20x30 Concrete Slab, 4 Inches Thick
A 20 × 30 ft slab (600 sq ft) at 4 in thick takes about 8.15 cubic yards of concrete, with 10% extra for waste. For ready-mix, order 8.25 cu yd for the 20x30, about $1,215 – $1,595 delivered, short-load fee included. Bagged, the same 20x30 is 367 80 lb or 489 60 lb bags, about $2,445 – $3,423 for the concrete alone.
- Ready-mix order
- 8.25 cu yd
- Ready-mix cost
- $1,215 – $1,595
- 80 lb bags
- 367
- Bag cost
- $2,445 – $3,423
Materials only for a 20x30 at 4 in, from national price ranges as of 2026-09; not a live quote.
What a 20x30 slab is used for
A 20x30 slab is a two-car garage with a 10 ft workshop or storage bay behind the cars, or a three-stall shed for a tractor, a mower and a trailer.
At 600 sq ft, it is a common size for a detached garage that needs room for bikes, a freezer and a workbench as well as the cars.
Pour it in one go if the truck can reach; a 30 ft run is long enough that the joints should be laid out before the concrete arrives.
Before you pour the 20x30 at 4 in
- Decide which end gets the doors, and keep the workshop end level for benches and machines.
- Plan floor drains or a slope to the door for melting snow and car washing.
- Mark the saw-cut lines on the forms so the cuts go in on time the next day.
Full plan for a 20x30 slab, 4 in
- Area
- 600 sq ft
- Thickness
- 4 in
- Volume
- 8.15 cu yd
Bagged concrete estimate (DIY)
$3,026 – $4,369
Est. as of 2026-09 — not a live quote
Ready-mix delivered, incl. short-load fee (materials only)
$1,215 – $1,595
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 734 × 40 lb, 587 × 50 lb or 367 × 80 lb bags. 489 bags
- Ready-mix concrete Alternative to bagged mix; most suppliers have a 1 cu yd order minimum. 8.25 cu yd
- Compactable gravel sub-base 4 in compacted base beneath the slab. 10 tons
- 6x6 welded wire mesh Reinforcement for crack control; not a substitute for engineered rebar on structural slabs. 660 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: 33,000 lb (16.5 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 20x30 slab, 4 in?
8.25 cu yd for the 20x30 is 2 truckloads at 8 cu yd each (the conservative end of the 8 to 10 a truck carries), about $1,215 – $1,595, short-load fee included. 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 600 sq ft at 4 in, and ask about a pump if the truck can't reach every corner of the 20x30. On the 20x30, ready-mix comes out cheaper by about $1,529 at the middle of each range, and it saves mixing 367 bags.
Is 4 in thick enough for a 20x30 slab?
Many residential garage floors the size of this 20x30 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 20x30 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 20x30 takes about 12.22 cu yd.
Control joints for a 20x30 slab at 4 in
A 20x30 slab at 4 in shrinks as it cures, and without joints it cracks wherever it chooses. ACI and PCA guidance spaces joints 24 to 36 times the thickness, which for this 20x30 at 4 in is every 8 to 12 ft, sawn 1 in deep (a quarter of the slab) once the surface holds up to the blade, usually within a day.
On the 20x30 at 4 in, a joint every 10 ft or less means 2 joints across the 30 ft side and 1 joint across the 20 ft side: 6 panels of about 10 × 10 ft and 70 ft of saw cut.
How much a 20x30 slab 4 in thick weighs
Wet concrete weighs about 150 lb per cubic foot, so the 20x30 pour is about 33,000 lb (16.5 tons). As bags, the 20x30 is 367 80 lb bags, 29,360 lb in all: about 20 trips in a pickup carrying 1,500 lb a load. Nobody hauls 29,360 lb of bags for a 20x30 in a pickup; it comes by ready-mix truck or as pallets on a delivery truck.
The 20x30 at 4 in against nearby sizes
Every row uses the same 10% waste and prices as the 20x30 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 |
|---|---|---|---|---|---|---|
| 24x24 slab, 4 in | 7.82 | 470 | 352 | 8 cu yd | $2,350 – $3,290 | $1,180 – $1,550 |
| 20x30 slab, 4 in (this page) | 8.15 | 489 | 367 | 8.25 cu yd | $2,445 – $3,423 | $1,215 – $1,595 |
| 24x30 slab, 4 in | 9.78 | 587 | 440 | 10 cu yd | $2,935 – $4,109 | $1,400 – $1,800 |
Questions about a 20x30 slab, 4 in
How many bags of concrete for a 20x30 slab 4 inches thick?
A 20x30 slab at 4 in takes about 367 80 lb bags or 489 60 lb bags (or 587 50 lb, 734 40 lb), with 10% added for waste: 220 cubic feet in all.
How many yards of concrete for a 20x30 slab 4 inches thick?
The 20x30 measures out at 7.41 cubic yards, or 8.15 with 10% for waste. Order 8.25 cu yd for the 20x30, since ready-mix is sold in quarter yards with a 1 yard minimum at most plants.
How much does a 20x30 concrete slab 4 inches thick cost?
Concrete alone for the 20x30 runs about $1,215 – $1,595 for 8.25 cu yd of ready-mix delivered, short-load fee included, or $2,445 – $3,423 for 489 60 lb bags. Add a 4 in gravel base and wire mesh and the bagged 20x30 plan comes to $3,026 – $4,369. Labor, forms and finishing for a 20x30 at 4 in come on top of these national ranges (as of 2026-09), which are not quotes.
How much does a 20x30 concrete slab 4 inches thick weigh?
About 33,000 lb (16.5 tons) of wet concrete for the 20x30, at 150 lb per cubic foot. The 367 80 lb bags for a 20x30 weigh 29,360 lb: about 20 trips in a pickup carrying 1,500 lb a load.
Where do control joints go on a 20x30 slab 4 inches thick?
On a 20x30 at 4 in, every 8 to 12 ft (24 to 36 times the thickness), sawn 1 in deep. On the 20x30 at 4 in, a joint every 10 ft or less means 2 joints across the 30 ft side and 1 joint across the 20 ft side: 6 panels of about 10 × 10 ft and 70 ft of saw cut.
Limitations
The 20x30 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 20x30 slab with a qualified professional.