How Much Does a 30x30 Concrete Slab Cost?
A 30x30 concrete slab costs $5,400–10,800 installed at $6–12 per square foot. At 900 square feet it's the footprint of a three-car garage, a workshop, or a small barn, and it's usually poured at 5–6 inches. That means 15–18 cubic yards of concrete, two or three ready-mix trucks, and a finishing crew with a power trowel.
A 30x30 concrete slab costs $5,400–10,800 installed — 900 square feet at the typical $6–12 per square foot. At this size the job changes character: it takes two or more trucks, it needs a power trowel to finish, and joint layout starts to matter as much as thickness. Price your exact dimensions in the concrete cost calculator.
What a 30x30 Slab Is Usually For
- Three-car garage — three bays at 10 feet each, with depth for storage.
- Workshop or hobby shop — room for a lift, a bench line, and a machine or two.
- Small barn or equipment shed — a tractor, mower, and implements.
These uses carry wheel loads and point loads, so most 30x30 slabs are 5 inches with #4 rebar, and 6 inches where a car lift or heavy machinery will sit. See slab-on-grade construction for how the thickened edge and base are built.
How Much Concrete a 30x30 Slab Takes
| Thickness | Cubic Yards (exact) | With 10% Waste | 80-lb Bags | Ready-Mix at $165–205/yd | Weight in Place |
|---|---|---|---|---|---|
| 4 in | 11.11 | 12.22 | 550 | $2,017–2,506 | 45,000 lb |
| 5 in | 13.89 | 15.28 | 688 | $2,521–3,132 | 56,250 lb |
| 6 in | 16.67 | 18.33 | 826 | $3,025–3,758 | 67,500 lb |
Every thickness exceeds one 8–10 yard truck, and the 6-inch pour can take three. If the trucks can't back up to the forms, budget for a concrete pump or a buggy crew — a 30-foot reach from the chute is more than a truck can cover without repositioning.
Joint Layout at 900 Square Feet
A 30x30 slab without control joints will crack; the question is only where. The usual rule is joint spacing of 2–3 times the slab thickness in inches, expressed in feet:
| Thickness | Max Joint Spacing | Layout on a 30x30 |
|---|---|---|
| 4 in | 8–12 ft | 10-ft grid, nine 10x10 panels |
| 5 in | 10–15 ft | 10-ft grid, nine panels |
| 6 in | 12–18 ft | 15-ft grid, four 15x15 panels |
Keep panels close to square, and saw-cut within the first day before shrinkage cracks start on their own.
30x30 Slab Cost in Four Cities
These ranges come from our slab foundation cost by city data at 900 square feet:
| Metro | Installed $/sq ft | 30x30 Slab (900 sq ft) |
|---|---|---|
| San Antonio, TX | $5.75–10.10 | $5,175–9,090 |
| Kansas City, MO | $8.26–15.25 | $7,434–13,725 |
| Portland, OR | $8.88–17.89 | $7,992–16,101 |
| Denver, CO | $9.32–16.57 | $8,388–14,913 |
Base and Rebar
- Gravel base: 4 inches of compacted stone under 900 sq ft is 11.11 cubic yards — about 15.6 tons, $278–556 at $25–50 per yard.
- Rebar: #4 bar at 18 inches each way is about 1,260 linear feet ($945–1,890). Use chairs so the steel sits in the middle of the slab, not on the gravel.
- Vapor barrier: Worth it under any shop that will be heated or have finished walls.
DIY vs. Contractor for a 30x30 Slab
Materials for a 5-inch slab run roughly $3,740–5,580: ready-mix ($2,521–3,132), gravel ($278–556), and rebar ($945–1,890), before forms, a pump, and a power trowel. That's roughly half to two-thirds of the installed price, and 900 square feet across two trucks is a crew job with little margin for error. For a slab of this size, get at least three bids and compare them against the calculator; see how to hire a concrete contractor.
Comparing sizes? See the 20x30 slab cost for a large two-car garage, or the 20x20 slab cost for a standard two-car garage.