20×20 Garage Slab: 4 Inch vs 6 Inch
A 20 ft square garage pad at 4 in, with the comparison table repeating the takeoff at 6 in.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
5.43 yd³
4.153 m³ placed+waste
Order weight
22,000 lb
9,979 kg
80 lb bags
245
6 pallets at 42 bags
60 lb bags
326
0.45 ft³ yield each
Rebar
560.00 ft
28 × 20 ft · 56 × 6 m · 411 lb
Ready-mix vs bags
$896.30
Truck $896.30 · 80 lb bags $1,653.75
Grid lines are equalized so the drawing stays readable. The order count uses on-center spacing with a bar at each end of the layout, then holds concrete cover in the field.
| Item | Net | Waste | Order | Unit price | Line total |
|---|---|---|---|---|---|
| Ready-mix concrete | 4.94 yd³ | 10% | 5.43 yd³ / 4.153 m³ | USD | $896.30 |
| Short-load fee | Full load | — | 1 truck @ 10 yd³ | USD | $0.00 |
| 80 lb pre-mix bags | 222.2 bags | 10% | 245 bags | USD | $1,653.75 |
| 60 lb pre-mix bags | 296.3 bags | 10% | 326 bags | USD | $1,760.40 |
| #4 rebar sticks, 20 ft | 28 sticks | 10% | 31 sticks / 56 × 6 m | USD | $434.00 |
| Rebar chairs | 196 crossings | 10% | 216 chairs | USD | $60.48 |
| Tie wire rolls | 196 ties | 10% | 1 rolls | USD | $8.00 |
Ready-mix truck
$896.30
0.54 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,653.75
60 lb alternative $1,760.40 · delivery weight 22,000 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 5.43 | 4.153 | 245 | 616 | $896.30 | $1,653.75 |
| 6 in | 8.15 | 6.230 | 367 | 616 | $1,344.44 | $2,477.25 |
Show the deterministic formula
Slab ft³ = L × W × thickness ÷ 12. Thickened-edge extra = footing band × (footing depth − slab thickness), with corner squares counted once.
yd³ = ft³ ÷ 27. m³ = ft³ × 0.0283168. Weight uses 150 lb/ft³, then the header converts that pound total at 1 lb = 0.45359237 kg. 80 lb bags yield 0.6 ft³ and 60 lb bags yield 0.45 ft³. Currency changes the price labels only.
Bars across a span = floor(span ÷ spacing) + 1. Order sticks = that count, plus 10, 5, or 15 percent waste, cut from 20 ft and 6 m stock. Chairs = one per grid intersection.
Contractor sign-off
Project: 20×20 Garage Slab: 4 Inch vs 6 Inch
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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.
Engineering notes
A 20 by 20 ft garage at 4 in is 4.94 cubic yards before waste. At 10 percent waste the order is about 5.43 yards, which is often a short load. The same footprint at 6 in is 50 percent more concrete. Use the thickness table on this page before you choose 4 in for a passenger car or 6 in for a heavier vehicle. The default grid is #4 bar at 18 in on center. Hold about 3 in of cover where concrete is cast against earth, and confirm the base compaction separately.
Code reference
IRC R506.1 sets a 3.5 in minimum for concrete slab-on-ground floors. Reinforcement cover and spacing follow ACI 318 practice and the local inspector. This quantity takeoff is not a soils or structural design.
Field questions
- Is 4 inches enough for a two-car garage?
- Many passenger-car garages are built at 4 in over a compacted base, but a heavier truck or poor soil often pushes the spec to 5 or 6 in. The table shows both quantities.
- How many yards is a 20x20 by 4 inch slab?
- The placed volume is 133.33 cubic feet, or 4.94 cubic yards, before the waste factor. At 10 percent the order is about 5.43 cubic yards.
- Does the 6 inch option change the rebar count?
- Thickness does not change the grid count. It changes volume, weight, bags, and the ready-mix price. The highlighted row is the thickness currently selected.