12 by 20 ft slab at 5 inches
Calculate the specifications for a residential slab measuring 12 ft by 20 ft at 5 inches thick, reinforced with #4 bars at 18 inches apart. Compliance with IRC R506. Soil design is excluded.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
4.07 yd³
3.115 m³ placed+waste
Order weight
16,500 lb
7,484 kg
80 lb bags
184
5 pallets at 42 bags
60 lb bags
245
0.45 ft³ yield each
Rebar
348.00 ft
23 × 20 ft · 32 × 6 m · 256 lb
Ready-mix vs bags
$672.22
Truck $672.22 · 80 lb bags $1,242.00
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 | Source Spec Material / Tool |
|---|---|---|---|---|---|---|
| Ready-mix concrete | 3.70 yd³ | 10% | 4.07 yd³ / 3.115 m³ | USD | $672.22 | Source Spec Material / Tool |
| Short-load fee | Full load | — | 1 truck @ 10 yd³ | USD | $0.00 | Source Spec Material / Tool |
| 80 lb pre-mix bags | 166.7 bags | 10% | 184 bags | USD | $1,242.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 222.2 bags | 10% | 245 bags | USD | $1,323.00 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 23 sticks | 10% | 26 sticks / 32 × 6 m | USD | $364.00 | Source Spec Material / Tool |
| Rebar chairs | 126 crossings | 10% | 139 chairs | USD | $38.92 | Source Spec Material / Tool |
| Tie wire rolls | 126 ties | 10% | 1 rolls | USD | $8.00 | Source Spec Material / Tool |
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Ready-mix truck
$672.22
0.41 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,242.00
60 lb alternative $1,323.00 · delivery weight 16,500 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 3.26 | 2.492 | 147 | 383 | $662.78 | $992.25 |
| 5 in | 4.07 | 3.115 | 184 | 383 | $672.22 | $1,242.00 |
| 6 in | 4.89 | 3.738 | 220 | 383 | $806.67 | $1,485.00 |
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: 12 by 20 ft slab at 5 inches
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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
This calculator provides an overview for designing a residential slab measuring 12 feet by 20 feet with a thickness of 5 inches. The slab is reinforced with #4 rebar spaced 18 inches apart, ensuring adequate structural integrity.
Note that this design does not include soil design considerations, which should be evaluated separately to ensure proper support and stability.
Code reference
IRC R506 - Concrete Floors on Ground
Field questions
- What is the purpose of using #4 bars in the slab?
- The #4 bars provide tensile strength to the concrete slab, helping to prevent cracking and improve overall durability.
- Why is soil design excluded from this calculation?
- Soil design is excluded because it requires specific geotechnical analysis based on the site's unique conditions, which is beyond the scope of this calculator.
- What thickness is recommended for a residential slab?
- A thickness of 5 inches is commonly recommended for residential slabs, providing a balance between strength and material usage.
- How far apart should the rebar be placed?
- In this design, the #4 rebar is placed 18 inches apart, which is a standard spacing for residential slabs to ensure adequate reinforcement.