18 by 20 ft slab at 4 inches
Calculate the requirements for a residential slab measuring 18 ft by 20 ft at 4 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
4.89 yd³
3.738 m³ placed+waste
Order weight
19,800 lb
8,981 kg
80 lb bags
220
6 pallets at 42 bags
60 lb bags
294
0.45 ft³ yield each
Rebar
512.00 ft
27 × 20 ft · 40 × 6 m · 376 lb
Ready-mix vs bags
$806.67
Truck $806.67 · 80 lb bags $1,485.00
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 | 4.44 yd³ | 10% | 4.89 yd³ / 3.738 m³ | USD | $806.67 | 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 | 200.0 bags | 10% | 220 bags | USD | $1,485.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 266.7 bags | 10% | 294 bags | USD | $1,587.60 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 27 sticks | 10% | 30 sticks / 40 × 6 m | USD | $420.00 | 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
$806.67
0.49 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,485.00
60 lb alternative $1,587.60 · delivery weight 19,800 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 4.89 | 3.738 | 220 | 563 | $806.67 | $1,485.00 |
| 5 in | 6.11 | 4.672 | 275 | 563 | $1,008.33 | $1,856.25 |
| 6 in | 7.33 | 5.607 | 330 | 563 | $1,210.00 | $2,227.50 |
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 4 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 that measures 18 ft by 20 ft with a thickness of 4 inches. The design incorporates #4 rebar spaced at 18 inches on center, ensuring structural integrity and compliance with IRC R506 guidelines.
Note that soil design considerations are excluded from this calculation. It is essential to evaluate the soil conditions separately to ensure proper support for the slab.
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 excluded because it requires a separate evaluation based on specific site conditions, which can vary significantly.
- 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.