30x40 Concrete Slab: Cost, Yards & Bags of Concrete
Quick answer: a 30 × 40 ft slab (1,200 sq ft) 6 inches thick needs 22.22 yd³ of concrete, 24.44 yd³ with 10% waste (order 24.5 yd³ of ready-mix) or 1,100 bags of 80-lb mix. Installed, it costs about $9,190 – $16,140 ($7.50 – $13.50 per sq ft); DIY materials run $4,090 – $6,010.
A 30 × 40 ft slab (1,200 sq ft) is the most common pole barn, metal building and barndominium size. At 6 in it is 24.44 yd³ of concrete in 4 trucks, a pour that needs a full crew, a plan for truck access, and usually a pump or a buggy.
Concrete for a 30x40 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 | 14.81 yd³ | 16.30 yd³ | 16.5 yd³ | 734 | 978 | 1,467 | $7,970–$14,620 |
| 5 in | 18.52 yd³ | 20.37 yd³ | 20.5 yd³ | 917 | 1,223 | 1,834 | $8,580–$15,380 |
| 6 in (typical) | 22.22 yd³ | 24.44 yd³ | 24.5 yd³ | 1,100 | 1,467 | 2,200 | $9,190–$16,140 |
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 (6 in slab) | Concrete | 80-lb bags |
|---|---|---|
| 0% (exact volume) | 22.22 yd³ | 1,000 |
| 5% | 23.33 yd³ | 1,050 |
| 10% | 24.44 yd³ | 1,100 |
| 15% | 25.56 yd³ | 1,150 |
Bags or a Ready-Mix Truck?
| Option | What you buy | Concrete bill |
|---|---|---|
| 80-lb bags | 1,100 bags at $6.50 + mixer rental $75 | $7,225 |
| Ready-mix | 24.5 yd³ at $150 + $120 delivery | $3,795 |
Ready-mix, without question. 1,100 bags would cost $7,225 and take about 44 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 (4 trucks of about 8–10 yd³ each). Try your own prices in the bags vs. ready-mix calculator.
What a 30x40 Slab Costs
| Line item (6 in slab) | Basis | Cost |
|---|---|---|
| Ready-mix concrete + delivery | 24.44 yd³ at $150 + $120 | $3,787 |
| Forms | 140 lin ft at $2.20 | $308 |
| Vapor barrier | 1,200 sq ft at $0.35 | $420 |
| #4 rebar at 18 in | 1,200 sq ft at $1.25 | $1,500 |
| Broom finish | 1,200 sq ft at $0.75 | $900 |
| Labor (place & finish) | 1,200 sq ft at $5.00 | $6,000 |
| Direct cost | Before overhead and profit | $12,915 |
- Installed price: $9,190 – $16,140. 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 $1,680 – $3,960 more.
- Thickened edge (12 in wide, 12–18 in deep) if the building sits on the slab: 2.8–5.5 yd³ more concrete, about $420 – $830, 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 30 × 40 ft Slab
| Concrete | 24.5 yd³ ready-mix, or 1,100 × 80-lb / 2,200 × 40-lb bags |
|---|---|
| Gravel base (4 in compacted) | 17.04 yd³, about 23.9 tons |
| Forms | 14 × 2x6 boards 12 ft long, 58 stakes, 116 screws (140 lin ft) |
| Rebar (#4 at 18 in) | 1,745 lin ft = 88 × 20-ft sticks, 154 chairs |
| Rebar (#4 at 12 in, heavy loads) | 2,602 lin ft = 131 × 20-ft sticks, 88 chairs |
| Or wire mesh (6x6) | 35 sheets of 5 × 10 ft, about $420 – $700 |
| Vapor barrier | 1,200 sq ft plus 6 in laps |
| Control joints | 6 panels of about 13.3 × 15 ft: 100 lin ft of saw cut, 1.5 in deep |
Recommended Specs for a Shop Or Barn Floor
| Thickness | 6 in for shops with heavy vehicles, lifts or machinery; 4-5 in for light storage |
|---|---|
| Strength | 4,000 PSI for heavy trucks and de-icing salt; 3,500 PSI air-entrained minimum |
| Base | 4 to 6 in of compacted crushed stone, plus a vapor barrier if the building will be heated |
| Reinforcement | #4 rebar at 12-18 in on chairs; heavier where an engineer specifies it |
| Edge | Thickened edge or footings at the walls; post-frame buildings set their posts on separate footings |
- Metal and post-frame buildings set their columns on separate footings or piers; pour those first and form the slab around them.
- For a barndominium or any heated space, add a vapor barrier and, where it freezes, perimeter insulation.
- Ask about a laser screed or a second finishing crew: 1,200 sq ft is a lot of surface to finish before it sets.
Pour Day for a 30x40 Slab
- Crew time: about 42 crew-hours to set forms, place, finish and cut joints (0.035 h per sq ft for a broom finish).
- Trucks: 24.5 yd³ is 4 trucks, about 123–196 minutes of unloading in total. Concrete must be placed within 90 minutes of batching.
- Reach: a truck chute reaches about 15–20 ft. At 30 ft wide, the middle is out of reach from one side, so plan access on two sides, a buggy, or a pump ($600 – $1,200).
- 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 30x40 Slab
The calculator below starts at 30 × 40 ft, 6 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.
30x40 Slab Questions
About $9,190 to $16,140 installed for a 6-inch slab with forms, a broom finish and labor ($7.50 to $13.50 per sq ft), at U.S. average prices. DIY materials run about $4,090 to $6,010. Site prep and a gravel base add about $1,680 to $3,960.
At 4 inches thick with 10% waste: 734 bags of 80-lb mix, 978 bags of 60-lb mix or 1,467 bags of 40-lb mix. At 6 inches: 1,100 bags of 80-lb mix.
At 4 inches, 14.81 cubic yards (order 16.50 with waste); at 5 inches, 18.52 (order 20.50); at 6 inches, 22.22 (order 24.50). Ready-mix is sold in quarter yards, so round up.
Ready-mix. 24.5 yards delivered costs about $3,795, while 1,100 bags would cost about $7,225 and take roughly 44 hours of mixing. Bags only win below about 1.3 cubic yards.
A #4 grid at 18 inches each way needs about 1,745 linear feet of bar including laps: 88 twenty-foot sticks and about 154 chairs. Welded wire mesh instead takes 35 sheets of 5 x 10 ft.
The installed range on this page is for a 6-inch slab with forms, vapor barrier, #4 rebar and a broom finish. Excavation, gravel, a thickened edge and a pump are extra; the page lists each.