A 30x40 slab poured 4 inches thick needs 14.81 cubic yards of concrete (400 cubic feet: 40 ft × 30 ft × 4 in ÷ 12). With 10 percent extra you order 16.5 cubic yards of ready-mix, or buy 734 bags of 80 lb mix and mix it yourself.
A 30x40 slab is 1,200 square feet: a three-car garage, a large workshop or a small barn floor. Trucks, a pump and a finishing crew all matter at this size.
How much concrete for a 30x40 slab, by thickness
Volume is the area times the thickness in feet. The 30x40 slab covers 1200 sq ft. Ordered volumes include 10 percent extra for uneven sub-base and spillage, the middle of the 4 to 10 percent NRMCA range, and ready-mix is sold in quarter yards.
| Build | Concrete, net | Order with 10% extra | Trucks | Weight |
|---|---|---|---|---|
| 4 in, plain | 14.81 cu yd (400 cu ft) | 16.5 cu yd | 2 | 61,600 lb |
| 4 in, wire mesh | 14.81 cu yd (400 cu ft) | 16.5 cu yd | 2 | 61,600 lb |
| 6 in, rebar | 22.22 cu yd (600 cu ft) | 24.5 cu yd | 3 | 92,400 lb |
The weight uses 140 lb per cubic foot, Quikrete's figure for the mix. A 6 inch slab is half again the concrete of a 4 inch one for the same footprint, which is worth spending on where a car or a hot tub will sit and not on a walkway.
How many bags of concrete for a 30x40 slab
Bag counts come from the yields on the manufacturers' data sheets: an 80 lb bag makes 0.60 cu ft, 60 lb 0.45, 50 lb 0.375 and 40 lb 0.30. They include the 10 percent allowance.
| Bag size | 4 in slab | 6 in slab |
|---|---|---|
| 80 lb | 734 bags | 1100 bags |
| 60 lb | 978 bags | 1467 bags |
| 50 lb | 1174 bags | 1760 bags |
| 40 lb | 1467 bags | 2200 bags |
734 bags of 80 lb is 29.4 tons of bagged mix to move and mix by hand, and a slab has to be poured before the first batch stiffens. Ready-mix is the practical route at this size. The concrete bag calculator converts any bag size and brand.
30x40 slab cost, three builds
Installed prices at US averages: ready-mix $110 to $150 per cu yd, delivery, fuel and a short-load fee below a full truck, placing and finishing $2 to $3 per sq ft (HomeAdvisor, June 2026), wire mesh $0.83 to $0.98 per sq ft (homewyse, September 2026) and rebar $2 to $3 per sq ft (This Old House, March 2026).
| Build | Materials only | Installed range | Typical per sq ft | Crew days |
|---|---|---|---|---|
| 4 in, plain | $2,250 to $3,100 | $4,650 to $6,700 (typical $5,700) | $5 | 2.5 |
| 4 in, wire mesh | $3,200 to $4,250 | $5,600 to $7,900 (typical $6,700) | $6 | 2.5 |
| 6 in, rebar | $5,500 to $7,900 | $7,900 to $11,500 (typical $9,700) | $8 | 2.5 |
A 16.5 cu yd order is well below the 8 to 12 yards a truck carries, so the estimate includes a short-load fee of about $330 ($260 to $390). Splitting a truck with a neighbor who is also pouring avoids it. The heavy build adds 109 bars of 20 ft No. 4 rebar in an 18 in grid, about 1,456 lb with laps and 10 percent spare; the rebar calculator lays out the grid.
Joints, thickness and permit
The 2024 IRC sets 3½ in as the minimum for a slab on ground (R506.1) over 4 in of granular base with a vapor retarder under floors. NRMCA advises control joints no farther apart than 24 times the thickness: 8 ft for a 4 in slab, 12 ft for a 6 in slab. On a 30x40 slab that means 4 joints each way at 4 in thick. Under IRC 2024 R105.2, sidewalks and driveways need no permit, and a free-standing patio slab is usually treated the same way. A slab that carries a building, such as a 30x40 garage or shed floor, is part of that building's permit.
Forming, placing and finishing is about 2.5 days for a two-person crew. Concrete reaches 2,500 psi at 7 days and 4,000 psi at 28 days (Quikrete data sheet), so keep vehicles off for the first week.
30x40 slab compared with other sizes
A 24x24 slab saves 7.7 cu yd less, $2,700 less installed.
Next steps for a 30x40 slab
- Change thickness, shape or add a thickened edge in the concrete slab calculator.
- Work out the rebar for the slab at your own spacing.
- Turn the yards into a truck order with the concrete yard calculator, and add pumping or finishes in the concrete cost calculator.
- Any other shape or a footing is in the concrete calculator. All size pages are grouped under cost calculators.