How Much Does a Concrete Slab Cost?
The concrete itself is often less than half of what a slab costs. Here is the full breakdown — excavation, base, forms, reinforcement, the pour, and finishing — and where the money actually goes.
Ask what a concrete slab costs and you will get a price per square foot. Ask what is in that price and the answers get vague — which is a problem, because the concrete itself is usually less than half of it, and the parts people leave out of their own estimates are exactly the parts that make slabs fail.
This is a component-by-component breakdown of a typical residential slab, what each part does, and where the money actually goes.
A worked example: a 20 × 30 ft slab
600 square feet, 4 inches thick, on prepared ground, with a compacted gravel base and a rebar mat. Figures are representative rather than a quote — regional variation is large, and labour in particular swings enormously.
| Component | Quantity | Typical cost | Share |
|---|---|---|---|
| Excavation and grading | ≈ 11 yd³ removed | $600 | 13% |
| Gravel base, 4 in compacted | ≈ 7.4 yd³ / 10.4 tons | $470 | 10% |
| Forms and stakes | 100 linear ft | $250 | 5% |
| Rebar, #4 at 18 in o.c. | ≈ 25 bars, 20 ft | $350 | 7% |
| Vapour barrier and misc. | 600 ft² | $180 | 4% |
| Concrete, ready-mix | 8.2 yd³ delivered | $1,350 | 29% |
| Pour, finish, and cure | Labour | $1,400 | 30% |
| Control joints and sealing | — | $100 | 2% |
| Total | — | ≈ $4,700 | 100% |
That is roughly $7.80 per square foot — and the concrete is 29% of it. Everything else is preparation, reinforcement, and the labour of placing and finishing.
Where each number comes from
Concrete: 8.2 cubic yards
600 ft² × 4 in ÷ 12 = 200 ft³ = 7.4 yd³ exact, plus the standard 10% for uneven subgrade and edge loss = 8.2 yd³. Ready-mix runs roughly $140–$180 per yard delivered in most markets, with short-load fees below about 3 yards.
This is the number the concrete calculator produces, and it is worth being generous with — running short mid-pour creates a cold joint, a weak and permanently visible seam where fresh concrete meets partly set concrete. There is no fixing it afterwards.
Base: 4 inches of compacted gravel
The base is doing more structural work than people credit. It spreads load, provides drainage so water does not sit under the slab and heave in winter, and gives a stable, uniform surface so the slab is a consistent thickness. A slab poured on soft or uneven ground cracks regardless of how good the concrete was.
600 ft² at 4 inches is 7.4 yd³, or about 10.4 tons of crusher run at 1.4 tons per yard — see the gravel calculator. It must be compacted, in lifts, with a plate compactor. Dumping gravel and raking it level does not count.
Rebar: about 25 sticks
A #4 mat at 18 inches on centre with 3 inches of edge cover works out to roughly 25 twenty-foot bars for a slab this size — the rebar calculator gives the exact count and the tie total.
Two details matter more than the quantity. The mat must sit on chairs in the middle of the slab, not on the ground — steel lying on the subgrade does essentially nothing, because it is below the neutral axis and surrounded by the wettest, dirtiest concrete in the pour. And the cover matters: 3 inches against earth, because steel too near the surface rusts, expands, and spalls the concrete face off.
Excavation: about 11 cubic yards
Digging 8 inches (4 in of base plus 4 in of slab) over 600 ft² is 400 ft³ = 14.8 yd³ bank. In common earth that swells about 25% when dug, so roughly 18.5 yd³ loose has to be hauled — two truck loads. The excavation calculator handles the swell, which is the part people get wrong when booking trucking.
Labour: the largest line
At 30% this is the biggest single component, and it is the one that varies most. Placing and finishing concrete is time-critical skilled work — the material is setting the whole time, the window for floating and troweling is short, and getting it wrong produces a surface that is permanently rough, dusty, or scaled.
It is also the component most people assume they can eliminate by doing it themselves. For a small pad that is reasonable. For 600 square feet it is a crew's worth of work in a few hours, and a first-timer finishing alone will not keep up with the concrete.
What moves the price most
| Factor | Effect |
|---|---|
| Thickness 4 in → 6 in | +50% on the concrete line, roughly +15% overall |
| Access for the truck | A pump adds $700–$1,200 when the truck cannot reach |
| Small pour (under 3 yd³) | Short-load fees of $50–$150 per yard short |
| Decorative finish | Stamped or coloured concrete can double the total |
| Soft or wet subgrade | Deeper excavation and more base — sometimes far more |
| Thickened edges or footings | Adds concrete volume the area calculation misses |
| Cold or hot weather pours | Admixtures, blankets, or early starts add cost |
Where slabs actually fail
Almost never because the concrete was too thin or the mix was weak. The recurring causes are:
- Inadequate base. Too thin, not compacted, or over ground that holds water. The slab spans the soft spot and cracks.
- Reinforcement in the wrong place. Mesh or rebar left lying on the subgrade contributes nothing.
- No control joints, or joints cut too late. Concrete cracks — that is not preventable. Control joints decide where. They should be cut within 6–18 hours and at a spacing of about 2 to 3 times the slab thickness in feet: a 4-inch slab wants joints every 8–12 feet.
- Poor curing. Concrete gains strength by hydration, not by drying out. A slab allowed to dry too fast in sun or wind ends up weaker and prone to surface crazing. Keep it damp or covered for the first several days.
- Water added at the truck. Adding water to make concrete easier to place lowers its strength substantially. If it is too stiff, that is a mix problem to raise with the supplier, not something to fix with a hose.
Estimating your own
- Square footage — measure the slab area.
- Excavation — dig depth is slab thickness plus base thickness; use loose volume for trucking.
- Gravel — base material, in tons, with waste.
- Rebar — bar count, ties, and weight at your spacing.
- Concrete — cubic yards with the waste factor.
Add forms, vapour barrier, and labour, and you have a real number rather than a per-square-foot guess. The exercise is worth doing even if you are hiring the work out — it tells you whether a quote is reasonable and which line a contractor is loading.
These guides are general information, not financial, medical, legal, or tax advice. See our editorial policy for how we research and review them.