8 by 30 ft slab at 4 inches
Calculate the specifications for an 8 ft by 30 ft residential slab with #4 bars at 18 in spacing. Ensure compliance with IRC R506 guidelines while excluding soil design considerations.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
3.26 yd³
2.492 m³ placed+waste
Order weight
13,200 lb
5,987 kg
80 lb bags
147
4 pallets at 42 bags
60 lb bags
196
0.45 ft³ yield each
Rebar
348.00 ft
33 × 20 ft · 33 × 6 m · 256 lb
Ready-mix vs bags
$662.78
Truck $662.78 · 80 lb bags $992.25
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 | 2.96 yd³ | 10% | 3.26 yd³ / 2.492 m³ | USD | $537.78 | 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 | 133.3 bags | 10% | 147 bags | USD | $992.25 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 177.8 bags | 10% | 196 bags | USD | $1,058.40 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 33 sticks | 10% | 37 sticks / 33 × 6 m | USD | $518.00 | Source Spec Material / Tool |
| Rebar chairs | 126 crossings | 10% | 139 chairs | USD | $38.92 | Source Spec Material / Tool |
| Tie wire rolls | 126 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
$662.78
0.33 nominal 10 yd³ loads · 1 to order
80 lb bags
$992.25
60 lb alternative $1,058.40 · delivery weight 13,200 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 3.26 | 2.492 | 147 | 383 | $662.78 | $992.25 |
| 5 in | 4.07 | 3.115 | 184 | 383 | $672.22 | $1,242.00 |
| 6 in | 4.89 | 3.738 | 220 | 383 | $806.67 | $1,485.00 |
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 30 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 provides guidance for designing a residential slab measuring 8 ft by 30 ft with a thickness of 4 inches, reinforced with #4 bars spaced at 18 inches. It is essential to follow the IRC R506 guidelines to ensure structural integrity.
Please note that soil design considerations are excluded from this calculation and should be addressed separately by a qualified professional.
Code reference
IRC R506 - Concrete Floors on Ground
Field questions
- What is the purpose of using #4 bars in slab construction?
- Using #4 bars in slab construction helps to reinforce the concrete, providing additional tensile strength and preventing cracking.
- Why is soil design excluded from this slab calculation?
- Soil design is excluded because it requires specific analysis of soil conditions, which should be conducted by a qualified engineer to ensure proper support for the slab.
- What does IRC R506 cover?
- IRC R506 covers the requirements for concrete floors on ground, including design, construction, and reinforcement specifications.
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