24×24 Concrete Slab Cost: Oversized 2-Car Garage & Shop Floor

Quick answer: a 24x24 concrete slab (576 sq ft) costs about $4,250 to $7,540 installed at the usual 5 inches for an oversized 2-car garage, and $6,710 to $8,560 as a 6-inch, 4,000 PSI shop floor with a troweled finish. A 5-inch slab needs 9.78 yd³ of concrete with 10% waste, which is 1 to 2 truckloads. Pouring a thickened edge with it (a monolithic slab) brings the total to roughly $6,610 to $9,240.

A 24x24 is the size most builders mean by a "real" two-car garage: room for two vehicles with the doors open, plus a workbench along one wall. It is also a popular footprint for a detached workshop. At this size the slab is no longer something you pour in an afternoon. You are ordering more than one truck, thinking about the footing under the walls, and choosing a concrete grade for what the floor will carry. This guide covers each of those choices and what they cost.

24x24 Slab Cost by Thickness

Five inches is the default for a garage this size. Four inches is legal under IRC section R506 (which sets a 3.5-inch minimum) and fine for a hobby room or storage, but a 576 sq ft floor that carries two vehicles is better at 5. Six inches suits a shop with heavy machines, a car lift or a work truck. Each extra inch adds about $293 at full scope, almost all of it concrete.

Where the Money Goes on a 5-Inch 24x24

That direct cost works out to $10.47 per sq ft, inside the $7 to $14 per sq ft range our garage slab cost guide uses. Not included: excavation and a gravel base ($806 to $1,901), a pump if the trucks cannot reach, and the footing under the walls, which is the next section.

Thickened Edge (Monolithic) Option

Garage walls need a footing. In mild climates the cheapest way to get one is to pour it with the floor: the slab edge is dug deeper and wider so the perimeter becomes a beam that carries the walls. This is a monolithic or thickened-edge slab. A common residential detail is a 12-inch-wide edge, 12 to 18 inches deep, with 2 continuous #4 bars running around it. The deeper the edge, the more concrete and digging:

Added to the full-scope 5-inch slab, a monolithic 24x24 comes to about $6,610 to $9,240 installed, from direct cost with a shallow edge to a deep edge plus overhead and profit. Whether you can use one is not your choice alone. Footing depth is set by the local frost depth under IRC section R403, and in cold regions the building department will usually require a footing below the frost line, often with a stem wall, instead of a shallow thickened edge (some areas allow a frost-protected shallow foundation with insulation). Ask the building department before pricing, and see the monolithic slab foundation guide. The footing calculator sizes a separate footing.

Shop Floor Spec: 6 Inches and Stronger Concrete

A shop floor takes point loads that a garage never sees: a two-post lift, a heavy table saw, an engine on a stand, a loaded trailer. For that, most owners go to 6 inches and a stronger mix. The jump in grade is cheap next to the rest of the job:

• 3,500 PSI: Garage floors, driveways, porches and steps in freeze-thaw areas. The IRC minimum for garage floors in severe-weathering areas (Table R402.2).
• 4,000 PSI: Driveways that see de-icing salt, RV and truck pads. On a 6-inch 24x24 it adds about $90 to $240 over 3,000 PSI.
• 4,500 PSI: Heavy equipment, shop floors with forklifts, commercial aprons. Expect about $147 to $190 per yd³.

Shops usually get a smooth steel-trowel finish so carts roll easily and spills wipe up, which adds about $288 over broom. Put together, a 6-inch, 4,000 PSI, troweled 24x24 runs $6,710 to $8,560 installed (from the full-scope direct cost up to that plus 25% overhead and profit, so the low end is higher than a plain 6-inch slab's). If you plan a car lift, check the lift maker's slab requirements before the pour; some call for a thicker or stronger slab under the posts. For a larger shop, the size table in our slab cost guide includes a 30x40.

Pour Day: Two Trucks, One Slab

The 5-inch slab alone is 9.78 yd³, which is about 19 tons of wet concrete. Some plants can send it as one full load; many will send two. Add a thickened edge or go to 6 inches and it is two loads for almost everyone. That changes the plan:

• Stagger the trucks. Ask for the second truck to arrive as the first one is finishing, not at the same time and not an hour later. A long gap leaves a cold joint where the two loads meet.
• Plan access for both. The chute reaches about 15 to 20 ft, and a 24-ft slab's far side is out of reach from one parking spot. If trucks cannot drive alongside, budget a line pump ($400 to $800) or a boom pump ($900 to $1,800).
• Crew size. About 20 crew-hours of placing and finishing, so a full day for a crew of 3 to 4.
• Joints. At 5 inches, joints every 10 to 12.5 ft work, so one joint each way at 12 ft makes four 12x12 panels. At 4 inches the limit drops to 8 to 10 ft and you need two joints each way. At 6 inches the range is 12 to 15 ft. Saw within about 4 to 12 hours of finishing.
• Curing. Keep cars off for about 7 days; the concrete reaches its design strength at about 28 days, so hold heavy shop equipment until then.

DIY or Hire a Pro?

For most people this is a pro job. Two trucks, a 24-ft screed, a footing that an inspector will check, and a finish that has to be done across 576 sq ft before it sets are a lot to manage with a first-time crew. The DIY materials range ($1,800 to $2,720 at 5 inches) is tempting next to the installed price, but a failed pour on a slab this size costs more than the labor you were trying to save. Doing the site prep and form work yourself and hiring a finishing crew for pour day is a reasonable middle ground; the labor cost calculator shows what that crew should cost. Before you sign, compare the bid with the quote checker or price your own layout in the slab cost calculator. Smaller footprint? See the 20x20 guide.

Sources and Method

Volumes use length × width × thickness plus 10% waste; thickened-edge volumes add the deepened perimeter ring. Costs use the calculator's line items and default rates (see the methodology) and vary by region and season. Code references: IRC section R506 (slabs on ground), section R403 (footings) and Table R402.2 (concrete strength). Slab and joint practice follows ACI 302.1R and ACI 332 (residential concrete); ready-mix ordering follows ASTM C94.