18 by 18 ft slab at 4 inches
Calculate the specifications for an 18 ft by 18 ft residential slab at 4 inches thick with #4 bars at 18 inches. Ensure compliance with IRC R506 standards.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
4.40 yd³
3.364 m³ placed+waste
Order weight
17,820 lb
8,083 kg
80 lb bags
198
5 pallets at 42 bags
60 lb bags
264
0.45 ft³ yield each
Rebar
468.00 ft
26 × 20 ft · 26 × 6 m · 344 lb
Ready-mix vs bags
$726.00
Truck $726.00 · 80 lb bags $1,336.50
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 | 4.00 yd³ | 10% | 4.40 yd³ / 3.364 m³ | USD | $726.00 | 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 | 180.0 bags | 10% | 198 bags | USD | $1,336.50 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 240.0 bags | 10% | 264 bags | USD | $1,425.60 | 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
$726.00
0.44 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,336.50
60 lb alternative $1,425.60 · delivery weight 17,820 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 4 inches
Prepared by
Date
Verified by (PE / electrician / inspector)
License number
Signature
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 design, specifically for an 18 ft by 18 ft slab with a thickness of 4 inches and #4 reinforcement bars spaced 18 inches apart. Please note that soil design considerations are excluded from this calculation.
Refer to the International Residential Code (IRC) Section R506 for guidelines on concrete slabs, including requirements for reinforcement and curing.
Code reference
IRC R506
Field questions
- What is the purpose of using #4 bars in the slab?
- The #4 bars provide necessary tensile strength to the concrete slab, helping to prevent cracking and ensuring structural integrity.
- Why is soil design excluded from this calculation?
- Soil design considerations vary widely based on local conditions and are typically addressed separately by a qualified engineer.
- What thickness is recommended for residential slabs?
- A thickness of 4 inches is commonly recommended for residential slabs, providing adequate support for typical loads.