18 by 30 ft slab at 8 inches
Calculate the specifications for a residential slab measuring 18 ft by 30 ft at 8 in thickness with #4 bars at 18 in spacing. Ensure compliance with IRC R506 guidelines.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
15.92 yd³
12.169 m³ placed+waste
Order weight
64,460 lb
29,239 kg
80 lb bags
717
18 pallets at 42 bags
60 lb bags
955
0.45 ft³ yield each
Rebar
960.00 ft
57 × 20 ft · 57 × 6 m · 705 lb
Ready-mix vs bags
$2,626.15
Truck $2,626.15 · 80 lb bags $4,839.75
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 | 14.47 yd³ | 10% | 15.92 yd³ / 12.169 m³ | USD | $2,626.15 | 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 | 651.1 bags | 10% | 717 bags | USD | $4,839.75 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 868.1 bags | 10% | 955 bags | USD | $5,157.00 | 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 |
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Ready-mix truck
$2,626.15
1.59 nominal 10 yd³ loads · 2 to order
80 lb bags
$4,839.75
60 lb alternative $5,157.00 · delivery weight 64,460 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 8 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 requirements for a residential concrete slab measuring 18 ft by 30 ft with a thickness of 8 inches. Reinforcement is provided using #4 bars spaced at 18 inches apart. Please note that soil design considerations are excluded from this calculation.
For compliance, refer to the International Residential Code (IRC) R506, which outlines the standards for concrete slabs in residential construction.
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 structural integrity.
- Why is soil design excluded from this calculation?
- Soil design is a separate consideration that depends on site-specific conditions and should be evaluated by a qualified professional.
- What is the significance of IRC R506?
- IRC R506 provides guidelines for the design and construction of concrete slabs in residential buildings, ensuring safety and compliance with building standards.