Concrete slab calculator: cubic yards, bags and thickness by use

Multiply the length by the width in feet, multiply by the thickness in inches, and divide by 324 for cubic yards; or convert everything to feet, multiply, and divide by 27. A 12 × 12 ft slab at 4 in is 144 × 4 ÷ 324 = 1.78 cubic yards, 1.96 with 10 percent waste. Do not round the thickness to 0.33 ft; keep the fraction or use 324. Add the waste to the volume before rounding up to the quarter yard or dividing into bags, and add any thickened edge as a separate footing volume. The calculator above does each step and prints them.

About 81 square feet at 4 in thick, 108 at 3 in, 65 at 5 in, 54 at 6 in and 40 at 8 in. The figure is 324 divided by the thickness in inches, because a cubic yard is 27 cubic feet and 27 × 12 is 324. Contractors run it backwards for a 4 in slab: square feet divided by 81 gives yards. A 9 × 9 ft pad at 4 in is one yard exactly. None of these include waste, so convert first and add the margin after; the full table by thickness, with bags and weight, is above.

Four inches for a patio, walkway, shed pad or interior floor without vehicles; six for a garage floor, a heavy driveway or an RV pad; five is the usual compromise for a car driveway. The code minimum for a floor is 3½ in, IRC 2021 R506.1, and everything above it is trade practice by load. Six inches is 50 percent more concrete than four, 7.41 against 4.94 cubic yards on a 20 × 20, and it does not compensate for a poor subgrade; a compacted base under 4 in outlasts 6 in on loose fill.

Often not. A 2×4 form board is 3½ in tall, so a slab screeded flush with the top of the form is 3½ in thick unless the form was set on a strip of base or a spacer. The difference is 12.5 percent of the concrete, 1.08 cubic yards instead of 1.23 on a 10 × 10, and it is why a 4 in order on a 2×4 form comes back with some left over. Three and a half inches still meets R506.1 for a floor. If you want a true 4 in, set the forms up half an inch or use a 2×6 form and a screed guide.

Because the ground was not at plan grade, the floor was sloped, or the edge was thickened. Half an inch of low subgrade across the pour adds 12.5 percent to a 4 in slab; a garage floor falling a quarter inch per foot over 24 ft averages 3 in thicker than its thin edge; a turned-down edge is a footing that was never in the slab arithmetic. Formula results are exact for the slab on paper. Enter the average thickness, add the turndown separately, shoot the grade before you order, and keep the 10 percent waste: it exists for the first two.

1.23 cubic yards at 4 in, 1.36 with 10 percent waste and 62 bags of 80 lb; the full answer at every thickness and bag size is on the concrete calculator page.

7.11 cubic yards at 4 in, which is 576 × 4 ÷ 324, or 7.82 with 10 percent waste, an order of 8 at the quarter-yard step. At 6 in it is 10.67 cubic yards. You will see 7.05 quoted for the 4 in slab; that comes from rounding 4 in to 0.33 ft and it is a third of a yard light on a slab this size. A 24 × 24 garage floor that slopes toward the door averages thicker than 4 in, so use the average in the tool, and add the turndown around the edge as a footing if there is one.

Only where the slab is below grade. R506.2.2 requires a 4 in base of clean graded sand, gravel, crushed stone, crushed concrete or slag passing a 2 in sieve where the slab is below grade, and the exception waives it on well-drained Group I soils. A patio at grade has no base requirement in the code. Most slabs get one anyway, because compacted base is the cheapest defence against a low subgrade, frost heave and standing water, and the trade puts 4 in under patios and 6 in or more under driveways and garage floors. The gravel is a separate order in tons that compacts by about a fifth.

Under a floor, yes: R506.2.3 requires a 10-mil vapor retarder to ASTM E1745 Class A with joints lapped 6 in between the slab and the base. Under flatwork, no: the exceptions cover garages and unheated accessory buildings, unheated storage rooms under 70 sq ft, carports, and driveways, walks, patios and other flatwork not likely to be enclosed and heated later. The last clause is the one to read twice. A shed pad or a garage slab you might insulate and heat someday is not flatwork, and the sheet is cheap next to a damp floor you cannot fix from above.

Two to three times the slab thickness in inches, as feet: 8 to 12 ft on a 4 in slab, with 10 ft the usual figure and 15 ft the practical ceiling for any thickness. The calculator prints 2½ times the thickness, rounded to the half foot and capped at 15 ft. This is a trade rule of thumb; the IRC sets no joint spacing for residential slabs on grade. Cut or tool the joints to a quarter of the slab depth as soon as the surface takes a blade cleanly, keep the panels near square, and put a joint at any re-entrant corner where a crack would otherwise start.

In trade practice, no. Reinforcement holds a shrinkage crack tight once it opens; a joint decides where the crack opens so it is straight and hidden. A thick slab with a tight bar grid will crack less visibly without joints, which is why some garage owners skip them and report no problems, but that is a gamble on the subgrade, not a rule. The one thing the IRC does say about reinforcement in a slab on ground is R506.2.4: where you use it, support it so it stays from the centre to the upper third of the slab during the pour.

About 150 lb per cubic foot for normal-weight concrete, which is 50 lb per square foot at 4 in, 62½ at 5 in, 75 at 6 in and 100 at 8 in. A 20 × 20 ft garage floor at 6 in is 200 cubic feet and roughly 30,000 lb, 15 tons. The density is a typical figure, not a code value; lightweight and heavily air-entrained mixes weigh less. It matters for a slab on a deck or a rooftop, which is an engineered job, and for hauling bags: a yard of 80 lb bags is 45 sacks and 3,600 lb in the back of a pickup.

IRC 2021 Table R402.2 sets 2,500 psi at 28 days for basement slabs and interior slabs on grade in every weathering zone. Garage floors, porches, carport slabs and exterior steps go to 3,000 psi where the weathering potential is moderate and 3,500 psi where it is severe, air-entrained at 5 to 7 percent. A patio is exterior flatwork and follows the same moderate or severe column in practice. Weathering potential is the map figure in Table R301.2 for your jurisdiction. The mix proportions and water behind the number are in concrete mix ratios, water and PSI.

At least 2 percent away from the house within 10 ft of the foundation, which is IRC 2021 R401.3 in the exception: impervious surfaces within 10 feet of the building foundation shall be sloped not less than 2 percent away from the building. Two percent is a quarter inch per foot, the trade figure for patios, walks and garage floors. Slope adds concrete if the top is graded and the base is flat: a quarter inch per foot over 24 ft is 6 in of extra depth at the far edge, 3 in on average. Grade the base to the same fall and the slab stays uniform.

Yes. A pad is what people call a small, isolated slab, an air-conditioner base, a generator pad, a hot tub pad or a shed pad, and it is calculated, poured and governed by the same rules as any other slab on ground. Use the calculator above with the pad dimensions; typical sizes are in the pad table. The one distinction that matters is heated or not: a pad under a shed you might later heat is a floor under R506, so the vapor retarder exception for flatwork stops applying, and a hot tub pad usually goes to 6 in for the loaded weight.

88 bags of 80 lb at 4 in with 10 percent waste, or 80 without; the bag question by shape and size is on the bags of concrete calculator and in how many bags of concrete do I need.