How Much Concrete for a 30x40 Slab? Yards and Trucks

How much concrete for a 30x40 slab is about 14.81 cubic yards at 4 inches thick, 18.52 cubic yards at 5 inches, and 22.22 cubic yards at 6 inches. That is 1,200 square feet, a popular size for shops, pole barns, and detached garages. With a 5 to 10 percent allowance, most builders order about 16 yards for a 4-inch floor and 23.5 to 24.5 yards for a 6-inch floor, which means two to three trucks.

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The calculation

Area: 30 × 40 = 1,200 square feet.
Thickness: 4 inches = 0.333 ft; 6 inches = 0.5 ft.
Volume at 4 inches: 1,200 × 0.333 = 400 cubic feet.
Yards: 400 ÷ 27 = 14.81 cubic yards.

The concrete calculator runs the same numbers and adds waste.

Yardage and trucks

Thickness	Cubic yards	With 8% extra	Typical trucks
4 inches	14.81	about 16	2
5 inches	18.52	about 20	2
6 inches	22.22	about 24	3

Truck capacity varies by region and axle count, commonly around 9 to 11 yards per load. Ask the plant what its trucks carry and how quickly it can send them back to back.

Thickness for a shop floor

4 inches: light storage, cars, and hobby use on a good base.
5 to 6 inches: pickups, small tractors, lifts, and heavy shelving.
Thicker or engineered: a two-post or four-post car lift often has its own anchor requirements, usually a minimum thickness and strength under each post. Check the lift manual before you pour.

Base under 1,200 square feet

A 4-inch compacted gravel base under a 30x40 slab is about 14.8 cubic yards, roughly 20 to 22 tons. Six inches of base is about 22 yards, or 30 to 33 tons. A vapor barrier is common under heated shops or floors that will get coatings. See gravel base under a concrete slab.

Joint layout

A 30x40 floor is too big to leave without joints. For a 5-inch slab, joints about 10 to 12 feet apart give a grid such as 10 × 10 panels (3 across, 4 down) or 10 × 13.3 panels. Saw-cut joints to one quarter of the depth as soon as the surface can take the saw without raveling, usually within 6 to 18 hours. Keep panels close to square.

Pour-day planning

Crew: 6 or more people for a 15 to 24 yard pour, including at least one experienced finisher.
Placement: a pump or a skid steer with a bucket can save hours on a big floor if trucks cannot reach all sides. See concrete pump truck cost.
Timing: in summer, start at first light. Retarding admixtures can help on hot days.
Ordering: ask the dispatcher to hold the last truck until you confirm the final quantity, so you do not pay for a full extra load.

Reinforcement

Rebar on chairs, welded wire on chairs, or fiber reinforcement are all used on shop floors. Rebar spaced 18 to 24 inches on center each way is common on 5 to 6 inch slabs. Reinforcement does not stop cracks but keeps them tight.

Bottom line

A 30x40 slab takes about 14.81 cubic yards at 4 inches, 18.52 at 5 inches, and 22.22 at 6 inches. Add about 8 to 10 percent, plan for two to three trucks, and line up a crew, a placement plan, and a joint layout before the first truck arrives.

Questions people ask

How many yards of concrete for a 30x40 slab?

About 14.81 cubic yards at 4 inches and 22.22 at 6 inches, before waste.

How many trucks of concrete for a 30x40 shop?

Usually two trucks for a 4-inch slab and three for a 6-inch slab, depending on truck capacity.

How thick should a 30x40 shop floor be?

Four inches is the minimum for light use. Many shops use 5 to 6 inches, and lifts may need more under the posts.

How much gravel do I need under a 30x40 slab?

About 14.8 cubic yards at 4 inches deep, roughly 20 to 22 tons.

How long does it take to pour a 30x40 slab?

Placement and finishing often take most of a day for a prepared crew, with trucks arriving 30 to 60 minutes apart.

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