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MaterialCalc — Concrete Slab Cost Calculator

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

Concrete Slab Cost Calculator

Four inches carries foot traffic. Vehicles want five or six.

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Covers subgrade that is not perfectly flat.

Result

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A slab is priced by volume, and volume is thickness times area — so the thickness field moves the cost more than anything else on this page. Going from 4 in to 6 in on a 20x20 slab adds half a cubic yard and roughly a third to the concrete bill. This gives you the volume, the bag count, the gravel base tonnage and what the whole thing costs.

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

Volume = length × width × thickness, converted to cubic yards by dividing cubic inches by 46,656. Add waste, then divide by the bag yield for a bag count or order that many yards of ready-mix. Gravel base is calculated the same way at the base depth, then converted to tons by density.

Worked examples

20x20 patio slab, 4 in thick

Slab length: 20 Slab width: 20 Thickness: 4 Waste: 10

About 5.4 cubic yards with waste. Far past bag territory — this is a ready-mix job, and the delivery charge is a rounding error against the concrete itself.

10x12 shed slab, 4 in thick

Slab length: 10 Slab width: 12 Thickness: 4 Waste: 10

Roughly 1.6 cubic yards. This is the crossover zone: just enough to justify a truck, and just small enough that the short-load fee still stings.

24x24 garage slab, 5 in thick

Slab length: 24 Slab width: 24 Thickness: 5 Waste: 10

About 9.8 cubic yards. The extra inch over a 4 in slab costs roughly two more yards, which is cheap insurance under a vehicle.

Choosing materials

  • Thickness is set by what drives on it, not by the size of the slab. Four inches for patios and sheds, five for a car, six for anything with a truck or trailer on it.
  • Order concrete by the yard and always round up. Coming up short mid-pour means a cold joint, which is a permanent weak line through the slab.
  • The gravel base does more for crack resistance than an extra inch of concrete does. It drains water out from under the slab, and water under a slab is what heaves it.
  • Wire mesh only works if it sits in the middle of the slab. Mesh lying on the ground under the pour does nothing at all — pull it up as you pour, or use chairs.
  • Above about a pallet of bags, ready-mix is both cheaper and dramatically faster. Mixing forty bags by hand is a full day; a truck is forty minutes.

Common questions

How many yards of concrete for a 20x20 slab?

A 20x20 slab at 4 in thick is 4.94 cubic yards before waste, or about 5.4 yards with a 10% allowance. At 5 in it is 6.2 yards, and at 6 in it is 7.4. Order to the quarter yard and round up.

How thick should my concrete slab be?

Four inches for patios, sheds and walkways. Five inches for a driveway or garage carrying passenger vehicles. Six inches or more if a loaded truck, RV or dumpster will sit on it. Thickness is about the load, not the area.

Do I need gravel under a concrete slab?

In almost every case yes. Four inches of compacted crushed stone gives the slab a uniform bearing surface and lets water drain away from underneath it. Pouring straight onto clay is the most common cause of a slab that cracks in its first winter.

Is wire mesh or rebar better for a slab?

For a residential slab on a good base, mesh is usually enough and it is much cheaper. Rebar earns its keep when the slab carries heavy point loads, spans poor soil, or is thicker than 5 in. Either way the reinforcement has to sit inside the slab, not under it.

What does a concrete slab cost per square foot?

Material alone typically runs $3 to $6 per square foot at 4 in, depending on thickness and local ready-mix prices. Formed, poured and finished by a contractor, expect roughly $8 to $14. Toggle labor on above to see both.

How much waste should I allow?

Ten percent is standard and is the default here. If the subgrade is rough or hand-graded, go to 15% — the low spots swallow concrete and you only find them once the truck is pouring.

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