18 by 18 ft slab at 8 inches
Calculate the requirements for a residential concrete slab measuring 18 ft by 18 ft and 8 in thick, reinforced with #4 bars at 18 in spacing. Reference IRC R506 for guidelines.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
9.72 yd³
7.434 m³ placed+waste
Order weight
39,380 lb
17,862 kg
80 lb bags
438
11 pallets at 42 bags
60 lb bags
584
0.45 ft³ yield each
Rebar
612.00 ft
34 × 20 ft · 34 × 6 m · 450 lb
Ready-mix vs bags
$1,604.37
Truck $1,604.37 · 80 lb bags $2,956.50
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 | 8.84 yd³ | 10% | 9.72 yd³ / 7.434 m³ | USD | $1,604.37 | 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 | 397.8 bags | 10% | 438 bags | USD | $2,956.50 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 530.4 bags | 10% | 584 bags | USD | $3,153.60 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 34 sticks | 10% | 38 sticks / 34 × 6 m | USD | $532.00 | Source Spec Material / Tool |
| Rebar chairs | 169 crossings | 10% | 186 chairs | USD | $52.08 | Source Spec Material / Tool |
| Tie wire rolls | 169 ties | 10% | 1 rolls | USD | $8.00 | Source Spec Material / Tool |
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Ready-mix truck
$1,604.37
0.97 nominal 10 yd³ loads · 1 to order
80 lb bags
$2,956.50
60 lb alternative $3,153.60 · delivery weight 39,380 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 6.25 | 4.776 | 282 | 673 | $1,030.74 | $1,903.50 |
| 5 in | 7.12 | 5.441 | 321 | 673 | $1,174.15 | $2,166.75 |
| 6 in | 7.99 | 6.105 | 360 | 673 | $1,317.56 | $2,430.00 |
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 18 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 is designed for a residential concrete slab measuring 18 ft by 18 ft with a thickness of 8 inches. The slab is reinforced with #4 rebar spaced at 18 inches on center. Please note that soil design considerations are excluded from this calculation.
Refer to IRC R506 for guidelines on concrete slab construction, including reinforcement and curing practices.
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.
- Is soil design included in this calculation?
- No, soil design considerations are excluded from this calculator. It is important to consult a professional for soil analysis.
- What thickness is recommended for a residential slab?
- An 8-inch thickness is commonly recommended for residential slabs, especially for areas with heavier loads.
- How far apart should the rebar be placed?
- In this design, the #4 rebar should be placed 18 inches on center.