18 by 30 ft slab at 4 inches
Calculate the requirements for an 18 ft by 30 ft residential slab at 4 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
7.33 yd³
5.607 m³ placed+waste
Order weight
29,700 lb
13,472 kg
80 lb bags
330
8 pallets at 42 bags
60 lb bags
440
0.45 ft³ yield each
Rebar
768.00 ft
47 × 20 ft · 47 × 6 m · 564 lb
Ready-mix vs bags
$1,210.00
Truck $1,210.00 · 80 lb bags $2,227.50
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 | 6.67 yd³ | 10% | 7.33 yd³ / 5.607 m³ | USD | $1,210.00 | 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 | 300.0 bags | 10% | 330 bags | USD | $2,227.50 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 400.0 bags | 10% | 440 bags | USD | $2,376.00 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 47 sticks | 10% | 52 sticks / 47 × 6 m | USD | $728.00 | Source Spec Material / Tool |
| Rebar chairs | 273 crossings | 10% | 301 chairs | USD | $84.28 | Source Spec Material / Tool |
| Tie wire rolls | 273 ties | 10% | 2 rolls | USD | $16.00 | 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
$1,210.00
0.73 nominal 10 yd³ loads · 1 to order
80 lb bags
$2,227.50
60 lb alternative $2,376.00 · delivery weight 29,700 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 7.33 | 5.607 | 330 | 845 | $1,210.00 | $2,227.50 |
| 5 in | 9.17 | 7.008 | 413 | 845 | $1,512.50 | $2,787.75 |
| 6 in | 11.00 | 8.410 | 495 | 845 | $1,815.00 | $3,341.25 |
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 30 ft slab at 4 inches
Prepared by
Date
Verified by (PE / electrician / inspector)
License number
Signature
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 slab measuring 18 ft by 30 ft with a thickness of 4 inches. The design incorporates #4 rebar spaced at 18 inches on center, ensuring structural integrity.
Please note that soil design considerations are excluded from this calculation. For accurate results, consult local building codes and a qualified engineer.
Code reference
IRC R506 - Concrete Floors on Ground
Field questions
- What is the purpose of using #4 bars in the slab?
- Using #4 bars provides reinforcement to the concrete slab, helping to prevent cracking and improve load-bearing capacity.
- Why is soil design excluded from this calculation?
- Soil design is excluded because it requires site-specific analysis and testing, which varies based on local conditions and cannot be generalized.
- What thickness is recommended for a residential slab?
- A thickness of 4 inches is commonly recommended for residential slabs, but this can vary based on load requirements and local building codes.
- How do I ensure compliance with IRC R506?
- To ensure compliance with IRC R506, follow the guidelines for concrete mix, reinforcement, and installation as specified in the code.