choice on large pours and on anything that cannot be watered regularly.

Concrete reaches roughly 70 percent of its design strength at 7 days and its specified strength at 28 days, which is why test cylinders are broken at 28 days. Light foot traffic is usually fine after 24 to 48 hours, furniture and foot loads after 7 days, and vehicles should stay off for the full 28 days. In cold weather, protect the pour from freezing for at least the first 3 days, since concrete that freezes before it sets loses strength permanently.

Thickened Edges

Slab edges carry more load per square foot than the middle, because there is concrete on only one side to spread it. Any edge that supports a wall, a garage door track, a shed frame or vehicle wheels should be thickened. A common detail is a 12 inch deep by 12 inch wide turned down edge on a 4 inch slab, with two runs of number 4 rebar in the bottom of the thickening.

Thickened edges add volume that is easy to forget when ordering. A 12 in by 12 in thickened edge adds 8 extra inches of depth over a 12 inch wide strip, which is 0.667 cubic feet per linear foot. Around a 12 ft by 20 ft slab, that is 64 linear feet, or about 42.7 cubic feet, which is 1.58 cubic yards on top of the 2.96 yards for the flat slab. On small slabs the thickening can be a third of the total order.

Those figures are the neat volume with no waste allowance. Add 5 to 10 percent before placing the order, and round up to the supplier's nearest quarter yard.