Concrete Slab Cost Calculator
Concrete Slab costs $3,200 to $4,850 for a typical 400 sq ft job in 2026, against a national benchmark of $3,900. Labor runs 25.2 crew hours at $38.54 an hour. Enter a ZIP code to apply that area's measured wages for this work and its regional price level.
What makes up the price
Cost by size
National averages, before any local adjustment. Computed from the same model as the calculator above, so the two always agree.
| Slab area | Sq ft | Typical | Range |
|---|---|---|---|
| 220 sq ft | 220 | $2,525 | $2,075 to $3,150 |
| 320 sq ft | 320 | $3,300 | $2,700 to $4,100 |
| 400 sq ft | 400 | $3,900 | $3,200 to $4,850 |
| 540 sq ft | 540 | $4,975 | $4,075 to $6,175 |
| 680 sq ft | 680 | $6,050 | $4,975 to $7,500 |
The sheet of plastic that decides how the slab behaves
If anything is going on top of this slab — a building, a floor covering, stored tools, a workshop — the most important thing in the quote is a vapor barrier: 10 to 15 mil polyethylene laid directly under the concrete, lapped six inches at the seams and taped, and run up the edge of the form.
Concrete is porous. Ground moisture moves up through a slab continuously, by capillary action, for as long as the slab exists. There is no curing period after which it stops. Without a barrier you get, in rough order of appearance:
- A floor that is permanently damp to the touch in humid weather.
- Flooring adhesive that releases, and vinyl or laminate that cups and lifts.
- Rust on anything metal stored on it, and on the underside of anything sitting on it.
- Mold in a heated or enclosed building, because the moisture has nowhere to go.
The barrier costs somewhere around $0.10 to $0.20 a square foot. Retrofitting one is impossible — the only fix after the fact is a topical moisture-blocking epoxy at several dollars a foot, and it is a partial fix.
One detail that gets argued about: the poly goes directly under the slab, not under the gravel with a layer of sand on top. The sand-cushion method was standard decades ago and is now understood to create a reservoir that holds water against the slab. Directly under the concrete is the current guidance.
Ask the question plainly: is there a vapor barrier, what thickness, and is it under the concrete or under the gravel?
The edge, if walls are landing on it
A slab sitting in the open can be a uniform four inches. A slab carrying a structure cannot — the perimeter has to take the weight of the walls and roof and get below the frost line, or the building moves with the seasons.
Three arrangements, and which one you need is a code question for your jurisdiction:
- Floating slab: uniform thickness, no footing. Fine for a shed under the size your code exempts, a pad, or anything freestanding and light.
- Monolithic slab: the slab and a thickened perimeter beam poured at once, typically 12 to 16 inches deep at the edge. The usual answer for a detached garage or a small addition in a mild climate.
- Slab with separate footings: footings poured below frost depth first, slab poured after. What a hard-freeze climate requires for anything heavy, and what most attached structures get.
Frost depth varies from nothing in the south to 48 inches or more across the northern tier. A garage on a floating slab in Minnesota is a building that lifts an inch every January and comes back down in April, and it will show in the walls within a few years.
This page prices a uniform slab. A thickened edge or separate footings is extra concrete and extra excavation, and a quote that includes them will be above the figure above — ask for it as a separate line so you can see it.
The bolts have to go in wet
If a wall plate is fastening to this slab, anchor bolts get set into the concrete while it is still plastic — pushed in, to the right depth, at the spacing the plans or code call for, within an inch or two of where the plate actually needs them.
The alternative is drilling and epoxying or expansion-bolting afterwards, which works but is slower, more expensive, and relies on the installer not hitting rebar. The common failure is not that they were forgotten; it is that nobody knew where the walls were going, so they were set by guesswork and half of them end up in the wrong place.
Have the building layout settled before the pour, not after. This is the single cheapest hour of planning on the whole project.
Why a small pad costs so much per square foot
For a 400 sq ft slab at four inches with mesh, at $3,900:
| Concrete, about 4.9 cubic yards | around $825 |
| Base stone, mesh, forms and curing | around $500 |
| Labor, about 25 crew hours | around $1,350 |
| Hauling the excavated soil | around $200 |
| Overhead and margin | the balance |
Now scale it down. A 120 sq ft shed pad needs a quarter of the concrete but the crew still mobilizes, excavates, forms, sets a barrier, pours, finishes, and comes back to cut joints. The total lands near $1,775 — about $14.80 a square foot against $9.75 for the 400, and $8.50 for a 1,000 sq ft slab.
