20x40 Concrete Slab, 4 Inches Thick
Poured 4 in thick, a 20 × 40 ft slab covering 800 sq ft needs about 10.86 cubic yards of concrete once 10% is added for waste. For ready-mix, order 11 cu yd for the 20x40, about $1,540 – $1,980 delivered. Bagged, the same 20x40 is 489 80 lb or 652 60 lb bags, about $3,260 – $4,564 for the concrete alone.
- Ready-mix order
- 11 cu yd
- Ready-mix cost
- $1,540 – $1,980
- 80 lb bags
- 489
- Bag cost
- $3,260 – $4,564
The 20x40 (4 in) prices are national material ranges as of 2026-09, not a quote from a supplier.
What a 20x40 slab parks
A 20x40 driveway or parking pad holds four cars: two side by side and two deep, which suits a two-car garage with a long drive.
Twenty feet is a typical double driveway width, enough for two cars to park with doors open between them.
Large driveways are usually poured with a joint down the middle and joints across every 10 ft or so, which also keeps the finishing manageable.
Before you pour the 20x40 at 4 in
- Check the town's rules for the apron and the curb cut before forming the street end.
- Plan where the water goes: a 20x40 sheds a lot of rain onto the lawn or the street.
- If a boat, an RV or a heavy truck will park here, go to 5 or 6 in.
Full plan for a 20x40 slab, 4 in
- Area
- 800 sq ft
- Thickness
- 4 in
- Volume
- 10.86 cu yd
Bagged concrete estimate (DIY)
$4,035 – $5,825
Est. as of 2026-09 — not a live quote
Ready-mix delivered (materials only)
$1,540 – $1,980
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 978 × 40 lb, 783 × 50 lb or 489 × 80 lb bags. 652 bags
- Ready-mix concrete Alternative to bagged mix; most suppliers have a 1 cu yd order minimum. 11 cu yd
- Compactable gravel sub-base 4 in compacted base beneath the slab. 13.33 tons
- 6x6 welded wire mesh Reinforcement for crack control; not a substitute for engineered rebar on structural slabs. 880 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: 44,000 lb (22 tons).
- Prices are national-average retail/delivery ranges as of 2026-09, not live quotes.
Bags or ready-mix for a 20x40 slab, 4 in?
The 20x40 at 4 in needs 2 trucks: 11 cu yd at 8 cu yd a load, the low end of what a truck carries, for about $1,540 – $1,980. 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 800 sq ft at 4 in, and ask about a pump if the truck can't reach every corner of the 20x40. On the 20x40, ready-mix comes out cheaper by about $2,152 at the middle of each range, and it saves mixing 489 bags.
Is 4 in thick enough for a 20x40 slab?
A 20x40 driveway at 4 in suits passenger cars on a well-compacted base. Heavy trucks, a camper or regular deliveries call for 5 to 6 in, and your town may set its own rule for the apron; at 6 in the 20x40 takes about 16.3 cu yd.
Control joints for a 20x40 slab at 4 in
A 20x40 slab at 4 in shrinks as it cures, and without joints it cracks wherever it chooses. The usual rule (ACI and PCA guidance) puts joints in a 20x40 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 20x40 at 4 in, a joint every 10 ft or less means 3 joints across the 40 ft side and 1 joint across the 20 ft side: 8 panels of about 10 × 10 ft and 100 ft of saw cut. The 20x40 is 2 times as long as it is wide, and long, narrow slabs crack across their width, so don't skip its cross joints.
How much a 20x40 slab 4 in thick weighs
At roughly 150 lb per cubic foot of wet concrete, a 20x40 at 4 in puts about 44,000 lb (22 tons) on the ground. As bags, the 20x40 is 489 80 lb bags, 39,120 lb in all: about 27 trips in a pickup carrying 1,500 lb a load. Nobody hauls 39,120 lb of bags for a 20x40 in a pickup; it comes by ready-mix truck or as pallets on a delivery truck.
The 20x40 at 4 in against nearby sizes
Every row uses the same 10% waste and prices as the 20x40 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 |
|---|---|---|---|---|---|---|
| 24x30 slab, 4 in | 9.78 | 587 | 440 | 10 cu yd | $2,935 – $4,109 | $1,400 – $1,800 |
| 20x40 slab, 4 in (this page) | 10.86 | 652 | 489 | 11 cu yd | $3,260 – $4,564 | $1,540 – $1,980 |
| 30x30 slab, 4 in | 12.22 | 734 | 550 | 12.25 cu yd | $3,670 – $5,138 | $1,715 – $2,205 |
Questions about a 20x40 slab, 4 in
How many bags of concrete for a 20x40 slab 4 inches thick?
A 20x40 slab at 4 in takes about 489 80 lb bags or 652 60 lb bags (or 783 50 lb, 978 40 lb), with 10% added for waste: 293.3 cubic feet in all.
How many yards of concrete for a 20x40 slab 4 inches thick?
The 20x40 measures out at 9.88 cubic yards, or 10.86 with 10% for waste. Order 11 cu yd for the 20x40, since ready-mix is sold in quarter yards with a 1 yard minimum at most plants.
How much does a 20x40 concrete slab 4 inches thick cost?
Concrete alone for the 20x40 runs about $1,540 – $1,980 for 11 cu yd of ready-mix delivered, or $3,260 – $4,564 for 652 60 lb bags. Add a 4 in gravel base and wire mesh and the bagged 20x40 plan comes to $4,035 – $5,825. Labor, forms and finishing for a 20x40 at 4 in come on top of these national ranges (as of 2026-09), which are not quotes.
How much does a 20x40 concrete slab 4 inches thick weigh?
About 44,000 lb (22 tons) of wet concrete for the 20x40, at 150 lb per cubic foot. The 489 80 lb bags for a 20x40 weigh 39,120 lb: about 27 trips in a pickup carrying 1,500 lb a load.
Where do control joints go on a 20x40 slab 4 inches thick?
On a 20x40 at 4 in, every 8 to 12 ft (24 to 36 times the thickness), sawn 1 in deep. On the 20x40 at 4 in, a joint every 10 ft or less means 3 joints across the 40 ft side and 1 joint across the 20 ft side: 8 panels of about 10 × 10 ft and 100 ft of saw cut.
Limitations
The 20x40 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 20x40 slab with a qualified professional.