18 by 8 ft slab at 4 inches
Calculate the requirements for a residential slab measuring 18 ft by 8 ft at 4 in thickness with #4 bars at 18 in spacing. Ensure compliance with IRC R506 standards.
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.
Contractor sign-off
Project: 18 by 8 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 the preliminary design of a residential slab. The specified dimensions are 18 ft by 8 ft with a thickness of 4 inches, reinforced with #4 bars spaced at 18 inches on center.
Note that soil design considerations are excluded from this calculation, and users should consult a qualified engineer for site-specific soil analysis and compliance with IRC R506.
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.
- Why is soil design excluded from this calculator?
- Soil design is site-specific and requires a detailed analysis by a qualified engineer to ensure proper load-bearing capacity and stability.
- What does IRC R506 cover?
- IRC R506 provides guidelines for the design and construction of concrete slabs on grade, including requirements for reinforcement, thickness, and curing.
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