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MaterialCalc — 30x40 Shop Slab Cost Calculator

Estimate prepared 09/02/2026. Estimates only; verify against your plans before ordering.

30x40 Shop Slab Cost Calculator

Five inches if equipment or a lift will sit on it.

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Result

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30x40 is the standard shop and pole barn footprint — 1,200 square feet, and at 4 inches roughly 15 cubic yards of concrete. That is two truckloads and a pour that has to be staged, which is where a slab this size stops being a weekend project.

Assumptions used

These are the numbers behind the result. Where one of them is soft, the cost range widens rather than pretending to a precision the data does not have.

80 lb bag yield
0.6 ft³

Yield of one 80 lb bag of standard concrete mix.

Quikrete product data sheet — Published product data, checked 09/01/2026

60 lb bag yield
0.45 ft³

Yield of one 60 lb bag of standard concrete mix.

Quikrete product data sheet — Published product data, checked 09/01/2026

Ready-mix short-load threshold
1 yd³

Below roughly one cubic yard, most suppliers add a short-load fee that erases the price advantage over bags.

Regional ready-mix supplier schedules — Rough figure — widens the estimate range, checked 09/01/2026

Bags per pallet
42 bags

Above roughly one pallet of 80 lb bags, ready-mix is usually cheaper and always faster.

Home Depot / Lowe's pallet quantities — Industry standard, varies by supplier, checked 09/01/2026

Compacted gravel base under slab
4 in

Drains water away from the underside of the slab. Skipping it is the usual cause of a slab that heaves.

International Residential Code (2021) — Industry standard, varies by supplier, checked 09/01/2026

Crushed gravel density
105 lb/ft³

Compacted 3/4 in crushed stone. Varies by quarry and moisture; loose stone runs lighter.

USGS aggregate bulk density range — Industry standard, varies by supplier, checked 09/01/2026

Compaction allowance
0.2

Loose stone loses roughly a fifth of its volume once compacted, so order above the finished volume.

Standard earthwork practice — Rough figure — widens the estimate range, checked 09/01/2026

Vapor barrier roll coverage
1000 ft²

Home Depot / Lowe's published product data — Published product data, checked 09/01/2026

Welded wire mesh sheet coverage
150 ft²

One 5x10 ft sheet, less the overlap at the seams.

Home Depot / Lowe's published product data — Industry standard, varies by supplier, checked 09/01/2026

Prices verified 09/01/2026 (US national average, big-box retail shelf price and regional ready-mix quotes). Regional adjustment: Blended from RSMeans city cost index bands and BLS construction wage differentials.

The formula

1,200 ft² × thickness in feet ÷ 27 = cubic yards. At 4 in that is 1,200 × 0.333 = 400 ft³ = 14.8 yd³ before waste. Base tonnage is the same footprint at the base depth, converted at 105 lb per cubic foot.

Worked examples

30x40 at 4 in

Length: 40 Width: 30 Thickness: 4 Waste: 10

About 16.3 cubic yards with waste and roughly 10 tons of base. Two trucks, and the base delivery is its own logistics problem.

30x40 at 5 in for equipment

Length: 40 Width: 30 Thickness: 5 Waste: 10

Just over 20 yards. Three inches of extra thickness across a footprint this large is where the money goes.

40x60 pole barn slab at 4 in

Length: 60 Width: 40 Thickness: 4 Waste: 10

About 32.6 yards — four trucks and a pump if access is awkward. At this size, hire the pour out.

Choosing materials

  • Five inches, not four, if a lift, a tractor or a loaded trailer will live in here. The extra inch is about 3.7 more yards on this footprint — cheap against a cracked slab you cannot fix.
  • Fifteen yards is two trucks. Stage them 30 to 40 minutes apart and have enough people to keep ahead of the concrete, because a slab this size cannot be finished by two.
  • Control joints every 12 to 15 feet each way. On a 30x40 that is a grid of nine panels, cut within 24 hours of the pour.
  • Decide the anchor bolt and post bracket layout before the truck arrives. Everything set into a shop slab is set permanently, and retrofitting through 5 inches of concrete is miserable.
  • A power trowel is worth renting at this size. Hand-floating 1,200 square feet before it sets is not realistic for a homeowner crew.

Common questions

How much concrete for a 30x40 slab?

At 4 inches thick, 14.8 cubic yards before waste and about 16.3 with a 10% allowance. At 5 inches it is 18.5 before waste, and at 6 inches 22.2.

How thick should a shop slab be?

Four inches for storage and light use, five for a workshop with equipment, six where a lift, a loaded trailer or a tractor will sit. Point loads, not total weight, are what crack a thin slab.

What does a 30x40 slab cost?

Material — concrete, base, mesh, vapor barrier — typically $4,500 to $7,000. Poured and finished by a contractor, roughly $9,600 to $18,000 depending on region and thickness.

Can I pour a 30x40 slab in sections?

Yes, and at this size many contractors do, using a construction joint with dowels between pours. Doing it accidentally — running out of concrete mid-pour — creates a cold joint with no dowels, which is a permanent weak line.

Do I need rebar in a shop slab?

Rebar on an 18 in grid is the usual specification for anything carrying vehicles or equipment. Mesh is acceptable for light storage on a well-compacted base, but the reinforcement must sit mid-slab either way.

How much gravel base for a 30x40 slab?

Four inches of compacted crushed stone is about 10 tons on this footprint. On clay or poor-draining soil, six inches and about 15 tons is money well spent.

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