8 by 18 ft slab at 6 inches
Calculate the specifications for an 8 ft by 18 ft residential slab at 6 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
3.91 yd³
2.990 m³ placed+waste
Order weight
15,840 lb
7,185 kg
80 lb bags
176
5 pallets at 42 bags
60 lb bags
235
0.45 ft³ yield each
Rebar
316.00 ft
25 × 20 ft · 25 × 6 m · 232 lb
Ready-mix vs bags
$770.33
Truck $770.33 · 80 lb bags $1,188.00
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 | 3.56 yd³ | 10% | 3.91 yd³ / 2.990 m³ | USD | $645.33 | 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 | 160.0 bags | 10% | 176 bags | USD | $1,188.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 213.3 bags | 10% | 235 bags | USD | $1,269.00 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 25 sticks | 10% | 28 sticks / 25 × 6 m | USD | $392.00 | 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
$770.33
0.39 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,188.00
60 lb alternative $1,269.00 · delivery weight 15,840 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 3.26 | 2.492 | 147 | 348 | $662.78 | $992.25 |
| 5 in | 3.59 | 2.741 | 162 | 348 | $716.56 | $1,093.50 |
| 6 in | 3.91 | 2.990 | 176 | 348 | $770.33 | $1,188.00 |
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: 8 by 18 ft slab at 6 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 assists in determining the requirements for a residential slab measuring 8 ft by 18 ft with a thickness of 6 inches. The design incorporates #4 rebar spaced at 18 inches, adhering to the guidelines set forth in IRC R506.
Please note that soil design considerations are excluded from this calculation and should be addressed separately by a qualified professional.
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, enhancing its tensile strength and helping to prevent cracking.
- Why is soil design excluded from this calculation?
- Soil design is a critical aspect that depends on specific site conditions and should be evaluated by a qualified engineer to ensure proper support for the slab.
- What thickness is recommended for a residential slab?
- For most residential applications, a thickness of 6 inches is commonly recommended, but this can vary based on load requirements and local codes.
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