20x20 Concrete Slab, 4 Inches Thick
A 20 × 20 ft slab (400 sq ft) at 4 in thick takes about 5.43 cubic yards of concrete, with 10% extra for waste. For ready-mix, order 5.5 cu yd for the 20x20, about $830 – $1,100 delivered, short-load fee included. Bagged, the same 20x20 is 245 80 lb or 326 60 lb bags, about $1,630 – $2,282 for the concrete alone.
- Ready-mix order
- 5.5 cu yd
- Ready-mix cost
- $830 – $1,100
- 80 lb bags
- 245
- Bag cost
- $1,630 – $2,282
Materials only for a 20x20 at 4 in, from national price ranges as of 2026-09; not a live quote.
Is a 20x20 slab a two-car garage?
Twenty by twenty is the smallest size usually sold as a two-car garage: two cars fit side by side under a 16 ft door, with little room left for storage or a workbench.
Many residential garage floors are poured 4 in thick, above the IRC minimum of 3-1/2 in, which suits passenger cars on firm, well-compacted soil.
A 20x20 also makes a large patio or a pad for a storage building, where two people can work on it in one day if the concrete arrives by truck.
Before you pour the 20x20 at 4 in
- Slope the floor toward the 16 ft door opening, about 1/8 in per foot, so water drains out.
- Ask the building department about frost footings or a thickened edge before forming.
- If a heavy pickup or a car lift will use it, look at the 6 in version of this slab instead.
Full plan for a 20x20 slab, 4 in
- Area
- 400 sq ft
- Thickness
- 4 in
- Volume
- 5.43 cu yd
Bagged concrete estimate (DIY)
$2,017 – $2,913
Est. as of 2026-09 — not a live quote
Ready-mix delivered, incl. short-load fee (materials only)
$830 – $1,100
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 489 × 40 lb, 392 × 50 lb or 245 × 80 lb bags. 326 bags
- Ready-mix concrete Alternative to bagged mix; most suppliers have a 1 cu yd order minimum. 5.5 cu yd
- Compactable gravel sub-base 4 in compacted base beneath the slab. 6.67 tons
- 6x6 welded wire mesh Reinforcement for crack control; not a substitute for engineered rebar on structural slabs. 440 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: 22,000 lb (11 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 20x20 slab, 4 in?
5.43 cu yd for the 20x20 fits on one ready-mix truck (they carry about 8 to 10 cu yd). In bags the 20x20 would be 245 80 lb bags weighing 19,600 lb, so ready-mix is the practical choice. Under 10 cu yd most suppliers add a short-load fee, and the $830 – $1,100 range for the 20x20 includes one. On the 20x20, ready-mix comes out cheaper by about $991 at the middle of each range, and it saves mixing 245 bags.
Is 4 in thick enough for a 20x20 slab?
For a 20x20 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 20x20 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 20x20 takes about 8.15 cu yd. See also: 20x20 slab at 6 in.
Control joints for a 20x20 slab at 4 in
A 20x20 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 20x20 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 20x20 at 4 in, a joint every 10 ft or less means 1 joint each way: 4 panels of about 10 × 10 ft and 40 ft of saw cut.
How much a 20x20 slab 4 in thick weighs
Wet concrete weighs about 150 lb per cubic foot, so the 20x20 pour is about 22,000 lb (11 tons). As bags, the 20x20 is 245 80 lb bags, 19,600 lb in all: about 14 trips in a pickup carrying 1,500 lb a load. Nobody hauls 19,600 lb of bags for a 20x20 in a pickup; it comes by ready-mix truck or as pallets on a delivery truck.
The 20x20 at 4 in against nearby sizes
Every row uses the same 10% waste and prices as the 20x20 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 |
|---|---|---|---|---|---|---|
| 12x30 slab, 4 in | 4.89 | 294 | 220 | 5 cu yd | $1,470 – $2,058 | $760 – $1,010 |
| 20x20 slab, 4 in (this page) | 5.43 | 326 | 245 | 5.5 cu yd | $1,630 – $2,282 | $830 – $1,100 |
| 24x24 slab, 4 in | 7.82 | 470 | 352 | 8 cu yd | $2,350 – $3,290 | $1,180 – $1,550 |
| 20x20 slab, 6 in | 8.15 | 489 | 367 | 8.25 cu yd | $2,445 – $3,423 | $1,215 – $1,595 |
Questions about a 20x20 slab, 4 in
How many bags of concrete for a 20x20 slab 4 inches thick?
Plan on 146.7 cubic feet for the 20x20, which is 245 80 lb bags, 326 60 lb bags, 392 50 lb or 489 40 lb bags once 10% waste is in.
How many yards of concrete for a 20x20 slab 4 inches thick?
The 20x20 measures out at 4.94 cubic yards, or 5.43 with 10% for waste. Order 5.5 cu yd for the 20x20, since ready-mix is sold in quarter yards with a 1 yard minimum at most plants.
How much does a 20x20 concrete slab 4 inches thick cost?
Concrete alone for the 20x20 runs about $830 – $1,100 for 5.5 cu yd of ready-mix delivered, short-load fee included, or $1,630 – $2,282 for 326 60 lb bags. Add a 4 in gravel base and wire mesh and the bagged 20x20 plan comes to $2,017 – $2,913. Labor, forms and finishing for a 20x20 at 4 in come on top of these national ranges (as of 2026-09), which are not quotes.
How much does a 20x20 concrete slab 4 inches thick weigh?
About 22,000 lb (11 tons) of wet concrete for the 20x20, at 150 lb per cubic foot. The 245 80 lb bags for a 20x20 weigh 19,600 lb: about 14 trips in a pickup carrying 1,500 lb a load.
Where do control joints go on a 20x20 slab 4 inches thick?
On a 20x20 at 4 in, every 8 to 12 ft (24 to 36 times the thickness), sawn 1 in deep. On the 20x20 at 4 in, a joint every 10 ft or less means 1 joint each way: 4 panels of about 10 × 10 ft and 40 ft of saw cut.
Limitations
The 20x20 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 20x20 slab with a qualified professional.