18 by 20 ft slab at 8 inches
Calculate the specifications for an 18 ft by 20 ft residential slab at 8 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
10.76 yd³
8.223 m³ placed+waste
Order weight
43,560 lb
19,758 kg
80 lb bags
484
12 pallets at 42 bags
60 lb bags
646
0.45 ft³ yield each
Rebar
664.00 ft
35 × 20 ft · 48 × 6 m · 488 lb
Ready-mix vs bags
$1,774.67
Truck $1,774.67 · 80 lb bags $3,267.00
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 | 9.78 yd³ | 10% | 10.76 yd³ / 8.223 m³ | USD | $1,774.67 | Source Spec Material / Tool |
| Short-load fee | Full load | — | 2 truck @ 10 yd³ | USD | $0.00 | Source Spec Material / Tool |
| 80 lb pre-mix bags | 440.0 bags | 10% | 484 bags | USD | $3,267.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 586.7 bags | 10% | 646 bags | USD | $3,488.40 | 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 |
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Ready-mix truck
$1,774.67
1.08 nominal 10 yd³ loads · 2 to order
80 lb bags
$3,267.00
60 lb alternative $3,488.40 · delivery weight 43,560 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 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 to assist in determining the specifications for a residential slab measuring 18 ft by 20 ft with a thickness of 8 inches. The reinforcement consists of #4 bars spaced 18 inches apart. Please note that soil design considerations are excluded from this calculation.
Ensure that your design adheres to the requirements outlined in IRC R506 for concrete slabs, which includes guidelines for thickness, reinforcement, and other structural considerations.
Code reference
IRC R506 - Concrete Floors on Ground
Field questions
- What is the purpose of using #4 bars in the slab?
- The #4 bars provide tensile strength to the concrete slab, helping to prevent cracking and ensuring structural integrity.
- Why is soil design excluded from this calculation?
- Soil design is a separate consideration that depends on the specific site conditions and soil characteristics, which are not covered in this calculator.
- What thickness is recommended for residential slabs?
- For residential slabs, a thickness of 4 to 6 inches is commonly recommended, but in this case, an 8-inch thickness is specified for added strength.
- How do I ensure compliance with IRC R506?
- To ensure compliance with IRC R506, follow the guidelines for slab thickness, reinforcement placement, and other structural requirements as outlined in the code.