18 by 12 ft slab at 5 inches
Calculate the requirements for an 18 ft by 12 ft residential slab at 5 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
3.67 yd³
2.803 m³ placed+waste
Order weight
14,850 lb
6,736 kg
80 lb bags
165
4 pallets at 42 bags
60 lb bags
220
0.45 ft³ yield each
Rebar
318.00 ft
22 × 20 ft · 22 × 6 m · 234 lb
Ready-mix vs bags
$730.00
Truck $730.00 · 80 lb bags $1,113.75
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 | 3.33 yd³ | 10% | 3.67 yd³ / 2.803 m³ | USD | $605.00 | 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 | 150.0 bags | 10% | 165 bags | USD | $1,113.75 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 200.0 bags | 10% | 220 bags | USD | $1,188.00 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 22 sticks | 10% | 25 sticks / 22 × 6 m | USD | $350.00 | Source Spec Material / Tool |
| Rebar chairs | 117 crossings | 10% | 129 chairs | USD | $36.12 | Source Spec Material / Tool |
| Tie wire rolls | 117 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
$730.00
0.37 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,113.75
60 lb alternative $1,188.00 · delivery weight 14,850 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 2.93 | 2.243 | 132 | 350 | $609.00 | $891.00 |
| 5 in | 3.67 | 2.803 | 165 | 350 | $730.00 | $1,113.75 |
| 6 in | 4.40 | 3.364 | 198 | 350 | $726.00 | $1,336.50 |
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 12 ft slab at 5 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 helps you determine the specifications for a residential concrete slab measuring 18 ft by 12 ft and 5 inches thick, reinforced with #4 bars spaced 18 inches apart. Please note that soil design considerations are excluded from this calculation.
For compliance, refer to the International Residential Code (IRC) Section R506, which outlines the requirements for concrete slabs in residential construction.
Code reference
IRC R506
Field questions
- What is the purpose of using #4 bars in the slab?
- The #4 bars provide tensile strength to the concrete slab, helping to resist cracking and improve overall structural integrity.
- Why is soil design excluded from this calculation?
- Soil design is a separate consideration that depends on the specific site conditions and soil characteristics, which are not covered by this calculator.
- What thickness is recommended for residential slabs?
- A thickness of 5 inches is commonly recommended for residential slabs, but it may vary based on load requirements and local building codes.