8 by 20 ft slab at 5 inches
Calculate the specifications for an 8 ft by 20 ft residential slab at 5 in thickness with #4 bars at 18 in spacing. Ensure compliance with IRC R506 guidelines.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
2.72 yd³
2.077 m³ placed+waste
Order weight
11,000 lb
4,990 kg
80 lb bags
123
3 pallets at 42 bags
60 lb bags
163
0.45 ft³ yield each
Rebar
232.00 ft
20 × 20 ft · 26 × 6 m · 170 lb
Ready-mix vs bags
$573.15
Truck $573.15 · 80 lb bags $830.25
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 | 2.47 yd³ | 10% | 2.72 yd³ / 2.077 m³ | USD | $448.15 | Source Spec Material / Tool |
| Short-load fee | Under 4 yd³ | — | 1 truck @ 10 yd³ | USD | $125.00 | Source Spec Material / Tool |
| 80 lb pre-mix bags | 111.1 bags | 10% | 123 bags | USD | $830.25 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 148.1 bags | 10% | 163 bags | USD | $880.20 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 20 sticks | 10% | 22 sticks / 26 × 6 m | USD | $308.00 | Source Spec Material / Tool |
| Rebar chairs | 84 crossings | 10% | 93 chairs | USD | $26.04 | Source Spec Material / Tool |
| Tie wire rolls | 84 ties | 10% | 1 rolls | USD | $8.00 | Source Spec Material / Tool |
Source links open a product search on the Amazon storefront for the selected currency. SiteSpecCalc may earn a commission. The link does not change the quantities above.
Ready-mix truck
$573.15
0.27 nominal 10 yd³ loads · 1 to order
80 lb bags
$830.25
60 lb alternative $880.20 · delivery weight 11,000 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 2.17 | 1.661 | 98 | 255 | $483.52 | $661.50 |
| 5 in | 2.72 | 2.077 | 123 | 255 | $573.15 | $830.25 |
| 6 in | 3.26 | 2.492 | 147 | 255 | $662.78 | $992.25 |
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: 8 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 is designed to assist in the preliminary design of a residential concrete slab measuring 8 ft by 20 ft with a thickness of 5 inches. The slab incorporates #4 rebar spaced at 18 inches on center, providing adequate reinforcement for typical residential applications.
It is important to note that this tool does not include soil design considerations, which must be evaluated separately to ensure proper support and stability of the slab. Always consult with a licensed engineer for comprehensive design and compliance with local codes.
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 reinforcement to the concrete slab, helping to control cracking and improve structural integrity under load.
- Why is soil design excluded from this calculator?
- Soil design is a critical aspect of foundation stability that requires site-specific analysis. This calculator focuses solely on the slab design parameters.
- What does IRC R506 cover?
- IRC R506 provides guidelines for the design and construction of concrete floors on ground, including thickness, reinforcement, and curing requirements.