20x40 Concrete Slab: Cost, Yards & Bags of Concrete
Quick answer: a 20 × 40 ft slab (800 sq ft) 5 inches thick needs 12.35 yd³ of concrete, 13.58 yd³ with 10% waste (order 13.75 yd³ of ready-mix) or 612 bags of 80-lb mix. Installed, it costs about $5,820 – $10,380 ($7.00 – $13.00 per sq ft); DIY materials run $2,420 – $3,700.
A 20 × 40 ft slab (800 sq ft) is a two-car-wide driveway: two cars side by side, two deep. At 5 in it is 13.58 yd³ with waste in 2 trucks, and the joint grid splits it into 8 panels.
Concrete for a 20x40 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 | 9.88 yd³ | 10.86 yd³ | 11 yd³ | 489 | 652 | 978 | $5,410–$9,870 |
| 5 in (typical) | 12.35 yd³ | 13.58 yd³ | 13.75 yd³ | 612 | 815 | 1,223 | $5,820–$10,380 |
| 6 in | 14.81 yd³ | 16.30 yd³ | 16.5 yd³ | 734 | 978 | 1,467 | $6,230–$10,890 |
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 (5 in slab) | Concrete | 80-lb bags |
|---|---|---|
| 0% (exact volume) | 12.35 yd³ | 556 |
| 5% | 12.96 yd³ | 584 |
| 10% | 13.58 yd³ | 612 |
| 15% | 14.20 yd³ | 639 |
Bags or a Ready-Mix Truck?
| Option | What you buy | Concrete bill |
|---|---|---|
| 80-lb bags | 612 bags at $6.50 + mixer rental $75 | $4,053 |
| Ready-mix | 13.75 yd³ at $150 + $120 delivery | $2,183 |
Ready-mix, without question. 612 bags would cost $4,053 and take about 24 hours of mixing, far longer than concrete stays workable, so the slab would end up with cold joints. Ready-mix trucks bring it as fast as the crew can place it (2 trucks of about 8–10 yd³ each). Try your own prices in the bags vs. ready-mix calculator.
What a 20x40 Slab Costs
| Line item (5 in slab) | Basis | Cost |
|---|---|---|
| Ready-mix concrete + delivery | 13.58 yd³ at $150 + $120 | $2,157 |
| Forms | 120 lin ft at $2.20 | $264 |
| Vapor barrier | 800 sq ft at $0.35 | $280 |
| #4 rebar at 18 in | 800 sq ft at $1.25 | $1,000 |
| Broom finish | 800 sq ft at $0.75 | $600 |
| Labor (place & finish) | 800 sq ft at $5.00 | $4,000 |
| Direct cost | Before overhead and profit | $8,301 |
- Installed price: $5,820 – $10,380. 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 $1,120 – $2,640 more.
- 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 20 × 40 ft Slab
| Concrete | 13.75 yd³ ready-mix, or 612 × 80-lb / 1,223 × 40-lb bags |
|---|---|
| Gravel base (4 in compacted) | 11.36 yd³, about 15.9 tons |
| Forms | 12 × 2x6 boards 12 ft long, 50 stakes, 100 screws (120 lin ft) |
| Rebar (#4 at 18 in) | 1,187 lin ft = 60 × 20-ft sticks, 98 chairs |
| Rebar (#4 at 12 in, heavy loads) | 1,727 lin ft = 87 × 20-ft sticks, 66 chairs |
| Or wire mesh (6x6) | 25 sheets of 5 × 10 ft, about $300 – $500 |
| Vapor barrier | 800 sq ft plus 6 in laps |
| Control joints | 8 panels of about 10 × 10 ft: 100 lin ft of saw cut, 1.25 in deep |
Recommended Specs for a Driveway
| Thickness | 4 in for cars; 5 in standard; 6 in for RVs and delivery trucks |
|---|---|
| Strength | 4,000 PSI air-entrained where de-icing salt is used, otherwise 3,500 PSI |
| Base | 4 to 6 in of compacted crushed stone |
| Reinforcement | #4 rebar at 18 in (12 in for heavy loads or cold climates) on chairs |
| Drainage | At least 1/8 in per foot, away from the garage door |
- Run a center joint down the length so each lane is its own row of panels.
- A driveway this size usually needs a permit and an approved street apron (curb cut); check before you form.
- Wait 7 days before parking on it, longer for trucks.
Pour Day for a 20x40 Slab
- Crew time: about 28 crew-hours to set forms, place, finish and cut joints (0.035 h per sq ft for a broom finish).
- Trucks: 13.75 yd³ is 2 trucks, about 69–110 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 20x40 Slab
The calculator below starts at 20 × 40 ft, 5 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.
20x40 Slab Questions
About $5,820 to $10,380 installed for a 5-inch slab with forms, a broom finish and labor ($7.00 to $13.00 per sq ft), at U.S. average prices. DIY materials run about $2,420 to $3,700. Site prep and a gravel base add about $1,120 to $2,640.
At 4 inches thick with 10% waste: 489 bags of 80-lb mix, 652 bags of 60-lb mix or 978 bags of 40-lb mix. At 5 inches: 612 bags of 80-lb mix.
At 4 inches, 9.88 cubic yards (order 11.00 with waste); at 5 inches, 12.35 (order 13.75); at 6 inches, 14.81 (order 16.50). Ready-mix is sold in quarter yards, so round up.
Ready-mix. 13.75 yards delivered costs about $2,183, while 612 bags would cost about $4,053 and take roughly 24 hours of mixing. Bags only win below about 1.3 cubic yards.
A #4 grid at 18 inches each way needs about 1,187 linear feet of bar including laps: 60 twenty-foot sticks and about 98 chairs. Welded wire mesh instead takes 25 sheets of 5 x 10 ft.
The installed range on this page is for a 5-inch driveway with forms, #4 rebar and a broom finish. Tear-out of an old driveway, excavation and the street apron are extra.