18 by 8 ft slab at 8 inches
Calculate the specifications for an 18 ft by 8 ft residential slab with #4 bars at 18 in spacing. Ensure compliance with IRC R506 while excluding soil design considerations.
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: 18 by 8 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 assists in determining the necessary specifications for a residential slab measuring 18 ft by 8 ft with a thickness of 8 in. The design incorporates #4 rebar spaced at 18 inches on center, in accordance with IRC R506 guidelines.
Please note that soil design considerations are excluded from this calculation. It is crucial to consult a qualified engineer for soil analysis and site-specific recommendations.
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 calculation?
- Soil design is site-specific and requires a geotechnical analysis to determine the appropriate foundation design based on soil conditions.
- What does IRC R506 cover?
- IRC R506 provides guidelines for the design and construction of concrete slabs on grade, including reinforcement requirements and placement.