How Much Concrete for a 20x30 Slab? Yards and Trucks
How much concrete for a 20x30 slab works out to about 7.41 cubic yards at 4 inches thick, 9.26 cubic yards at 5 inches, and 11.11 cubic yards at 6 inches. The slab covers 600 square feet, a common size for a two-car garage, a pole barn floor, or a large patio. After a 5 to 10 percent allowance, plan on roughly 8 to 8.25 yards for a 4-inch slab and 12 yards for a 6-inch slab.
Concrete calculator →How the number is calculated
- Area: 20 × 30 = 600 square feet.
- Thickness in feet: 6 inches = 0.5 ft.
- Volume: 600 × 0.5 = 300 cubic feet.
- Cubic yards: 300 ÷ 27 = 11.11 yards.
For other thicknesses, swap in the thickness in feet, or use the concrete calculator.
Yards by thickness
| Thickness | Cubic yards | With 5% | With 10% |
|---|---|---|---|
| 4 inches | 7.41 | 7.78 | 8.15 |
| 5 inches | 9.26 | 9.72 | 10.19 |
| 6 inches | 11.11 | 11.67 | 12.22 |
How many trucks
A standard mixer truck usually carries up to about 9 to 11 cubic yards, depending on the truck and local weight limits. That means:
- 4-inch slab: one truck in most cases.
- 5-inch slab: one full truck, sometimes with a small second load.
- 6-inch slab: usually two trucks, or one full load plus a short load.
When two trucks are needed, ask the plant to space them so the second arrives before the first section stiffens. Our page on how many yards are in a concrete truck covers capacity in more detail.
Thickness for a garage or barn
A 20x30 garage is usually poured at 4 to 6 inches, with many builders choosing 5 or 6 inches for pickups, SUVs, and heavy storage. A pole barn that will see a tractor or loaded trailer leans toward 6 inches or more, plus reinforcement. Thickened edges under walls or a perimeter footing poured with the slab add volume, often 1.5 to 3 yards for a 20x30 footprint.
Joints and reinforcement
At 600 square feet, joints matter. For a 5-inch slab, a common joint spacing is about 10 to 12 feet, which could mean a grid of roughly 10 by 10 or 10 by 15 foot panels. Cut joints to about one quarter of the slab depth within the first day. Rebar or welded wire set mid-depth keeps any cracks tight. See control joint spacing for the rules of thumb.
Getting concrete to the forms
If the truck can back up to the slab, the chute can reach most of a 20x30 pour with some wheelbarrow work. If the slab is behind a house, inside a building shell, or up a slope, a line pump or boom pump may pay for itself in labor and finish quality. See concrete pump truck cost for what drives that price.
Base and grade check
A 600 square foot slab needs about 7.4 cubic yards of gravel base at 4 inches, which is roughly 10 to 11 tons depending on the stone. A level, compacted base matters as much as the concrete order: an inch of extra depth across the whole slab adds almost 2 yards. Check grade with a string line or laser before ordering.
Crew and timing
Plan a crew of at least four to five for a 7 to 12 yard slab: someone on the chute, two or more moving and screeding concrete, and someone floating and edging. Pour early in the day in hot weather, and have curing compound or wet burlap ready.
Bottom line
A 20x30 slab needs about 7.41 cubic yards at 4 inches, 9.26 at 5 inches, and 11.11 at 6 inches. Add 5 to 10 percent, round up to the plant’s next increment, and count trucks before the pour. A 4-inch slab is usually one truck; a 6-inch garage or barn floor often needs two.
Questions people ask
How many yards of concrete for a 20x30 slab 4 inches thick?
About 7.41 cubic yards, or about 8 to 8.25 yards after a 5 to 10 percent allowance.
How many yards for a 20x30 slab 6 inches thick?
About 11.11 cubic yards, or roughly 12 yards with extra. That usually takes two trucks.
How thick should a 20x30 garage slab be?
Four to six inches is common, with 5 or 6 inches preferred for heavier vehicles. Follow local code.
How much gravel goes under a 20x30 slab?
About 7.4 cubic yards at 4 inches deep, roughly 10 to 11 tons depending on the material.
Can I pour a 20x30 slab myself?
It is possible with an experienced crew, but 7 to 12 yards sets quickly. Many homeowners hire a finisher for slabs this size.
Related guides
- How Much Concrete for a 24x24 Garage Slab?
- How Many Yards in a Concrete Truck? Mixer Sizes
- Concrete Pump Truck Cost: Line vs Boom Pumps
- Control Joint Spacing for a Concrete Driveway
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