12x20 Concrete Slab: Cost, Yards & Bags of Concrete
Quick answer: a 12 × 20 ft slab (240 sq ft) 4 inches thick needs 2.96 yd³ of concrete, 3.26 yd³ with 10% waste (order 3.5 yd³ of ready-mix) or 147 bags of 80-lb mix. Installed, it costs about $1,770 – $3,140 ($7.00 – $13.50 per sq ft); DIY materials run $750 – $1,130.
A 12 × 20 ft slab (240 sq ft) is a good-sized patio for a dining table and a lounge area, or the floor of a large shed or single carport. On a patio, the 12-ft depth from the house gives room to pull chairs out; the 20-ft run gets two control joints so the panels stay close to square.
Concrete for a 12x20 Slab at 4, 5 and 6 Inches
| Thickness | Net volume | With 10% waste | Ready-mix order | 80-lb bags | 60-lb bags | 40-lb bags | Installed |
|---|---|---|---|---|---|---|---|
| 4 in (typical) | 2.96 yd³ | 3.26 yd³ | 3.5 yd³ | 147 | 196 | 294 | $1,770–$3,140 |
| 5 in | 3.70 yd³ | 4.07 yd³ | 4.25 yd³ | 184 | 245 | 367 | $1,890–$3,290 |
| 6 in | 4.44 yd³ | 4.89 yd³ | 5 yd³ | 220 | 294 | 440 | $2,010–$3,450 |
How Much Extra to Order
The table above adds 10%, the usual allowance for a formed slab on a graded base. Pick less only for rigid forms on a laser-flat base, more for soft or uneven ground:
| Waste allowance (4 in slab) | Concrete | 80-lb bags |
|---|---|---|
| 0% (exact volume) | 2.96 yd³ | 134 |
| 5% | 3.11 yd³ | 140 |
| 10% | 3.26 yd³ | 147 |
| 15% | 3.41 yd³ | 154 |
Bags or a Ready-Mix Truck?
| Option | What you buy | Concrete bill |
|---|---|---|
| 80-lb bags | 147 bags at $6.50 + mixer rental $75 | $1,031 |
| Ready-mix | 3.5 yd³ at $150 + $120 delivery + $140 short-load fee | $785 |
Ready-mix is $246 cheaper here and saves about 5.9 hours of mixing 147 bags. Bags only win below about 1.3 yd³, once the delivery and short-load fees are counted. Try your own prices in the bags vs. ready-mix calculator.
What a 12x20 Slab Costs
| Line item (4 in slab) | Basis | Cost |
|---|---|---|
| Ready-mix concrete + delivery | 3.26 yd³ at $150 + $120 | $609 |
| Forms | 64 lin ft at $2.20 | $141 |
| Vapor barrier | 240 sq ft at $0.35 | $84 |
| #4 rebar at 18 in | 240 sq ft at $1.25 | $300 |
| Broom finish | 240 sq ft at $0.75 | $180 |
| Labor (place & finish) | 240 sq ft at $5.00 | $1,200 |
| Direct cost | Before overhead and profit | $2,514 |
- Installed price: $1,770 – $3,140. The low end is a plain slab with lower-cost labor; the high end is the full scope above plus 25% contractor overhead and profit.
- Site prep and gravel base (excavation and 4 in of compacted stone): about $340 – $790 more.
- Stamped finish instead of broom: about $3,390 – $5,170 installed.
- Permit, where one is needed: about $50 – $350.
Have a contractor's price? Put it into the concrete quote checker, or compare two or three bids side by side there.
