8 by 18 ft slab at 4 inches
Calculate the specifications for an 8 ft by 18 ft residential slab with #4 bars at 18 in spacing. Ensure compliance with IRC R506 standards. Soil design is excluded.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
1.96 yd³
1.495 m³ placed+waste
Order weight
7,920 lb
3,592 kg
80 lb bags
88
3 pallets at 42 bags
60 lb bags
118
0.45 ft³ yield each
Rebar
212.00 ft
19 × 20 ft · 19 × 6 m · 156 lb
Ready-mix vs bags
$447.67
Truck $447.67 · 80 lb bags $594.00
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 | 1.78 yd³ | 10% | 1.96 yd³ / 1.495 m³ | USD | $322.67 | 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 | 80.0 bags | 10% | 88 bags | USD | $594.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 106.7 bags | 10% | 118 bags | USD | $637.20 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 19 sticks | 10% | 21 sticks / 19 × 6 m | USD | $294.00 | Source Spec Material / Tool |
| Rebar chairs | 78 crossings | 10% | 86 chairs | USD | $24.08 | Source Spec Material / Tool |
| Tie wire rolls | 78 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
$447.67
0.20 nominal 10 yd³ loads · 1 to order
80 lb bags
$594.00
60 lb alternative $637.20 · delivery weight 7,920 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 1.96 | 1.495 | 88 | 233 | $447.67 | $594.00 |
| 5 in | 2.44 | 1.869 | 110 | 233 | $528.33 | $742.50 |
| 6 in | 2.93 | 2.243 | 132 | 233 | $609.00 | $891.00 |
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.
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Project: 8 by 18 ft slab at 4 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 determining the specifications for a residential concrete slab measuring 8 ft by 18 ft and 4 inches thick, reinforced with #4 rebar spaced at 18 inches on center. Please note that soil design considerations are excluded from this calculation.
Ensure that all construction practices adhere to the guidelines set forth in the International Residential Code (IRC) R506 for concrete slabs.
Code reference
IRC R506
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.
- Is soil design included in this calculation?
- No, soil design considerations are excluded from this calculation. It is important to consult a geotechnical engineer for soil-related issues.
- What thickness is recommended for a residential slab?
- For most residential applications, a thickness of 4 inches is commonly recommended, but this may vary based on specific load requirements and local codes.
- What spacing is recommended for rebar in a residential slab?
- In this case, #4 bars are spaced at 18 inches on center, which is a common practice for residential slabs to ensure adequate reinforcement.
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