18 by 18 ft slab at 5 inches
Calculate the requirements for an 18 ft by 18 ft residential slab with #4 bars at 18 in spacing. Ensure compliance with IRC R506 standards. Soil design is excluded.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
5.50 yd³
4.205 m³ placed+waste
Order weight
22,275 lb
10,104 kg
80 lb bags
248
6 pallets at 42 bags
60 lb bags
330
0.45 ft³ yield each
Rebar
468.00 ft
26 × 20 ft · 26 × 6 m · 344 lb
Ready-mix vs bags
$907.50
Truck $907.50 · 80 lb bags $1,674.00
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 | 5.00 yd³ | 10% | 5.50 yd³ / 4.205 m³ | USD | $907.50 | 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 | 225.0 bags | 10% | 248 bags | USD | $1,674.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 300.0 bags | 10% | 330 bags | USD | $1,782.00 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 26 sticks | 10% | 29 sticks / 26 × 6 m | USD | $406.00 | 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 |
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
$907.50
0.55 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,674.00
60 lb alternative $1,782.00 · delivery weight 22,275 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 4.40 | 3.364 | 198 | 515 | $726.00 | $1,336.50 |
| 5 in | 5.50 | 4.205 | 248 | 515 | $907.50 | $1,674.00 |
| 6 in | 6.60 | 5.046 | 297 | 515 | $1,089.00 | $2,004.75 |
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 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 is designed for residential slab construction, specifically for an 18 ft by 18 ft slab with a thickness of 5 inches. The use of #4 reinforcing bars spaced at 18 inches is specified to enhance the structural integrity of the slab.
Note that this design does not include soil design considerations, which should be evaluated separately based on site-specific conditions.
Code reference
IRC R506
Field questions
- What is the purpose of using #4 bars in the slab?
- The #4 bars provide reinforcement to the slab, helping to control cracking and improve load-bearing capacity.
- Is soil design included in this calculation?
- No, soil design is excluded and should be assessed separately based on local soil conditions.
- What thickness is recommended for the slab?
- A thickness of 5 inches is specified for this residential slab design.
- What are the dimensions of the slab?
- The slab dimensions are 18 feet by 18 feet.