18 by 20 ft slab at 6 inches
Calculate the specifications for a residential slab measuring 18 ft by 20 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
8.80 yd³
6.728 m³ placed+waste
Order weight
35,640 lb
16,166 kg
80 lb bags
396
10 pallets at 42 bags
60 lb bags
528
0.45 ft³ yield each
Rebar
664.00 ft
35 × 20 ft · 48 × 6 m · 488 lb
Ready-mix vs bags
$1,452.00
Truck $1,452.00 · 80 lb bags $2,673.00
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 | 8.00 yd³ | 10% | 8.80 yd³ / 6.728 m³ | USD | $1,452.00 | 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 | 360.0 bags | 10% | 396 bags | USD | $2,673.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 480.0 bags | 10% | 528 bags | USD | $2,851.20 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 35 sticks | 10% | 39 sticks / 48 × 6 m | USD | $546.00 | Source Spec Material / Tool |
| Rebar chairs | 182 crossings | 10% | 201 chairs | USD | $56.28 | Source Spec Material / Tool |
| Tie wire rolls | 182 ties | 10% | 1 rolls | USD | $8.00 | 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
$1,452.00
0.88 nominal 10 yd³ loads · 1 to order
80 lb bags
$2,673.00
60 lb alternative $2,851.20 · delivery weight 35,640 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 6.84 | 5.233 | 308 | 730 | $1,129.33 | $2,079.00 |
| 5 in | 7.82 | 5.981 | 352 | 730 | $1,290.67 | $2,376.00 |
| 6 in | 8.80 | 6.728 | 396 | 730 | $1,452.00 | $2,673.00 |
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 20 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 is designed to help you determine the specifications for a residential slab measuring 18 ft by 20 ft with a thickness of 6 inches. The design includes #4 rebar spaced at 18 inches on center, which is crucial for structural integrity.
Please note that soil design considerations are excluded from this calculation. Always consult a licensed engineer for site-specific soil conditions and compliance with local codes.
Code reference
IRC R506
Field questions
- What is the purpose of using #4 bars in the slab?
- Using #4 bars provides 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 site-specific and depends on various factors such as soil type and load-bearing capacity, which are not covered in 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.