18 by 30 ft slab at 5 inches
Calculate the requirements for an 18 ft by 30 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
9.17 yd³
7.008 m³ placed+waste
Order weight
37,125 lb
16,840 kg
80 lb bags
413
10 pallets at 42 bags
60 lb bags
550
0.45 ft³ yield each
Rebar
768.00 ft
47 × 20 ft · 47 × 6 m · 564 lb
Ready-mix vs bags
$1,512.50
Truck $1,512.50 · 80 lb bags $2,787.75
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 | 8.33 yd³ | 10% | 9.17 yd³ / 7.008 m³ | USD | $1,512.50 | 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 | 375.0 bags | 10% | 413 bags | USD | $2,787.75 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 500.0 bags | 10% | 550 bags | USD | $2,970.00 | 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
$1,512.50
0.92 nominal 10 yd³ loads · 1 to order
80 lb bags
$2,787.75
60 lb alternative $2,970.00 · delivery weight 37,125 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 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 18 ft by 30 ft with a thickness of 5 inches. The design incorporates #4 rebar spaced 18 inches apart, ensuring structural integrity and compliance with IRC R506.
Please note that soil design considerations are excluded from this calculation and should be addressed separately by a qualified professional.
Code reference
IRC R506
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 load-bearing capacity.
- Why is soil design excluded from this calculation?
- Soil design is a critical aspect that varies based on local conditions and requires specific analysis by a qualified engineer, which is beyond the scope of this calculator.
- What does IRC R506 cover?
- IRC R506 provides guidelines for the design and construction of concrete slabs on grade, ensuring they meet safety and performance standards.