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 a complex process that depends on various factors including soil type, load-bearing capacity, and local conditions, which are not covered in this calculator.

What thickness is recommended for a residential slab?

For most residential applications, a thickness of 4 to 6 inches is commonly recommended, depending on the load and usage.

18 by 8 ft slab at 6 inches

Calculate the specifications for a residential slab measuring 18 ft by 8 ft at 6 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

3.91 yd³

2.990 m³ placed+waste

Order weight

15,840 lb

7,185 kg

80 lb bags

176

5 pallets at 42 bags

60 lb bags

235

0.45 ft³ yield each

Rebar

316.00 ft

25 × 20 ft · 25 × 6 m · 232 lb

Ready-mix vs bags

$770.33

Truck $770.33 · 80 lb bags $1,188.00

Slab footprint and rebar gridLive scaled blueprint

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.

Bill of materials and shopping list
Item	Net	Waste	Order	Unit price	Line total	Source Spec Material / Tool
Ready-mix concrete	3.56 yd³	10%	3.91 yd³ / 2.990 m³	USD	$645.33	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	160.0 bags	10%	176 bags	USD	$1,188.00	Source Spec Material / Tool
60 lb pre-mix bags	213.3 bags	10%	235 bags	USD	$1,269.00	Source Spec Material / Tool
#4 rebar sticks, 20 ft	25 sticks	10%	28 sticks / 25 × 6 m	USD	$392.00	Source Spec Material / Tool
Rebar chairs	78 crossings	10%	86 chairs	USD	$24.08	Source Spec Material / Tool
Tie wire rolls	78 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

$770.33

0.39 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,188.00

60 lb alternative $1,269.00 · delivery weight 15,840 lb

Side-by-side thickness comparison at the same footprint
Thickness	Order yd³	Order m³	80 lb bags	Rebar ft	Ready-mix	80 lb cost
4 in	3.26	2.492	147	348	$662.78	$992.25
5 in	3.59	2.741	162	348	$716.56	$1,093.50
6 in	3.91	2.990	176	348	$770.33	$1,188.00

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 8 ft slab at 6 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 helps you determine the necessary specifications for a residential concrete slab measuring 18 ft by 8 ft and 6 inches thick, reinforced with #4 bars spaced at 18 inches. Please note that soil design considerations are excluded from this calculation.

For compliance, refer to the International Residential Code (IRC) R506, which provides guidelines for concrete slabs, including reinforcement and thickness requirements.

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 a complex process that depends on various factors including soil type, load-bearing capacity, and local conditions, which are not covered in this calculator.
What thickness is recommended for a residential slab?: For most residential applications, a thickness of 4 to 6 inches is commonly recommended, depending on the load and usage.