This is why published figures of $4 to $7 a square foot for a shed slab do not describe a contractor job. At 120 sq ft the ready-mix alone is $315 to $405 once the short-load fee is in, and the base, mesh, poly and forms take it past $500 before anybody has been paid. A $500 to $800 shed pad is a concrete order and your own weekend.
Two ways to make a small slab cost less per foot:
- Combine it with other work. A shed pad poured the same day as a patio or a walkway shares one mobilization and one delivery. This can take 30% off the small piece.
- Make it bigger. Counterintuitive, but the step from 120 to 200 square feet adds much less than the per-foot figure implies, because almost all of the increase is concrete.
Thickness, honestly
- Four inches with welded wire mesh: a shed, an air conditioner or generator pad, a base under something light. The default above.
- Five inches with a #3 rebar grid: a garage floor, a workshop, anything cars drive onto.
- Six inches with a #4 grid: a garage with a lift, a barn, farm or contractor equipment, or a genuinely poor subgrade.
The steel matters more than the inch. A four inch slab with #3 rebar at 18 inches on center resists cracking at two to three times the load of the same slab with mesh alone, so if you are choosing between an extra inch and better reinforcement, take the reinforcement.
Specify 4,000 psi for anything exterior or unheated, with air entrainment where it freezes. Three thousand psi is a thin spec for a slab that has to take vehicles or freeze-thaw cycling, and the difference in the concrete bill is small.
Slope, flatness and drains
A slab under a building is not a patio and the rules invert:
- A garage floor slopes to the door, about an eighth of an inch per foot — enough that meltwater and washdown leave, shallow enough that nothing rolls.
- A shed or equipment pad can be dead level, and usually should be.
- A slab with a floor drain needs the slope set toward it from every direction, which is real work and should be priced as such. In many jurisdictions a garage floor drain also needs an oil separator before it can connect to anything.
- Flatness matters more than on outdoor flatwork because a building sits on it. Ask what tolerance they work to; a quarter inch over ten feet is a reasonable residential expectation.
The curing timetable
- Strip forms and walk on it: 24 to 48 hours.
- Build on it: 7 days for framing a light structure, and confirm with whoever is building.
- Drive on it: 7 days for a car, 28 days for anything heavy.
- Coat it with epoxy or a sealer: 28 days, and most coatings also want a moisture test first.
Keep the surface damp for the first few days in hot, dry weather or confirm that a curing compound went on. Concrete does not dry, it reacts with water, and a slab whose surface loses its water too early ends up with a weak, dusty top layer — which on a garage floor is the layer you are going to be standing on and parking on.
Questions worth asking
- Is there a vapor barrier, what thickness, and is it directly under the concrete?
- Thickened edge, separate footings, or uniform slab — and what does the local code require for what is going on top?
- What is the frost depth here, and does the design get below it?
- Are anchor bolts being set in the pour, and is the building layout final?
- What psi mix, and is it air entrained?
- What base is going in, how deep, and is it compacted?
- Mesh or rebar, and at what spacing?
- Which way does it slope, and is a drain involved?
- Is the excavated soil in the price or left on site?
- Can this be poured the same day as anything else you have planned?
Common questions
How much does a concrete slab cost?
Around $3,900 for a 400 sq ft slab at four inches with wire mesh, with most jobs that size falling between $3,200 and $4,850 — about $9.75 per square foot. Five inches with a rebar grid adds roughly $625 and six inches about $1,400. Size moves the per-foot figure a lot: a 120 sq ft pad works out near $14.80 a square foot and a 1,000 sq ft slab near $8.50, because the crew mobilizes, forms, pours and returns to cut joints either way.
How much does a 24x24 garage slab cost?
About $7,250 at six inches with a #4 rebar grid, or $6,150 at five inches with a #3 grid — 576 square feet either way. Six inches is the right spec if a lift is going in or the subgrade is poor; five is enough for ordinary cars and light trucks. Neither figure includes a thickened edge or separate footings for the walls, which a detached garage will need and which should appear as its own line on a quote.
Does a concrete slab need a vapor barrier?
Yes, if anything is going on top of it — a building, a floor covering, stored tools, a workshop. Ground moisture moves up through concrete continuously by capillary action and never stops, so without a barrier the floor stays damp, flooring adhesive releases, metal rusts and an enclosed building grows mold. Use 10 to 15 mil polyethylene laid directly under the concrete, lapped six inches and taped, at roughly $0.10 to $0.20 a square foot. It cannot be retrofitted.
