8 by 20 ft slab at 4 inches
Calculate the requirements for a residential concrete slab measuring 8 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
2.17 yd³
1.661 m³ placed+waste
Order weight
8,800 lb
3,992 kg
80 lb bags
98
3 pallets at 42 bags
60 lb bags
131
0.45 ft³ yield each
Rebar
232.00 ft
20 × 20 ft · 26 × 6 m · 170 lb
Ready-mix vs bags
$483.52
Truck $483.52 · 80 lb bags $661.50
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 | 1.98 yd³ | 10% | 2.17 yd³ / 1.661 m³ | USD | $358.52 | Source Spec Material / Tool |
| Short-load fee | Under 4 yd³ | — | 1 truck @ 10 yd³ | USD | $125.00 | Source Spec Material / Tool |
| 80 lb pre-mix bags | 88.9 bags | 10% | 98 bags | USD | $661.50 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 118.5 bags | 10% | 131 bags | USD | $707.40 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 20 sticks | 10% | 22 sticks / 26 × 6 m | USD | $308.00 | Source Spec Material / Tool |
| Rebar chairs | 84 crossings | 10% | 93 chairs | USD | $26.04 | Source Spec Material / Tool |
| Tie wire rolls | 84 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
$483.52
0.22 nominal 10 yd³ loads · 1 to order
80 lb bags
$661.50
60 lb alternative $707.40 · delivery weight 8,800 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 2.17 | 1.661 | 98 | 255 | $483.52 | $661.50 |
| 5 in | 2.72 | 2.077 | 123 | 255 | $573.15 | $830.25 |
| 6 in | 3.26 | 2.492 | 147 | 255 | $662.78 | $992.25 |
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.
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Project: 8 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 concrete slab measuring 8 ft by 20 ft with a thickness of 4 inches. The design incorporates #4 rebar placed at 18-inch intervals to enhance structural integrity.
Please note that soil design considerations are excluded from this calculation. For complete compliance, refer to the International Residential Code (IRC) R506 for guidelines on concrete slabs.
Code reference
IRC R506
Field questions
- What is the purpose of using #4 bars in the slab?
- Using #4 bars helps to reinforce the concrete, providing additional strength and preventing cracking under load.
- Why is soil design excluded from this calculation?
- Soil design is a separate consideration that depends on the specific site conditions and should be evaluated by a qualified professional.
- What thickness is recommended for a residential slab?
- A thickness of 4 inches is commonly recommended for residential slabs, but this can vary based on load requirements and local codes.
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