8 by 18 ft slab at 8 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 guidelines. Soil design considerations are excluded.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
4.56 yd³
3.489 m³ placed+waste
Order weight
18,480 lb
8,382 kg
80 lb bags
206
5 pallets at 42 bags
60 lb bags
274
0.45 ft³ yield each
Rebar
316.00 ft
25 × 20 ft · 25 × 6 m · 232 lb
Ready-mix vs bags
$752.89
Truck $752.89 · 80 lb bags $1,390.50
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 | 4.15 yd³ | 10% | 4.56 yd³ / 3.489 m³ | USD | $752.89 | 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 | 186.7 bags | 10% | 206 bags | USD | $1,390.50 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 248.9 bags | 10% | 274 bags | USD | $1,479.60 | 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
$752.89
0.46 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,390.50
60 lb alternative $1,479.60 · delivery weight 18,480 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 8 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 slab measuring 8 ft by 18 ft with a thickness of 8 inches. The slab reinforcement consists of #4 bars placed at 18 inches on center, ensuring structural integrity.
Please note that while this calculator follows the guidelines set forth in IRC R506, it does not include soil design considerations, which should be evaluated 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, helping to control cracking and improve load-bearing capacity.
- Why is soil design excluded from this calculator?
- Soil design is a complex aspect that requires site-specific analysis and should be performed by a qualified engineer to ensure proper foundation support.
- What does IRC R506 cover?
- IRC R506 outlines the requirements for concrete slabs on grade, including thickness, reinforcement, and construction practices to ensure safety and durability.
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