20x20 Concrete Slab, 6 Inches Thick
Poured 6 in thick, a 20 × 20 ft slab covering 400 sq ft needs about 8.15 cubic yards of concrete once 10% is added for waste. For ready-mix, order 8.25 cu yd for the 20x20, about $1,215 – $1,595 delivered, short-load fee included. Bagged, the same 20x20 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
The 20x20 (6 in) prices are national material ranges as of 2026-09, not a quote from a supplier.
When a 20x20 slab should be 6 inches thick
Going to 6 in on a 20x20 makes sense for a two-car garage that will hold heavy pickups, a loaded work truck or a small tractor, or where the soil is clay or not well compacted.
Many two-post car lifts list a minimum slab thickness and concrete strength in their installation manuals, and a 6 in floor clears most of them; check yours before you pour.
At this thickness, plenty of builders switch from wire mesh to a rebar grid, and the joint spacing stretches to about 12 to 18 ft, so a 20x20 can be cut in quarters.
A 6 in 20x20 pour: check first
- Order 4,000 psi concrete if a lift, heavy trucks or road salt are in the plan; ask the supplier what they recommend locally.
- Size the rebar grid with the rebar calculator before you order.
- Keep the slab level where a lift will stand, and slope only the drive-in area toward the door.
Full plan for a 20x20 slab, 6 in
- Area
- 400 sq ft
- Thickness
- 6 in
- Volume
- 8.15 cu yd
Bagged concrete estimate (DIY)
$2,832 – $4,054
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. 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: 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 20x20 slab, 6 in?
The 20x20 at 6 in needs 2 trucks: 8.25 cu yd at 8 cu yd a load, the low end of what a truck carries, for 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 400 sq ft at 6 in, and ask about a pump if the truck can't reach every corner of the 20x20. Ready-mix wins for this 20x20 by roughly $1,529 (range midpoints), and nobody has to mix 367 bags by hand.
Is 6 in thick enough for a 20x20 slab?
6 in is the step up for a 20x20 that will carry heavy pickups, a loaded trailer or a car lift, or that sits on soft or clay soil. Check your local code and the equipment maker's slab specs; at 4 in the same 20x20 would take about 5.43 cu yd. For a 20x20 at 6 in, many builders use rebar rather than wire mesh; the rebar calculator sizes a grid for it. See also: 20x20 slab at 4 in.
Control joints for a 20x20 slab at 6 in
Like any slab, a 6 in 20x20 shrinks while it cures; control joints decide where its cracks go. ACI and PCA guidance spaces joints 24 to 36 times the thickness, which for this 20x20 at 6 in is every 12 to 18 ft, sawn 1.5 in deep (a quarter of the slab) once the surface holds up to the blade, usually within a day.
On the 20x20 at 6 in, a joint every 15 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 6 in thick weighs
At roughly 150 lb per cubic foot of wet concrete, a 20x20 at 6 in puts about 33,000 lb (16.5 tons) on the ground. As bags, the 20x20 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 20x20 in a pickup; it comes by ready-mix truck or as pallets on a delivery truck.
The 20x20 at 6 in against nearby sizes
Every row uses the same 10% waste and prices as the 20x20 at 6 in; costs are for the concrete only.
| Slab | Cu yd | 60 lb bags | 80 lb bags | Ready-mix order | Bags cost | Ready-mix cost |
|---|---|---|---|---|---|---|
| 20x20 slab, 6 in (this page) | 8.15 | 489 | 367 | 8.25 cu yd | $2,445 – $3,423 | $1,215 – $1,595 |
| 30x30 slab, 6 in | 18.33 | 1,100 | 825 | 18.5 cu yd | $5,500 – $7,700 | $2,590 – $3,330 |
| 20x20 slab, 4 in | 5.43 | 326 | 245 | 5.5 cu yd | $1,630 – $2,282 | $830 – $1,100 |
Questions about a 20x20 slab, 6 in
How many bags of concrete for a 20x20 slab 6 inches thick?
A 20x20 slab at 6 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 20x20 slab 6 inches thick?
8.15 cubic yards with waste (7.41 without) for a 20x20 at 6 in. Round the 20x20 order to 8.25 cu yd: plants batch in quarter yards and most won't send less than a yard.
How much does a 20x20 concrete slab 6 inches thick cost?
Concrete alone for the 20x20 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 20x20 plan comes to $2,832 – $4,054. Labor, forms and finishing for a 20x20 at 6 in come on top of these national ranges (as of 2026-09), which are not quotes.
How much does a 20x20 concrete slab 6 inches thick weigh?
About 33,000 lb (16.5 tons) of wet concrete for the 20x20, at 150 lb per cubic foot. The 367 80 lb bags for a 20x20 weigh 29,360 lb: about 20 trips in a pickup carrying 1,500 lb a load.
Where do control joints go on a 20x20 slab 6 inches thick?
On a 20x20 at 6 in, every 12 to 18 ft (24 to 36 times the thickness), sawn 1.5 in deep. On the 20x20 at 6 in, a joint every 15 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 6 in are material estimates and typical price ranges, not a quote or a structural design. Confirm that 6 in, the reinforcement and the joint layout suit your soil, loads and local code for a 20x20 slab with a qualified professional.