How Much Concrete for a 30×40 Slab?
Quick Answer
A 30×40 foot slab poured 6 inches thick needs 22.2 cubic yards of concrete. Order about 24.5 cubic yards with a 10% waste factor. This is a large commercial-scale pour that requires multiple ready-mix trucks.
- 6-inch slab: 22.2 cubic yards (600 cubic feet)
- Order ~24.5 cubic yards with 10% waste
- 4-inch version: 14.8 cubic yards
- Ready-mix cost: $3,330–$3,885 delivered
Formula
Multiply the slab dimensions in feet by the thickness in feet, then divide by 27. A 30×40 shop or barn slab is usually poured 5 to 6 inches thick.
Step-by-Step Calculation
- 1
Slab area
30 × 40 = 1,200 square feet
- 2
Volume at 6 inches
1,200 × 0.5 = 600 cubic feet
- 3
Convert to cubic yards
600 ÷ 27 = 22.2 cubic yards
- 4
Add 10% waste
22.2 × 1.10 = 24.5 cubic yards
30×40 Slab Concrete by Thickness
| Thickness | Cubic Feet | Cubic Yards | With 10% Waste | Ready-Mix Cost* |
|---|---|---|---|---|
| 4 inches | 400 | 14.8 | 16.3 | $2,220–$2,590 |
| 5 inches | 500 | 18.5 | 20.4 | $2,775–$3,240 |
| 6 inches | 600 | 22.2 | 24.5 | $3,330–$3,885 |
*Ready-mix at $150–$175 per cubic yard. Slabs over 10 cubic yards need multiple trucks and often a concrete pump for placement.
Pro Tips
- At 22+ cubic yards, this is a commercial-scale pour. You need 3 mixer trucks staged to arrive in sequence (10 yards, 10 yards, 4.5 yards) with 20-minute intervals. Have a dispatcher cell number and confirm arrival windows the morning of the pour.
- Hire a concrete pump truck ($400–$800 for this job) — at 1,200 square feet, there is no feasible way to wheel-barrow or chute this volume fast enough to prevent cold joints. The pump pays for itself in labor savings and pour quality.
- For a 30×40 shop or barn slab, install control joints every 10–12 feet in both directions (2 joints across the 30-foot width, 3 across the 40-foot length). Without these, the slab will crack randomly — with them, cracks follow the pre-scored lines.
- This slab size requires a building permit in most jurisdictions. Check setback requirements and verify the grade plan with your local building department before pouring — repositioning 24.5 cubic yards of concrete is not an option.
Key Cost Factors
- Ready-mix for 24.5 yards (with waste) at $110–$150/yd runs $2,700–$3,675. You get better per-yard pricing at this volume — most plants offer a discount above 20 yards. Rebar grid for 1,200 sq ft costs $600–$900. Sub-base gravel costs $500–$800. Total materials: $4,000–$5,700.
- Professional installation for a 30×40 slab runs $7–$12 per sq ft ($8,400–$14,400). At this scale, the labor component is significant — a crew of 6–8 people works for a full day. This is firmly in the 'hire a contractor' category unless you have commercial concrete experience.
- An often-overlooked cost: site preparation for a slab this size usually requires a bobcat or excavator for grading ($250–$400/day rental or $500–$1,000 for a contractor). Hand-grading 1,200 sq ft is impractical.
Common Mistakes to Avoid
- Assuming one truck covers it. A 30×40 slab at 4 inches is about 14.8 cubic yards — that is more than one standard truck holds. Coordinate multiple trucks so the pour stays continuous.
- Underbuilding the sub-base on 1,200 sq ft. At this scale, base preparation and compaction are everything. Any weak area becomes a crack or a low spot that holds water.
- Skipping engineered reinforcement. A slab this large for a shop or pole barn needs a designed rebar grid, and often a thickened footing under bearing walls.
- No control joint plan. Without a proper grid of saw-cut joints, a 1,200 sq ft slab will crack randomly and unpredictably.
Related Project Considerations
- Get the pour engineered. For a slab this size supporting a building, have the thickness, reinforcement, and footings designed for the loads and soil rather than guessing.
- Plan a concrete pump. Reaching all corners of a 30×40 pour is difficult with chutes alone; a pump keeps the pour fast and continuous, avoiding cold joints.
- Line up a finishing crew. Placing and finishing 15 yards before it sets requires several experienced hands and power troweling equipment — not a solo weekend job.
Frequently Asked Questions
How many concrete trucks for a 30×40 slab?
A standard ready-mix truck carries about 8 to 10 cubic yards. A 6-inch thick 30×40 slab needing about 24.5 cubic yards (with waste) takes 3 truckloads. Coordinate deliveries so trucks arrive as you finish placing each load — typically every 30–45 minutes. Late trucks cause cold joints between pours, which weaken the slab and create visible seams. Confirm scheduling with the batch plant the day before.
How thick should a 30×40 shop slab be?
Use 5 to 6 inches for a workshop or garage, and 6 inches or more with #4 rebar on 18-inch centers for slabs holding trucks, lifts, or heavy machinery. A 4-inch slab is only suitable for light foot-traffic buildings like garden sheds. For vehicle lifts, pour a thickened pad (8–12 inches) at the lift locations to handle the concentrated point loads without cracking the surrounding slab.
How much does a 30×40 concrete slab cost?
Materials run $3,330–$3,885 at 6 inches thick, covering concrete, reinforcement, and gravel. A fully installed 30×40 slab costs $7,200–$18,000 (about $6–$15 per sq ft) including grading, compacted gravel base, reinforcement, forming, labor, finishing, and control joints. Large slabs need control joints every 10–12 feet in both directions to manage shrinkage cracking — include these in the finishing spec.