MaterialCalc — Concrete Slab Cost Calculator
Estimate prepared 10/31/2026. Estimates only; verify against your plans before ordering.
Concrete Slab 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.
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
- 60 lb bag yield
- 0.45 ft³
- Ready-mix short-load threshold
- 1 yd³
- Bags per pallet
- 42 bags
- Compacted gravel base under slab
- 4 in
- Crushed gravel density
- 105 lb/ft³
- Compaction allowance
- 0.2
- Vapor barrier roll coverage
- 1000 ft²
- Welded wire mesh sheet coverage
- 150 ft²
Yield of one 80 lb bag of standard concrete mix.
SolidPublished product dataQuikrete product data sheet, checked 09/01/2026
Yield of one 60 lb bag of standard concrete mix.
SolidPublished product dataQuikrete product data sheet, 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
Above roughly one pallet of 80 lb bags, ready-mix is usually cheaper and always faster.
StandardIndustry standard, varies by supplierHome Depot / Lowe's pallet quantities, 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
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
SolidPublished product dataHome Depot / Lowe's published product data, 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
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.
In this topic
Concrete Slab Calculators — 7 calculators
- 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.
- Slab-on-Grade Foundation Cost CalculatorConcrete, thickened edge and cost for a monolithic slab foundation.
- 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.
Related guides
- How Thick Should a Concrete Slab Be?Four inches for foot traffic, five to six where vehicles park. What each inch costs, and why the base under the slab matters more than the last inch of concrete.
- How Much Concrete Do I Need?Volume in cubic yards, the waste allowance nobody adds, and the point where bags stop making sense and a truck starts. With the arithmetic shown.
- Concrete Slab vs Pier FoundationA slab is more concrete and a finished floor; piers are less concrete, faster on a slope, and leave the services accessible. Which suits which build.
Related calculators
- 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.
- Slab-on-Grade Foundation Cost CalculatorConcrete, thickened edge and cost for a monolithic slab foundation.
- 20x40 Concrete Driveway Cost CalculatorConcrete, base and cost for a 20 by 40 driveway.
- 30x40 Shop Slab Cost CalculatorConcrete, base and cost for a 30 by 40 shop slab.
- Concrete Driveway Cost CalculatorConcrete, base and cost for a driveway slab.