Should the vapor barrier go under the gravel or under the slab?
Directly under the slab. Putting poly under the gravel with a sand cushion on top was standard practice decades ago and is now understood to create a reservoir that holds water against the concrete. Current guidance is the barrier immediately beneath the concrete, on top of the compacted base. It is worth asking the question specifically, because the old method is still used by crews who learned it that way.
How thick should a concrete slab be?
Four inches with welded wire mesh for a shed, an equipment pad or a base under something light. Five inches with a #3 rebar grid for a garage floor or a workshop — anything cars drive onto. Six inches with a #4 grid for a garage with a lift, a barn, or farm equipment. The steel matters more than the inch: a four inch slab with #3 rebar at 18 inches on center resists cracking at two to three times the load of the same slab with mesh alone.
Does a slab for a garage need footings?
Almost always something more than a uniform slab, and which arrangement depends on your climate and code. A monolithic slab — the floor and a thickened perimeter beam 12 to 16 inches deep poured at once — is the usual answer for a detached garage in a mild climate. Where the ground freezes hard, separate footings below frost depth are required, and frost depth runs from nothing in the south to 48 inches or more across the northern tier. A garage on a floating slab in a cold climate lifts every winter and it shows in the walls within a few years.
Why is a small concrete slab so expensive per square foot?
Because almost none of the cost scales with area. A 120 sq ft shed pad uses a quarter of the concrete of a 400 sq ft slab, but the crew still mobilizes, excavates, forms, lays the barrier and steel, pours, finishes and comes back to cut joints. The total lands near $1,775, or about $14.80 a square foot against $9.75 for the 400. Published figures of $4 to $7 a square foot for a shed slab describe a materials order and your own labor — at that size the ready-mix alone is $315 to $405 once the short-load fee is in.
How can I make a small slab cheaper?
Two things work. Combine it with other concrete work so one mobilization and one delivery covers both — a shed pad poured the same day as a patio or walkway can come in 30% below its standalone price. And consider making it slightly bigger: the step from 120 to 200 square feet adds much less than the per-square-foot figure suggests, because nearly all of the increase is concrete rather than labor.
Do anchor bolts go in before or after the pour?
Into the wet concrete, pushed in to the right depth at the spacing the plans or code require. The alternative — drilling and epoxying or expansion-bolting afterwards — works but is slower, more expensive and depends on the installer missing the rebar. The common failure is not forgetting them; it is setting them before anybody knew where the walls were going. Have the building layout final before the truck arrives.
What psi concrete should a slab be?
Four thousand psi for anything exterior or unheated, with air entrainment where the ground freezes. Three thousand psi is a thin spec for a slab that takes vehicles or freeze-thaw cycling, and the difference in the concrete bill is small enough that there is no good reason to economize there. Air entrainment is the part people skip and it is what stops the surface spalling off after a few winters.
Which way should a garage floor slope?
Toward the door, at about an eighth of an inch per foot — enough that meltwater and washdown leave, shallow enough that nothing rolls on its own. A shed or equipment pad can be dead level and usually should be. If a floor drain is involved, the slope has to be set toward it from every direction, which is real work and belongs on the quote as such; many jurisdictions also require an oil separator before a garage drain can connect to anything.
How long before I can build on a new slab?
Walk on it after 24 to 48 hours, frame a light structure on it after 7 days, and drive a car on it after 7 — but keep anything heavy off for the full 28 days. Epoxy coatings and most sealers also want 28 days, plus a moisture test. Keep the surface damp for the first few days in hot weather or confirm that a curing compound went on, because a slab that loses its surface water early ends up with a weak, dusty top layer, and on a garage floor that is the layer you park on.
Where these numbers come from
Nothing here is copied from a contractor directory. Labor is priced from what the trades are actually paid, materials from published producer prices, and both are adjusted to your area using federal cost-of-living data.
- Poured concrete foundation and structure contractors, United States: $38.54/hour, August 2026 (BLS CES CES2023811003)
- Ready-mix concrete, delivered: $170.00 per cubic yard (price recorded October 2026, not index-adjusted)
- Crushed stone base, delivered: $45.00 per cubic yard (price recorded October 2026, not index-adjusted)
- Welded wire reinforcing mesh: $0.25 per sq ft of slab (price recorded October 2026, not index-adjusted)
- Forms, stakes, joint filler and curing compound: $0.45 per sq ft of slab (price recorded October 2026, not index-adjusted)