Concrete for a 30×30 slab
A 30×30 ft slab at 4″ thick needs about 550 × 80 lb bags of concrete mix (with 10% waste) — or 12.25 cubic yards of ready-mix. Estimated materials cost ≈ $2,021 (ready-mix). This is a truck pour. Budget joints, reinforcement, and labour before you budget bags.
| Area | 900 sq ft |
|---|---|
| Volume | 300 ft³ = 11.11 cu yd |
| 80 lb bags (no waste) | 500 |
| 80 lb bags (10% waste) | 550 |
| 60 lb bags (10% waste) | 734 |
| Ready-mix | 12.25 cu yd |
| Finished slab weight | ≈ 42,000 lb |
| Recommended | Order ready-mix |
What a 30×30 slab is usually for
- Large shop floor
- Barn or stable floor
- Multi-bay garage
- Equipment yard pad
900 square feet is firmly professional territory. The volume alone means multiple truck loads coordinated so each arrives as the previous is placed, and the joint grid below will run to a dozen panels or more. Reinforcement at this scale is engineered — bar size, spacing and placement chosen for the actual loads — rather than picked from a residential rule of thumb.
Adjust it
Change the size, thickness, bag size, or waste % below:
How much concrete do I need?
| Bags (with overage) | |
|---|---|
| Bags (no overage) | |
| Ready-mix (delivered) | |
| Estimated materials cost |
Estimate for standard concrete mix (e.g. Quikrete #1101 / Sakrete High-Strength). Bag counts round up and include your overage %. Cost is an estimate (as of June 2026, US national average) — confirm bag price and ready-mix delivery (often a 1 yd³ minimum + short-load fees) with your supplier. Not a substitute for a structural engineer on load-bearing work.
📘 Cost at this footprint: slab cost by size and DIY vs hiring a pro
A 30×30 slab is usually a contractor job
A professionally installed basic slab runs roughly $4–$8 per square foot including labour, so for 900 sq ft:
| Route | Estimated total | What you spend |
|---|---|---|
| DIY | ≈ $2,121–$2,321 | Concrete, forms, base, reinforcement, tool rental — plus your weekend |
| Hire a contractor | ≈ $3,600–$7,200 | Everything, done to a finish, usually in a day |
The gap is labour, and it is real — but so is the risk transfer. A contractor who gets the finish wrong fixes it; if you do, you own it. See DIY vs hiring a pro and slab cost by size for the full breakdown of where each route makes sense.
What a truck pour actually looks like
| Wheelbarrow loads | ≈ 100 (at 3 ft³ each — weight-limited, not volume-limited) |
|---|---|
| Mixer batches | ≈ 275 at 2 bags per batch |
| Rough mixing time | ≈ 22.9 hours of continuous mixing, one person |
| Finished weight | ≈ 42,000 lb on the ground |
Be honest about that number.550 bags is past the point where a one-person mix-and-pour produces a good slab. Concrete starts stiffening within about 30 to 60 minutes in warm weather, so a long mixing session leaves cold joints between what you placed first and what you placed last — visible lines that are also weak lines. If you are set on bags, get help and run two mixers; otherwise price the truck.
Budget for the tools as well as the concrete: a mixer runs about $70/day, a plate compactor for the base about $80/day, and a concrete saw for the joints about $60/day.
Reinforcement takeoff for 30×30
Reinforcement does not stop concrete cracking; it holds the crack tight so the slab keeps acting as one piece. For a residential slab-on-grade of 900 sq ft, either of these is conventional:
| Option | Takeoff for 30×30 |
|---|---|
| Welded wire mesh (6×6) | 22 sheets of 5 × 10 ft, allowing a 6″ lap all round |
| Rebar grid at 16″ o.c. | 24 + 24 bars = 1,440 linear ft ≈ 72 × 20 ft sticks |
| Tie wire | ≈ 576 intersections to tie |
Whichever you use, it has to sit in the middle of the slab to do anything. Mesh laid on the ground and walked over during the pour is the most common wasted material on a DIY slab — it ends up at the bottom, where it carries no tension at all. Use chairs or bar supports, and lift as you go if you have no other option.
