8 by 30 ft slab at 5 inches
Calculate the design for an 8 ft by 30 ft residential slab with #4 bars at 18 in spacing. Ensure compliance with IRC R506. Soil design considerations are excluded.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
4.07 yd³
3.115 m³ placed+waste
Order weight
16,500 lb
7,484 kg
80 lb bags
184
5 pallets at 42 bags
60 lb bags
245
0.45 ft³ yield each
Rebar
348.00 ft
33 × 20 ft · 33 × 6 m · 256 lb
Ready-mix vs bags
$672.22
Truck $672.22 · 80 lb bags $1,242.00
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 | 3.70 yd³ | 10% | 4.07 yd³ / 3.115 m³ | USD | $672.22 | 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 | 166.7 bags | 10% | 184 bags | USD | $1,242.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 222.2 bags | 10% | 245 bags | USD | $1,323.00 | 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
$672.22
0.41 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,242.00
60 lb alternative $1,323.00 · delivery weight 16,500 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 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 assists in determining the specifications for a residential slab measuring 8 ft by 30 ft with a thickness of 5 inches. The design incorporates #4 rebar spaced at 18 inches on center, ensuring structural integrity.
Note that this design does not include soil design considerations, which should be evaluated separately to ensure proper support and stability.
Code reference
IRC R506 - Concrete Floors on Ground
Field questions
- What is the purpose of using #4 bars in the slab?
- Using #4 bars helps to reinforce the concrete slab, providing additional tensile strength and preventing cracking.
- Why is soil design excluded from this calculation?
- Soil design is excluded because it requires specific geotechnical analysis based on the site conditions, which varies by location.
- What thickness is recommended for a residential slab?
- A thickness of 5 inches is commonly recommended for residential slabs, but it may vary based on load requirements and local codes.
- How often should the rebar be spaced in this slab design?
- In this design, the rebar is spaced at 18 inches on center, which is a standard spacing for residential slabs.
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