8 by 20 ft slab at 8 inches
Calculate the specifications for an 8 ft by 20 ft residential slab with #4 bars at 18 in spacing. Ensure compliance with IRC R506 guidelines while excluding soil design considerations.
Formula SSC-2026.1
Formula SSC-2026.1 · 4,000 PSI
Order volume
5.05 yd³
3.862 m³ placed+waste
Order weight
20,460 lb
9,280 kg
80 lb bags
228
6 pallets at 42 bags
60 lb bags
304
0.45 ft³ yield each
Rebar
344.00 ft
26 × 20 ft · 32 × 6 m · 253 lb
Ready-mix vs bags
$833.56
Truck $833.56 · 80 lb bags $1,539.00
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 | 4.59 yd³ | 10% | 5.05 yd³ / 3.862 m³ | USD | $833.56 | 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 | 206.7 bags | 10% | 228 bags | USD | $1,539.00 | Source Spec Material / Tool |
| 60 lb pre-mix bags | 275.6 bags | 10% | 304 bags | USD | $1,641.60 | Source Spec Material / Tool |
| #4 rebar sticks, 20 ft | 26 sticks | 10% | 29 sticks / 32 × 6 m | USD | $406.00 | 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
$833.56
0.51 nominal 10 yd³ loads · 1 to order
80 lb bags
$1,539.00
60 lb alternative $1,641.60 · delivery weight 20,460 lb
| Thickness | Order yd³ | Order m³ | 80 lb bags | Rebar ft | Ready-mix | 80 lb cost |
|---|---|---|---|---|---|---|
| 4 in | 3.59 | 2.741 | 162 | 378 | $716.56 | $1,093.50 |
| 5 in | 3.95 | 3.021 | 178 | 378 | $777.06 | $1,201.50 |
| 6 in | 4.32 | 3.302 | 195 | 378 | $712.56 | $1,316.25 |
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.
Contractor sign-off
Project: 8 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 assists in determining the necessary specifications for a residential concrete slab measuring 8 ft by 20 ft and 8 inches thick, reinforced with #4 bars spaced 18 inches apart. Please note that soil design considerations are excluded from this calculation.
For compliance, refer to the International Residential Code (IRC) Section R506, which outlines the requirements for concrete floors and slabs.
Code reference
IRC R506
Field questions
- What is the purpose of using #4 bars in the slab?
- The #4 bars provide reinforcement to the concrete slab, helping to control cracking and improve structural integrity.
- Why is soil design excluded from this calculation?
- Soil design is site-specific and requires a geotechnical assessment, which is beyond the scope of this calculator.
- What thickness is recommended for a residential slab?
- An 8-inch thickness is commonly recommended for residential slabs, especially in areas with heavier loads.
- How does IRC R506 affect my slab design?
- IRC R506 provides guidelines for the construction of concrete slabs, including thickness, reinforcement, and curing practices to ensure safety and durability.