A 24 × 20 ft garage slab poured 4 inches thick with a 12 × 12 in thickened edge takes 216 cubic feet of concrete: 160 in the slab and 56 in the edge. That is 8.0 cubic yards, 8.8 with 10 % waste, so you order 9 yards of ready-mix. Underneath go about 6 yards of gravel base and one roll of 6-mil poly; inside, 36 #4 bars at 18 in on center or twelve sheets of wire mesh. The calculator above works out the whole slab, including a check of the pressure on the soil.
How the calculator works
Enter the slab size and thickness and choose whether it has a thickened edge. Pick the reinforcement, the base, the vapor retarder and any foam, then how you will buy the concrete. Feet and inches can be mixed freely, and every quantity follows the market in the header.
Concrete
The slab itself is length × width × thickness. A thickened (turned-down) edge adds a strip around the perimeter below the slab: its extra depth times its width times the perimeter, with the corners counted once. For the default slab that is 8 in × 12 in × 84 ft, or 56 cubic feet. The waste allowance covers an uneven base, form deflection and what stays in the truck; 5 to 10 % is usual. Concrete comes in 80, 60 or 40 lb bags for small pours or by the truck, ordered in quarter-yard steps, for anything over a yard or two.
The IRC sets 3½ in as the minimum thickness of a concrete floor slab on the ground (R506.1). Driveways and slabs that carry vehicles are usually 4 to 6 in; check local requirements.
Rebar or wire mesh
Slabs on ground are reinforced to hold cracks tight, not to carry the load. A rebar grid uses the same method as the Rebar Calculator: bars each way from the spacing and edge cover, laps where a bar is longer than a 20 ft stock bar, and the bars to buy from a cutting plan. Welded wire mesh, most often 6×6-W1.4×W1.4, comes in 5 × 10 ft sheets or 5 × 150 ft rolls; pieces overlap one mesh spacing plus 2 in each way (ACI 318-19 25.5.4), which is 8 in for 6 in mesh. Either one only works if it sits in the slab, on chairs or dobies, not on the base.
Base and vapor retarder
The IRC calls for a 4 in base course of clean graded sand, gravel or crushed stone under slabs below grade, except on well-drained or sand-gravel soils (R506.2.2), and a 6-mil polyethylene vapor retarder with joints lapped at least 6 in between the base and the slab (R506.2.3). The calculator lays the poly in strips of the roll width along the slab length and counts the rolls; the base is area × thickness, also given in tons.
Pressure on the soil
Normal-weight concrete weighs about 150 lb per cubic foot, so a 4 in slab presses on the ground with only 50 psf; with thickened edges spread over the whole area the default slab averages about 68 psf. The IRC presumes clay and silt soils can carry 1,500 psf, sand 2,000 psf and gravel 3,000 psf (Table R401.4.1). If the slab carries walls, storage racks or heavy equipment, enter that load and the calculator compares the total with the soil you choose. Point loads and wall lines need footings; a uniform check does not replace that design.
Common mistakes
- Forgetting the thickened edge. On a garage slab it can be a quarter of the concrete.
- Ordering exact. A short load costs far more than half a yard of extra concrete. Add waste and round up.
- Mesh on the ground. Pulling mesh up during the pour rarely works; set it on chairs.
- Skipping the vapor retarder. Moisture through the slab ruins flooring and adhesives later.
For footings, use the Footing Calculator; for the base stone alone, the Gravel Calculator.