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MaterialCalc — Slab-on-Grade Foundation Cost Calculator

Estimate prepared 09/01/2026. Estimates only; verify against your plans before ordering.

Slab-on-Grade Foundation Cost Calculator

The perimeter beam that carries the walls. 12 to 16 in is typical.

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Result

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A slab-on-grade foundation is a field slab with a beam poured around its perimeter to carry the walls, and the beam is the part homeowner estimates always miss. On a 30x40 building the thickened edge adds well over a cubic yard on top of the flat slab. This prices both parts, plus the base, mesh and vapor barrier.

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.

Thickened edge extra depth
8 in

The perimeter of a monolithic slab drops below the field to carry wall loads.

International Residential Code (2021) — 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

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

Vapor barrier roll coverage
1000 ft²

Home Depot / Lowe's published product data — Published product data, 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

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

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

Field volume = length × width × slab thickness. Edge volume = perimeter × edge width × extra depth below the slab. Add them, add waste, divide by 27 for cubic yards. Base tonnage is the footprint at base depth, times density, allowing for compaction.

Worked examples

30x40 shop building, 4 in slab, 12 in edge

Building length: 30 Building width: 40 Slab thickness: 4 Thickened edge width: 12 Waste: 10

About 17.5 cubic yards — roughly 14.8 in the field and 2.6 in the edge beam. Two trucks, and the edge is 18% of the concrete.

24x24 garage, 5 in slab, 16 in edge

Building length: 24 Building width: 24 Slab thickness: 5 Thickened edge width: 16 Waste: 10

Roughly 11.7 yards. A wider edge on a smaller footprint means the beam is a larger share — over a fifth of the total here.

40x60 pole building slab, 4 in, 12 in edge

Building length: 40 Building width: 60 Slab thickness: 4 Thickened edge width: 12 Waste: 10

About 34 yards, four trucks, and a pour that has to be staged. At this size a concrete pump and a power trowel stop being luxuries.

Choosing materials

  • A monolithic slab is only appropriate where the frost line is shallow. In cold climates the code wants a footing below frost depth, poured separately, and this calculation no longer applies.
  • The edge beam carries the wall loads, so its rebar is structural rather than crack control. Two continuous bars is the usual minimum, and it must lap properly at corners.
  • The vapor barrier goes under the slab and over the base, sealed at every seam and lapped up the edge. A slab without one wicks ground moisture into the building for its whole life.
  • Set all under-slab plumbing before the pour and photograph it against a tape measure. Everything buried in a slab-on-grade is buried permanently.
  • Post-tensioned slabs are common on expansive soils in the South and are a different animal entirely — do not substitute rebar for tendons without an engineer.

Common questions

What is a slab-on-grade foundation?

A single concrete pour that acts as both the floor and the foundation, with a thickened beam around the perimeter carrying the wall loads. It is faster and cheaper than a stem wall or basement, and it is the standard approach in warm climates.

How thick is the edge of a monolithic slab?

Typically 12 to 16 inches wide and extending 8 to 12 inches below the field slab, so the beam is 12 to 16 inches deep in total. Your local code sets the minimum, and it goes up with wall loads and down with better soil.

How much concrete for a 30x40 slab foundation?

At 4 in thick with a 12 in thickened edge, about 15.9 cubic yards before waste and 17.5 with a 10% allowance. The field slab alone is 14.8; the edge beam adds the rest.

Can I build a slab-on-grade in a cold climate?

Only as a frost-protected shallow foundation, which uses rigid insulation extending outward from the slab edge to keep frost away from the bearing soil. It is engineered, it is detailed, and it is not the same as simply pouring a monolithic slab.

What does a slab-on-grade foundation cost?

Material typically runs $4 to $7 per square foot including base, mesh and vapor barrier. Formed, poured and finished by a contractor, expect $8 to $15 per square foot, more where soils need engineering.

Do I need insulation under the slab?

If the building is heated, yes. Rigid foam under the slab or at least around the perimeter pays for itself quickly, and retrofitting it is impossible. Two inches at the edge is the highest-value place to put it.

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