18 by 30 ft slab at 6 inches
Calculate the requirements for a residential slab measuring 18 ft by 30 ft at 6 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
12.87 yd³
9.843 m³ placed+waste
Order weight
52,140 lb
23,650 kg
80 lb bags
580
14 pallets at 42 bags
60 lb bags
773
0.45 ft³ yield each
Rebar
960.00 ft
57 × 20 ft · 57 × 6 m · 705 lb
Ready-mix vs bags
$2,124.22
Truck $2,124.22 · 80 lb bags $3,915.00
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 | 11.70 yd³ | 10% | 12.87 yd³ / 9.843 m³ | USD | $2,124.22 | Source Spec Material / Tool |
| Short-load fee | Full load | — | 2 truck @ 10 yd³ | USD | $0.00 | Source Spec Material / Tool |
| 80 lb pre-mix bags | 526.7 bags | 10% | 580 bags | USD | $3,915.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 702.2 bags | 10% | 773 bags | USD | $4,174.20 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 57 sticks | 10% | 63 sticks / 57 × 6 m | USD | $882.00 | 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
$2,124.22
1.29 nominal 10 yd³ loads · 2 to order
80 lb bags
$3,915.00
60 lb alternative $4,174.20 · delivery weight 52,140 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 9.83 | 7.517 | 443 | 1,056 | $1,622.30 | $2,990.25 |
| 5 in | 11.35 | 8.680 | 511 | 1,056 | $1,873.26 | $3,449.25 |
| 6 in | 12.87 | 9.843 | 580 | 1,056 | $2,124.22 | $3,915.00 |
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 30 ft slab at 6 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 concrete slab measuring 18 feet by 30 feet with a thickness of 6 inches. The design incorporates #4 reinforcing bars spaced at 18 inches on center, following the guidelines set forth in IRC R506.
Please note that soil design considerations are excluded from this calculation. It is essential to consult a qualified engineer for soil analysis and site-specific conditions.
Code reference
IRC R506 - Concrete Floors on Ground
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 overall structural integrity.
- Why is soil design excluded from this calculation?
- Soil design is site-specific and requires a geotechnical analysis to determine the appropriate bearing capacity and other factors affecting the slab's performance.
- What thickness is recommended for residential slabs?
- A thickness of 6 inches is commonly recommended for residential slabs, providing adequate strength for typical loads.
- How do I ensure compliance with IRC R506?
- To ensure compliance with IRC R506, follow the guidelines for concrete mix, reinforcement, and placement as outlined in the code.