Materials List for a 12 × 20 ft Slab
| Concrete | 3.5 yd³ ready-mix, or 147 × 80-lb / 294 × 40-lb bags |
|---|---|
| Gravel base (4 in compacted) | 3.41 yd³, about 4.8 tons |
| Forms | 6 × 2x4 boards 12 ft long, 24 stakes, 48 screws (64 lin ft) |
| Rebar (#4 at 18 in) | 349 lin ft = 18 × 20-ft sticks, 35 chairs |
| Or wire mesh (6x6) | 9 sheets of 5 × 10 ft, about $108 – $180 |
| Vapor barrier | 240 sq ft plus 6 in laps |
| Control joints | 6 panels of about 6.7 × 6 ft: 44 lin ft of saw cut, 1 in deep |
Recommended Specs for a Patio
| Thickness | 4 in |
|---|---|
| Strength | 3,000 PSI; 3,500 PSI air-entrained where it freezes |
| Base | 4 to 6 in of compacted crushed stone |
| Reinforcement | 6x6 wire mesh or fiber mix; #3-#4 rebar on expansive clay |
| Drainage | 1/8 to 1/4 in per foot away from the house |
- Slope the patio away from the house, about 1/4 in per foot, so the 12-ft depth drops roughly 3 in.
- Want a stamped or colored finish? The stamped cost row below shows what it adds for this size.
- Keep the patio top 1-2 in below the door threshold and 6 in below wood siding.
Pour Day for a 12x20 Slab
- Crew time: about 8 crew-hours to set forms, place, finish and cut joints (0.035 h per sq ft for a broom finish).
- Trucks: 3.5 yd³ is 1 truck, about 18–28 minutes of unloading in total. Concrete must be placed within 90 minutes of batching.
- Reach: a truck chute reaches about 15–20 ft, enough for this slab if the truck can park beside it. If it can't, budget wheelbarrows or a line pump.
- Joints: saw-cut 4–12 hours after finishing with a wet saw, or 1–4 hours with an early-entry saw.
- Curing: foot traffic after 24–48 hours, cars after 7 days, full strength at 28 days.
Price Your Own 12x20 Slab
The calculator below starts at 12 × 20 ft, 4 inches. Change the thickness, finish or reinforcement, pick your state for local prices, and the Live Estimate updates with a blueprint of the joint and rebar layout.
Concrete Slab Cost & Volume Estimator
Precision multi-layer project budgeting: Materials, Sub-Base, Reinforcement, Labor & Equipment
Picking a location sets the ready-mix, excavation and labor rates below to that area's typical level. You can still type your own local prices afterwards. Slab prices by state.
Closer spacing uses more steel and raises the rebar cost.
Not charged when bagged concrete is selected.
A scaled plan of your slab: rebar grid at the spacing you chose, support chairs about every 3 to 4 ft, and the recommended control-joint saw cuts (2.5 times the thickness in inches, in feet, capped at 15 ft). It redraws as you change the inputs.
| Scope Item / Material Description | Amount |
|---|
Enter the total your contractor quoted. We compare it with a benchmark for the same slab and scope you set in the calculator: materials at your entered prices plus labor at the regional standard rate, before any overhead or profit. Make sure the calculator scope (base, rebar, finish, pump, demolition) matches what the bid includes.
Optional: enter the bid's line items to see where the markup sits
Enter the forecast for pour day. The check follows ACI 306R (cold weather) and ACI 305R (hot weather) and estimates the surface evaporation rate that drives plastic shrinkage cracking.
12x20 Slab Questions
About $1,770 to $3,140 installed for a 4-inch slab with forms, a broom finish and labor ($7.00 to $13.50 per sq ft), at U.S. average prices. DIY materials run about $750 to $1,130. Site prep and a gravel base add about $340 to $790.
At 4 inches thick with 10% waste: 147 bags of 80-lb mix, 196 bags of 60-lb mix or 294 bags of 40-lb mix. At 6 inches: 220 bags of 80-lb mix.
At 4 inches, 2.96 cubic yards (order 3.50 with waste); at 5 inches, 3.70 (order 4.25); at 6 inches, 4.44 (order 5.00). Ready-mix is sold in quarter yards, so round up.
Ready-mix. 3.5 yards delivered costs about $785, while 147 bags would cost about $1,031 and take roughly 6 hours of mixing. Bags only win below about 1.3 cubic yards.
A #4 grid at 18 inches each way needs about 349 linear feet of bar including laps: 18 twenty-foot sticks and about 35 chairs. Welded wire mesh instead takes 9 sheets of 5 x 10 ft.
A stamped finish runs about $6 to $12 per sq ft, against $0.50 to $0.80 for a broom finish. The cost section on this page shows the full installed range for a stamped 12 × 20 slab.