30x40 Concrete Slab, 4 Inches Thick
Poured 4 in thick, a 30 × 40 ft slab covering 1,200 sq ft needs about 16.3 cubic yards of concrete once 10% is added for waste. For ready-mix, order 16.5 cu yd for the 30x40, about $2,310 – $2,970 delivered. Bagged, the same 30x40 is 734 80 lb or 978 60 lb bags, about $4,890 – $6,846 for the concrete alone.
- Ready-mix order
- 16.5 cu yd
- Ready-mix cost
- $2,310 – $2,970
- 80 lb bags
- 734
- Bag cost
- $4,890 – $6,846
Materials only for a 30x40 at 4 in, from national price ranges as of 2026-09; not a live quote.
The 30x40 shop slab
Thirty by forty is one of the most built shop and pole-barn sizes: room for two vehicles, a workbench wall and storage, or for a boat and a camper side by side.
At 1,200 sq ft, the slab is a two-truck pour at 4 in, and most owners hire a finishing crew for the day.
If any of the building will be heated, a vapor retarder under the slab (6 mil poly is the usual choice) helps keep moisture from coming up through the floor.
A 4 in 30x40 pour: check first
- Decide where the overhead doors go before you pour, and form a clean edge at each one.
- Plan floor drains, plumbing and power sleeves now; cutting the slab later is expensive.
- For a lift or heavy equipment, consider 5 or 6 in in that bay.
Full plan for a 30x40 slab, 4 in
- Area
- 1,200 sq ft
- Thickness
- 4 in
- Volume
- 16.3 cu yd
Bagged concrete estimate (DIY)
$6,052 – $8,738
Est. as of 2026-09 — not a live quote
Ready-mix delivered (materials only)
$2,310 – $2,970
Est. as of 2026-09 — not a live quote
For a pour this size, ready-mix delivery is typically more practical than mixing bags by hand.
Shopping list
- Concrete mix (60 lb bags) Or 1,467 × 40 lb, 1,174 × 50 lb or 734 × 80 lb bags. 978 bags
- Ready-mix concrete Alternative to bagged mix; most suppliers have a 1 cu yd order minimum. 16.5 cu yd
- Compactable gravel sub-base 4 in compacted base beneath the slab. 20 tons
- 6x6 welded wire mesh Reinforcement for crack control; not a substitute for engineered rebar on structural slabs. 1,320 sq ft
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Buy bagged concrete
Pick up bagged mix and reinforcement from a hardware store for smaller pours.
View shopping listAssumptions
- Waste allowance: 10% applied to account for over-excavation and spillage.
- Approx. wet weight: 66,000 lb (33 tons).
- Prices are national-average retail/delivery ranges as of 2026-09, not live quotes.
Bags or ready-mix for a 30x40 slab, 4 in?
The 30x40 at 4 in needs 3 trucks: 16.5 cu yd at 8 cu yd a load, the low end of what a truck carries, for about $2,310 – $2,970. Ask the supplier to space the trucks so each load goes down before the last one stiffens (a wet edge), line up enough hands to place and finish 1,200 sq ft at 4 in, and ask about a pump if the truck can't reach every corner of the 30x40. Ready-mix wins for this 30x40 by roughly $3,228 (range midpoints), and nobody has to mix 734 bags by hand.
Is 4 in thick enough for a 30x40 slab?
For a 30x40 garage or shop floor that carries passenger cars, 4 in is common and above the IRC minimum of 3-1/2 in. Go to 5 or 6 in if the 30x40 will hold heavy pickups, an RV, a car lift or a tractor, or sits on soft or clay soil, and check your local code first. At 6 in the same 30x40 takes about 24.44 cu yd.
Control joints for a 30x40 slab at 4 in
Like any slab, a 4 in 30x40 shrinks while it cures; control joints decide where its cracks go. The usual rule (ACI and PCA guidance) puts joints in a 30x40 at 4 in every 8 to 12 ft, 24 to 36 times the thickness, cut 1 in deep (a quarter of the thickness) as soon as the surface takes the saw without raveling, usually within the first day.
On the 30x40 at 4 in, a joint every 10 ft or less means 3 joints across the 40 ft side and 2 joints across the 30 ft side: 12 panels of about 10 × 10 ft and 170 ft of saw cut.
How much a 30x40 slab 4 in thick weighs
At roughly 150 lb per cubic foot of wet concrete, a 30x40 at 4 in puts about 66,000 lb (33 tons) on the ground. As bags, the 30x40 is 734 80 lb bags, 58,720 lb in all: about 40 trips in a pickup carrying 1,500 lb a load. Nobody hauls 58,720 lb of bags for a 30x40 in a pickup; it comes by ready-mix truck or as pallets on a delivery truck.
The 30x40 at 4 in against nearby sizes
Every row uses the same 10% waste and prices as the 30x40 at 4 in; costs are for the concrete only.
| Slab | Cu yd | 60 lb bags | 80 lb bags | Ready-mix order | Bags cost | Ready-mix cost |
|---|---|---|---|---|---|---|
| 24x40 slab, 4 in | 13.04 | 783 | 587 | 13.25 cu yd | $3,915 – $5,481 | $1,855 – $2,385 |
| 30x40 slab, 4 in (this page) | 16.3 | 978 | 734 | 16.5 cu yd | $4,890 – $6,846 | $2,310 – $2,970 |
| 30x50 slab, 4 in | 20.37 | 1,223 | 917 | 20.5 cu yd | $6,115 – $8,561 | $2,870 – $3,690 |
Questions about a 30x40 slab, 4 in
How many bags of concrete for a 30x40 slab 4 inches thick?
Plan on 440 cubic feet for the 30x40, which is 734 80 lb bags, 978 60 lb bags, 1,174 50 lb or 1,467 40 lb bags once 10% waste is in.
How many yards of concrete for a 30x40 slab 4 inches thick?
16.3 cubic yards with waste (14.81 without) for a 30x40 at 4 in. Round the 30x40 order to 16.5 cu yd: plants batch in quarter yards and most won't send less than a yard.
How much does a 30x40 concrete slab 4 inches thick cost?
Concrete alone for the 30x40 runs about $2,310 – $2,970 for 16.5 cu yd of ready-mix delivered, or $4,890 – $6,846 for 978 60 lb bags. Add a 4 in gravel base and wire mesh and the bagged 30x40 plan comes to $6,052 – $8,738. Labor, forms and finishing for a 30x40 at 4 in come on top of these national ranges (as of 2026-09), which are not quotes.
How much does a 30x40 concrete slab 4 inches thick weigh?
About 66,000 lb (33 tons) of wet concrete for the 30x40, at 150 lb per cubic foot. The 734 80 lb bags for a 30x40 weigh 58,720 lb: about 40 trips in a pickup carrying 1,500 lb a load.
Where do control joints go on a 30x40 slab 4 inches thick?
On a 30x40 at 4 in, every 8 to 12 ft (24 to 36 times the thickness), sawn 1 in deep. On the 30x40 at 4 in, a joint every 10 ft or less means 3 joints across the 40 ft side and 2 joints across the 30 ft side: 12 panels of about 10 × 10 ft and 170 ft of saw cut.
Limitations
The 30x40 figures at 4 in are material estimates and typical price ranges, not a quote or a structural design. Confirm that 4 in, the reinforcement and the joint layout suit your soil, loads and local code for a 30x40 slab with a qualified professional.