12x20 Concrete Slab, 4 Inches Thick
A 12 × 20 ft slab (240 sq ft) at 4 in thick takes about 3.26 cubic yards of concrete, with 10% extra for waste. For ready-mix, order 3.5 cu yd for the 12x20, about $550 – $740 delivered, short-load fee included. Bagged, the same 12x20 is 147 80 lb or 196 60 lb bags, about $980 – $1,372 for the concrete alone.
- Ready-mix order
- 3.5 cu yd
- Ready-mix cost
- $550 – $740
- 80 lb bags
- 147
- Bag cost
- $980 – $1,372
The 12x20 (4 in) prices are national material ranges as of 2026-09, not a quote from a supplier.
The 12x20 one-car garage or carport
Twelve by twenty is about the smallest footprint for a one-car garage: a midsize car fits, with a couple of feet on each side to open a door, but not much room for storage.
It is also the standard size for a single carport, a boat or camper pad, or a long shed that holds a lawn tractor and garden tools in a row.
A typical single garage door is 8 or 9 ft wide, so on a 12 ft wall that leaves only a foot or two on each side of the opening for framing.
A 4 in 12x20 pour: check first
- Slope the floor toward the door or a drain, as codes require for garages; 1/8 in per foot is common.
- Ask the town whether the walls need footings below the frost line or a thickened edge.
- If you plan a workbench or shelves, consider 12x24 instead: the extra 4 ft is a small share of the total cost.
Full plan for a 12x20 slab, 4 in
- Area
- 240 sq ft
- Thickness
- 4 in
- Volume
- 3.26 cu yd
Bagged concrete estimate (DIY)
$1,212 – $1,750
Est. as of 2026-09 — not a live quote
Ready-mix delivered, incl. short-load fee (materials only)
$550 – $740
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 294 × 40 lb, 235 × 50 lb or 147 × 80 lb bags. 196 bags
- Ready-mix concrete Alternative to bagged mix; most suppliers have a 1 cu yd order minimum. 3.5 cu yd
- Compactable gravel sub-base 4 in compacted base beneath the slab. 4 tons
- 6x6 welded wire mesh Reinforcement for crack control; not a substitute for engineered rebar on structural slabs. 264 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: 13,200 lb (6.6 tons).
- Prices are national-average retail/delivery ranges as of 2026-09, not live quotes.
- Ready-mix orders under 10 cu yd usually carry a short-load fee; the ready-mix range includes a flat $60-$110. Some suppliers charge per yard instead, so ask for the full fee schedule.
Bags or ready-mix for a 12x20 slab, 4 in?
3.26 cu yd for the 12x20 fits on one ready-mix truck (they carry about 8 to 10 cu yd). In bags the 12x20 would be 147 80 lb bags weighing 11,760 lb, so ready-mix is the practical choice. Under 10 cu yd most suppliers add a short-load fee, and the $550 – $740 range for the 12x20 includes one. Ready-mix wins for this 12x20 by roughly $531 (range midpoints), and nobody has to mix 147 bags by hand.
Is 4 in thick enough for a 12x20 slab?
Many residential garage floors the size of this 12x20 are poured 4 in thick, which clears the IRC minimum of 3-1/2 in for cars and light trucks. Go to 5 or 6 in if the 12x20 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 12x20 takes about 4.89 cu yd.
Control joints for a 12x20 slab at 4 in
Like any slab, a 4 in 12x20 shrinks while it cures; control joints decide where its cracks go. The usual rule (ACI and PCA guidance) puts joints in a 12x20 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 12x20 at 4 in, a joint every 10 ft or less means 2 joints across the 20 ft side and 1 joint across the 12 ft side: 6 panels of about 6.7 × 6 ft and 44 ft of saw cut. For the 12x20 that's more joints than the spacing alone calls for, so that no panel is more than 1.5 times as long as it is wide. The 12x20 is 1.7 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 12x20 slab 4 in thick weighs
Wet concrete weighs about 150 lb per cubic foot, so the 12x20 pour is about 13,200 lb (6.6 tons). As bags, the 12x20 is 147 80 lb bags, 11,760 lb in all: about 8 trips in a pickup carrying 1,500 lb a load. Nobody hauls 11,760 lb of bags for a 12x20 in a pickup; it comes by ready-mix truck or as pallets on a delivery truck.
The 12x20 at 4 in against nearby sizes
Every row uses the same 10% waste and prices as the 12x20 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 |
|---|---|---|---|---|---|---|
| 10x20 slab, 4 in | 2.72 | 163 | 123 | 2.75 cu yd | $815 – $1,141 | $445 – $605 |
| 12x20 slab, 4 in (this page) | 3.26 | 196 | 147 | 3.5 cu yd | $980 – $1,372 | $550 – $740 |
| 16x16 slab, 4 in | 3.48 | 209 | 157 | 3.5 cu yd | $1,045 – $1,463 | $550 – $740 |
Questions about a 12x20 slab, 4 in
How many bags of concrete for a 12x20 slab 4 inches thick?
A 12x20 slab at 4 in takes about 147 80 lb bags or 196 60 lb bags (or 235 50 lb, 294 40 lb), with 10% added for waste: 88 cubic feet in all.
How many yards of concrete for a 12x20 slab 4 inches thick?
3.26 cubic yards with waste (2.96 without) for a 12x20 at 4 in. Round the 12x20 order to 3.5 cu yd: plants batch in quarter yards and most won't send less than a yard.
How much does a 12x20 concrete slab 4 inches thick cost?
Concrete alone for the 12x20 runs about $550 – $740 for 3.5 cu yd of ready-mix delivered, short-load fee included, or $980 – $1,372 for 196 60 lb bags. Add a 4 in gravel base and wire mesh and the bagged 12x20 plan comes to $1,212 – $1,750. Labor, forms and finishing for a 12x20 at 4 in come on top of these national ranges (as of 2026-09), which are not quotes.
How much does a 12x20 concrete slab 4 inches thick weigh?
About 13,200 lb (6.6 tons) of wet concrete for the 12x20, at 150 lb per cubic foot. The 147 80 lb bags for a 12x20 weigh 11,760 lb: about 8 trips in a pickup carrying 1,500 lb a load.
Where do control joints go on a 12x20 slab 4 inches thick?
On a 12x20 at 4 in, every 8 to 12 ft (24 to 36 times the thickness), sawn 1 in deep. On the 12x20 at 4 in, a joint every 10 ft or less means 2 joints across the 20 ft side and 1 joint across the 12 ft side: 6 panels of about 6.7 × 6 ft and 44 ft of saw cut.
Limitations
The 12x20 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 12x20 slab with a qualified professional.