MaterialCalc — Slab-on-Grade Foundation Cost Calculator
Estimate prepared 10/31/2026. Estimates only; verify against your plans before ordering.
Slab-on-Grade Foundation Cost Calculator
Enter your measurements and get both numbers: how much material the job takes, and what it costs in your state.
01 / 03Measurements
How big is it?
Measure to the outside. Round to the nearest half foot — nothing on a job site is exact.
The perimeter beam that carries the walls. 12 to 16 in is typical.
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
- Compacted gravel base under slab
- 4 in
- Welded wire mesh sheet coverage
- 150 ft²
- Vapor barrier roll coverage
- 1000 ft²
- Crushed gravel density
- 105 lb/ft³
- Compaction allowance
- 0.2
- Ready-mix short-load threshold
- 1 yd³
The perimeter of a monolithic slab drops below the field to carry wall loads.
StandardIndustry standard, varies by supplierInternational Residential Code (2021), checked 09/01/2026
Drains water away from the underside of the slab. Skipping it is the usual cause of a slab that heaves.
StandardIndustry standard, varies by supplierInternational Residential Code (2021), checked 09/01/2026
One 5x10 ft sheet, less the overlap at the seams.
StandardIndustry standard, varies by supplierHome Depot / Lowe's published product data, checked 09/01/2026
SolidPublished product dataHome Depot / Lowe's published product data, checked 09/01/2026
Compacted 3/4 in crushed stone. Varies by quarry and moisture; loose stone runs lighter.
StandardIndustry standard, varies by supplierUSGS aggregate bulk density range, checked 09/01/2026
Loose stone loses roughly a fifth of its volume once compacted, so order above the finished volume.
RoughRough figure — widens the estimate rangeStandard earthwork practice, checked 09/01/2026
Below roughly one cubic yard, most suppliers add a short-load fee that erases the price advantage over bags.
RoughRough figure — widens the estimate rangeRegional ready-mix supplier schedules, 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.
In this topic
Concrete Slab Calculators — 7 calculators
- Concrete Slab Cost CalculatorConcrete volume, bag count and slab cost.
- 20x20 Concrete Slab Cost CalculatorConcrete, base and cost for a 20 by 20 foot slab.
- 24x24 Garage Slab Concrete & Cost CalculatorConcrete, base and cost for a two-car garage slab.
- 30x40 Shop Slab Cost CalculatorConcrete, base and cost for a 30 by 40 shop slab.
- Concrete Patio Slab Cost CalculatorConcrete, base and cost for a patio slab.
- Rebar Grid & Spacing CalculatorBar count, total footage and cost for a rebar mat.
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