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.
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 30×40 slab needing 24.5 cubic yards takes 3 truckloads. Coordinate deliveries so trucks arrive as you finish placing each load.
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 rebar for slabs holding trucks, lifts, or heavy machinery. A 4-inch slab is only suitable for light foot-traffic buildings.
How much does a 30×40 concrete slab cost?
Materials run $3,330–$3,885 at 6 inches. A fully installed 30×40 slab costs $7,200–$18,000 (about $6–$15 per sq ft) including grading, gravel base, reinforcement, labor, and finishing.