20x30 Concrete Slab: Cost, Yards & Bags of Concrete
Quick answer: a 20 × 30 ft slab (600 sq ft) 5 inches thick needs 9.26 yd³ of concrete, 10.19 yd³ with 10% waste (order 10.25 yd³ of ready-mix) or 459 bags of 80-lb mix. Installed, it costs about $4,420 – $7,850 ($7.00 – $13.50 per sq ft); DIY materials run $1,870 – $2,830.
A 20 × 30 ft slab (600 sq ft) is a two-car garage with a full-width workshop at the back, or a large patio and outdoor kitchen. At 5 in thick it is 10.19 yd³ with waste, more than one standard truck carries, so the plant sends 2 trucks and the timing between them matters.
Concrete for a 20x30 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 | 7.41 yd³ | 8.15 yd³ | 8.25 yd³ | 367 | 489 | 734 | $4,110–$7,470 |
| 5 in (typical) | 9.26 yd³ | 10.19 yd³ | 10.25 yd³ | 459 | 612 | 917 | $4,420–$7,850 |
| 6 in | 11.11 yd³ | 12.22 yd³ | 12.25 yd³ | 550 | 734 | 1,100 | $4,720–$8,230 |
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) | 9.26 yd³ | 417 |
| 5% | 9.72 yd³ | 438 |
| 10% | 10.19 yd³ | 459 |
| 15% | 10.65 yd³ | 480 |
Bags or a Ready-Mix Truck?
| Option | What you buy | Concrete bill |
|---|---|---|
| 80-lb bags | 459 bags at $6.50 + mixer rental $75 | $3,059 |
| Ready-mix | 10.25 yd³ at $150 + $120 delivery | $1,658 |
Ready-mix, without question. 459 bags would cost $3,059 and take about 18 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 20x30 Slab Costs
| Line item (5 in slab) | Basis | Cost |
|---|---|---|
| Ready-mix concrete + delivery | 10.19 yd³ at $150 + $120 | $1,648 |
| Forms | 100 lin ft at $2.20 | $220 |
| Vapor barrier | 600 sq ft at $0.35 | $210 |
| #4 rebar at 18 in | 600 sq ft at $1.25 | $750 |
| Broom finish | 600 sq ft at $0.75 | $450 |
| Labor (place & finish) | 600 sq ft at $5.00 | $3,000 |
| Direct cost | Before overhead and profit | $6,278 |
- Installed price: $4,420 – $7,850. 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 $840 – $1,980 more.
- Thickened edge (12 in wide, 12–18 in deep) if the building sits on the slab: 2.3–4.2 yd³ more concrete, about $340 – $640, plus trenching and two #4 bars around the edge.
- 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 × 30 ft Slab
| Concrete | 10.25 yd³ ready-mix, or 459 × 80-lb / 917 × 40-lb bags |
|---|---|
| Gravel base (4 in compacted) | 8.52 yd³, about 11.9 tons |
| Forms | 10 × 2x6 boards 12 ft long, 40 stakes, 80 screws (100 lin ft) |
| Rebar (#4 at 18 in) | 883 lin ft = 45 × 20-ft sticks, 77 chairs |
| Or wire mesh (6x6) | 20 sheets of 5 × 10 ft, about $240 – $400 |
| Vapor barrier | 600 sq ft plus 6 in laps |
| Control joints | 6 panels of about 10 × 10 ft: 70 lin ft of saw cut, 1.25 in deep |
Recommended Specs for a Garage Floor
| Thickness | 4 in minimum, 5 in typical; 6 in for heavy trucks or a lift |
|---|---|
| Strength | 3,500 PSI air-entrained (IRC minimum for garage floors in severe weathering areas) |
| Base | 4 to 6 in of compacted crushed stone, plus a 10-mil vapor barrier under heated or finished garages |
| Reinforcement | #4 rebar at 18 in on chairs |
| Edge | Monolithic thickened edge (12 in wide, 12-18 in deep) or a stem wall on footings, per your permit |
- Ask the plant to space the second truck so the first load is still workable when it arrives; a cold joint between loads is a weak spot.
- The 30-ft length gets two interior control joints; mark them on the forms before the pour.
- For a workshop end, plan floor drains or a sleeve for a future air line before the pour.
Pour Day for a 20x30 Slab
- Crew time: about 21 crew-hours to set forms, place, finish and cut joints (0.035 h per sq ft for a broom finish).
- Trucks: 10.25 yd³ is 2 trucks, about 51–82 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 20x30 Slab
The calculator below starts at 20 × 30 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.
20x30 Slab Questions
About $4,420 to $7,850 installed for a 5-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 $1,870 to $2,830. Site prep and a gravel base add about $840 to $1,980.
At 4 inches thick with 10% waste: 367 bags of 80-lb mix, 489 bags of 60-lb mix or 734 bags of 40-lb mix. At 5 inches: 459 bags of 80-lb mix.
At 4 inches, 7.41 cubic yards (order 8.25 with waste); at 5 inches, 9.26 (order 10.25); at 6 inches, 11.11 (order 12.25). Ready-mix is sold in quarter yards, so round up.
Ready-mix. 10.25 yards delivered costs about $1,658, while 459 bags would cost about $3,059 and take roughly 18 hours of mixing. Bags only win below about 1.3 cubic yards.
A #4 grid at 18 inches each way needs about 883 linear feet of bar including laps: 45 twenty-foot sticks and about 77 chairs. Welded wire mesh instead takes 20 sheets of 5 x 10 ft.
At 4 inches, 7.41 cubic yards net (8.15 with waste). At 5 inches, 9.26 net and 10.19 with waste, so order 10.25 yards, which takes 2 trucks.