20×20 Concrete Slab Cost: 2-Car Garage & Large Patio Guide

Quick answer: a 20x20 concrete slab (400 sq ft) costs about $2,810 to $5,060 installed as a 4-inch patio, and $3,010 to $5,320 as a 5-inch 2-car garage floor. That is $7 to $13 per sq ft. A patio needs 5.43 yd³ of concrete with 10% waste; a garage floor needs 6.79 yd³. At either thickness one ready-mix truck is far cheaper than bags.

A 20x20 is the size where a backyard slab turns into a real construction job. It is the footprint of a small two-car garage and of a large patio with room for a dining set, a grill and a seating area. The two uses look alike on a plan, but a garage floor is thicker, stronger and built to a different standard. This guide prices both side by side, explains why bags stop making sense at this size, and walks through what pour day looks like.

Patio vs. Garage Floor: Side by Side

The extra inch for a garage costs only about $204 at full scope, because it changes the concrete line and nothing else. That is cheap insurance. IRC section R506 allows a 3.5-inch floor slab, but car tires put concentrated loads near the edges and joints, so most builders treat 5 inches as the practical minimum for a garage floor (our thickness guide explains why). If the garage will hold a heavy pickup, a camper or a lift, look at 6 inches ($3,220 to $5,570 installed).

Line Items for Each Version

Both columns use the full scope from our slab cost calculator: ready-mix, forms, vapor barrier, a #4 rebar grid, broom finish and labor at $5.00/sq ft. A plain patio would drop the vapor barrier and might swap rebar for mesh.

Two garage extras are not in that table. Moving from 3,000 to 3,500 PSI adds about $20 to $70 for this volume. A smooth steel-trowel finish, which many people prefer inside a garage because it sweeps clean, adds about $200 over broom. Labor is about 49% of the direct cost on the patio and 47% on the garage floor, which is why the DIY savings at this size are real but so is the workload.

Why Ready-Mix Clearly Wins at 20x20

At 4 inches, a 20x20 needs 245 bags of 80-lb mix. That costs about $1,593, against $935 for 5.43 yd³ of ready-mix with delivery. The truck saves about $658 before you even count the mixer rental, and it saves you from lifting about 9.8 tons of bags. At 5 inches the gap grows to $850 (306 bags).

There is a bigger reason than price. Hand-mixing hundreds of bags takes hours, so the first batches start to stiffen before the last ones go in. You end up with cold joints and a patchy finish across a slab that should look like one piece. A 5.43 yd³ order is also at or near most plants' usual minimum load of 5–6 yards, so short-load fees are small or zero. If your supplier's minimum is 6 yards, the per-yard short-load charge on a 4-inch patio is only about $17 to $26. The ready-mix calculator rounds your order to what the plant will sell.

Pour Day for a 20x20

• One truck. Mixer trucks carry about 8 to 10 yd³, so either version fits in a single load. Unloading takes about 25 to 45 minutes for the patio and 35 to 55 minutes for the garage floor at a normal pace. Most suppliers include a set amount of unloading time and charge for extra waiting, so be ready when the truck arrives.
• Access. The chute reaches about 15 to 20 ft from where the truck parks. The far side of a 20-ft slab is often beyond that, so plan for wheelbarrows or a line pump ($400 to $800 for the job).
• Crew. A contractor spends about 14 crew-hours placing and finishing 400 sq ft, so a crew of 3 to 4 is busy most of a day.
• Joints. One joint each way at the 10-ft mark gives four 10x10 panels, inside the spacing limit for both 4 and 5 inches. That is 40 linear ft of joint, cut to a quarter of the slab depth. With a conventional saw, cut within about 4 to 12 hours of finishing.
• Curing. Foot traffic after 24 to 48 hours; wait about 7 days before parking a car on a new garage floor.

Large Patio Upgrades

Finish choice moves the price of a 400 sq ft patio more than thickness does. Exposed aggregate adds about $1,100 in finish cost over broom. A fully stamped 20x20 runs about $13 to $22 per sq ft installed, or $5,200 to $8,800. Many homeowners stamp a border and broom-finish the middle to get most of the look for less. Over 20 ft, the drainage slope drops the outer edge about 2.5 to 5 inches below the house side. More in the patio slab cost guide.

What Else Changes the Price

• Site prep: excavation and a compacted gravel base add about $560 to $1,320. A garage floor should always have a compacted base; the gravel base calculator sizes it.
• Removing an old slab: about $3 to $4 per sq ft, or $1,200 to $1,600.
• Permits: a garage almost always needs one; a detached patio often does not. Flatwork permits run roughly $50 to $350, and a garage building permit costs more.

DIY or Hire a Pro?

A 20x20 is a big step up from a shed pad. Screeding a 20-ft span needs a long straightedge and two people working in step, and a full truck has to be placed and finished before it sets. DIY can save the labor line, roughly $1,400 to $2,000, but only if you have experienced help. For a garage floor tied to footings and an inspection, most homeowners hire out. Check any quote with the concrete quote checker, and see our DIY guide. Other sizes: 12x12 and 24x24.

Sources and Method

Quantities use length × width × thickness plus 10% waste. Costs use the calculator's line items and default rates (see our methodology) and vary by region and season. Concrete strength for garage floors follows IRC Table R402.2; slab and base guidance follows IRC section R506 and ACI 302.1R; ready-mix delivery practice follows ASTM C94.