Concrete slab cost calculator
Estimate the installed cost of a poured concrete slab from its area, your price per square foot, the sub-base and the rebar — with an adjustable contingency buffer.
Calculator
A 400 sq ft slab at $7.00/sq ft plus sub-base and rebar is about $3,575.00 with 10% contingency.
"Slab" covers a lot of jobs — a shed pad, an AC or generator pad, a basketball key, the floor of a garage or workshop, or the base for an addition. What they share is the same cost skeleton: the flatwork priced by the square foot, a compacted sub-base underneath, and reinforcement (rebar grid or welded wire mesh) tying it together. This calculator prices those three pieces from the numbers on your quote and adds a contingency so the estimate carries a realistic buffer.
Reinforcement here is a cost line, not a structural design. The rebar field is simply what the contractor charges for material and placement; sizing load-bearing concrete, footings and reinforcement is the job of a licensed engineer and your local code. Use this to budget and to compare bids — not to engineer a slab.
Formula
The total is the sum of the priced items scaled by the contingency factor:
flatwork = area_sqft × price_per_sqft
subtotal = flatwork + sub_base + rebar
total = subtotal × (1 + contingency%)
Sub-base is the excavation, gravel and grading; rebar is the reinforcement material and labor as quoted; contingency is a fraction (0.10 = 10%).
Worked example
A 400 sq ft slab at $7/sq ft, with $250 of sub-base and $200 of rebar, at a 10% contingency:
- Flatwork = 400 × $7 = $2,800
- Subtotal = $2,800 + $250 + $200 = $3,250
- Total = $3,250 × 1.10 = $3,575
That puts a plain reinforced slab of this size around $3,575. Thicker slabs for heavier loads, a thickened edge, or a smoother finish will each add to the per-foot price.
Thickness, reinforcement & quantity checks
Thickness is the quiet cost driver. A 4" residential slab is the common default; step up to 5–6" for a driveway apron, an RV pad or a slab that will carry vehicles and you use more concrete per square foot and often heavier reinforcement. That is a planning choice guided by the labeled thickness typicals — see the slab-on-grade thickness reference — and confirmed with a professional, never guessed.
To quantity-check a quote, pair this with the concrete slab calculator to see the cubic yards and bag count the pour actually needs, and the rebar calculator for the linear feet of steel at your spacing. Installed plain slabs commonly fall in the labeled band below — a sanity guide only, since mix, thickness, access and local labor move the real figure. Get itemized written quotes from licensed, insured concrete contractors before you commit.
Reference table
Labeled planning references (you enter your real price):
| Item | Typical |
|---|---|
| Plain slab, installed | $5–$10/sq ft |
| Residential slab thickness | 4" |
| Garage / shed slab thickness | 4–5" |
| Driveway / heavy thickness | 5–6" |