Concrete for a 12×20 slab

What a 12×20 slab is usually for

• Single-car garage floor
• Two-car parking pad (tandem)
• Long workshop
• Carport slab

This is the standard single-car garage footprint — 12 feet of width and 20 of depth is what most residential plans use, giving door clearance on both sides and room for a workbench across the end. Once a structure is going on top, the slab stops being a simple pad: garage slabs normally want a thickened perimeter edge to carry the wall load, which is concrete over and above the flat-slab volume calculated here.

Thickness is the decision on a 12×20 parking slab

Thickness is the one input that changes the order most, and it is chosen by what will sit on the slab rather than by budget. Here is the same 240 sq ft footprint at each common depth:

Going from 4″ to 6″ on this slab adds roughly $248 in materials — about 43% more. That is the entire cost of the decision, and it is why the advice for anything carrying a vehicle is to just pour the extra inches. Removing and replacing a cracked slab costs many times the difference.

The 3.5″ row is not a recommendation, it is a reality check: a “4 inch” slab poured on a subgrade that is a little high in places genuinely finishes at 3.5″ in those spots. If your base is not dead flat, order to the thicker figure. More on this in concrete slab thickness: 4″ vs 6″.

Hiring this out vs pouring it yourself

A professionally installed basic slab runs roughly $4–$8 per square foot including labour, so for 240 sq ft:

The gap is labour, and it is real — but so is the risk transfer. A contractor who gets the finish wrong fixes it; if you do, you own it. See DIY vs hiring a pro and slab cost by size for the full breakdown of where each route makes sense.

Reinforcement on a 12×20 vehicle slab

Reinforcement does not stop concrete cracking; it holds the crack tight so the slab keeps acting as one piece. For a residential slab-on-grade of 240 sq ft, either of these is conventional:

Whichever you use, it has to sit in the middle of the slab to do anything. Mesh laid on the ground and walked over during the pour is the most common wasted material on a DIY slab — it ends up at the bottom, where it carries no tension at all. Use chairs or bar supports, and lift as you go if you have no other option.

Joint layout for a 12×20 drive or parking pad

Concrete shrinks as it cures and it will crack — the only question is whether it cracks where you decided or where it chose. Control joints are the decision. The rule of thumb is a maximum spacing in feet of about two to three times the slab thickness in inches, so at 4″ that is 8–12 ft.

That layout keeps every panel at 1.2:1 or better, inside the 1.5:1 limit that stops a long thin panel cracking across its middle. Cut or tool the joints the same day — within roughly 6 to 12 hours of finishing, once the surface will carry a saw without ravelling but before shrinkage has done its work.

Bags vs ready-mix once wheels are involved

At 2.96 cubic yards, this slab sits in the awkward middle band where the short-load fee decides it. Here is the comparison with the fees included:

Ready-mix is the better call at this size, even carrying the short-load fee. Beyond the price, a truck delivers the whole slab as one continuous placement — no cold joints between batches, no risk of the first section setting while you mix the last, and a consistent mix throughout. Mixing 147 bags to match it is a full day of heavy work with a worse result.

The short-load fee is what people miss. Suppliers charge it below roughly 4 cubic yards, typically $50–$150, because sending a truck out costs them the same whether it is full or not. Always ask for the delivered total including fees rather than the per-yard rate. The full reasoning is in bags vs ready-mix concrete.

The pour itself

Be honest about that number.147 bags is past the point where a one-person mix-and-pour produces a good slab. Concrete starts stiffening within about 30 to 60 minutes in warm weather, so a long mixing session leaves cold joints between what you placed first and what you placed last — visible lines that are also weak lines. If you are set on bags, get help and run two mixers; otherwise price the truck.

Budget for the tools as well as the concrete: a mixer runs about $70/day, a plate compactor for the base about $80/day, and a concrete saw for the joints about $60/day.

Watch out on a slab this size

Every number on this page is for a flat slab of constant thickness. A thickened edge or a footing detail can add 15 percent or more to the order, and it is not optional under a load-bearing wall. Get the footing detail from your plans or your building department before you order.

The arithmetic

Volume = 12 × 20 × (4 ÷ 12) = 80 ft³, then ÷ 27 = 2.96 cubic yards. An 80 lb bag yields 0.60 ft³, so 80 ÷ 0.60 = 134 bags before waste; add 10% and round up to 147 bags. Ready-mix rounds up to the quarter yard, giving 3.5 yd³. Every constant behind those steps is documented in how we calculate, and the reasoning is in how much concrete do I need?

Other slab sizes

• Concrete for a 10×16 slab
• Concrete for a 10×20 slab
• Concrete for a 12×12 slab
• Concrete for a 12×14 slab
• Concrete for a 12×16 slab
• Concrete for a 12×24 slab
• Concrete for a 16×16 slab
• Concrete for a 16×20 slab
• Concrete for a 20×20 slab
• Concrete for a 20×24 slab

See all slab sizes →