Joint layout at 30×30
Concrete shrinks as it cures and it will crack — the only question is whether it cracks where you decided or where it chose. Control joints are the decision. The rule of thumb is a maximum spacing in feet of about two to three times the slab thickness in inches, so at 4″ that is 8–12 ft.
| Panel grid | 3 × 3 = 9 panels |
|---|---|
| Panel size | ≈ 10 × 10 ft |
| Cuts needed | 4 (120 linear ft) |
| Cut depth | 1″ (a quarter of the slab) |
That layout keeps every panel at 1:1 or better, inside the 1.5:1 limit that stops a long thin panel cracking across its middle. Cut or tool the joints the same day — within roughly 6 to 12 hours of finishing, once the surface will carry a saw without ravelling but before shrinkage has done its work.
Thickness still changes the order
Thickness is the one input that changes the order most, and it is chosen by what will sit on the slab rather than by budget. Here is the same 900 sq ft footprint at each common depth:
| Thickness | 80 lb bags | Ready-mix | Est. materials | Typical use |
|---|---|---|---|---|
| 3.5″ | 482 | 10.75 yd³ | ≈ $1,774 | What a nominal 4″ pour often actually measures |
| 4″ (this page) | 550 | 12.25 yd³ | ≈ $2,021 | Patios, sheds, walkways, foot traffic |
| 5″ | 688 | 15.5 yd³ | ≈ $2,558 | Cars, light trailers, frequent vehicle use |
| 6″ | 825 | 18.5 yd³ | ≈ $3,053 | Trucks, RVs, machinery, heavy loads |
Going from 4″ to 6″ on this slab adds roughly $1,031 in materials — about 51% more. That is the entire cost of the decision, and it is why the advice for anything carrying a vehicle is to just pour the extra inches. Removing and replacing a cracked slab costs many times the difference.
The 3.5″ row is not a recommendation, it is a reality check: a “4 inch” slab poured on a subgrade that is a little high in places genuinely finishes at 3.5″ in those spots. If your base is not dead flat, order to the thicker figure. More on this in concrete slab thickness: 4″ vs 6″.
Why bags lose at this size
At 11.11 cubic yards, this slab sits clearly past the point where bags stop being sensible. Here is the comparison with the fees included:
| Route | What you order | Estimated cost |
|---|---|---|
| Bags (80 lb) | 550 bags at ≈ $6.50 each | ≈ $3,575 |
| Ready-mix delivered | 12.25 yd³ at ≈ $165/yd³ | ≈ $2,021 |
Ready-mix is the better call at this size, even carrying the short-load fee. Beyond the price, a truck delivers the whole slab as one continuous placement — no cold joints between batches, no risk of the first section setting while you mix the last, and a consistent mix throughout. Mixing 550 bags to match it is a full day of heavy work with a worse result.
The short-load fee is what people miss. Suppliers charge it below roughly 4 cubic yards, typically $50–$150, because sending a truck out costs them the same whether it is full or not. Always ask for the delivered total including fees rather than the per-yard rate. The full reasoning is in bags vs ready-mix concrete.
Watch out on a slab this size
Treat the figures on this page as a budgeting estimate for the volume of concrete only. A slab of this size and use needs a designed specification covering thickness, reinforcement, base, vapour barrier and jointing. The calculator tells you roughly what it will cost; an engineer tells you what to build.
The arithmetic
Volume = 30 × 30 × (4 ÷ 12) = 300 ft³, then ÷ 27 = 11.11 cubic yards. An 80 lb bag yields 0.60 ft³, so 300 ÷ 0.60 = 500 bags before waste; add 10% and round up to 550 bags. Ready-mix rounds up to the quarter yard, giving 12.25 yd³. Every constant behind those steps is documented in how we calculate, and the reasoning is in how much concrete do I need?
Other slab sizes
Last updated: 2026-08-21.
Bag yields from the
QUIKRETE #1101
data sheet. Joint spacing, panel aspect and cut depth follow standard residential
practice (2–3 × slab thickness, 1.5:1 max panel ratio, cut to ¼ depth). Cost
figures are US-national-average estimates and vary by region and supplier.
Planning estimates only — see terms.