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 improve overall structural integrity.

Why is soil design excluded from this calculator?

Soil design is a complex process that depends on various factors such as soil type, load-bearing capacity, and moisture conditions, which require site-specific analysis.

What thickness is recommended for a residential slab?

A thickness of 5 inches is commonly recommended for residential slabs, but this can vary based on load requirements and local building codes.

12 by 24 ft slab at 5 inches

Calculate the specifications for a residential slab measuring 12 ft by 24 ft at 5 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

4.89 yd³

3.738 m³ placed+waste

Order weight

19,800 lb

8,981 kg

80 lb bags

220

6 pallets at 42 bags

60 lb bags

294

0.45 ft³ yield each

Rebar

420.00 ft

35 × 20 ft · 35 × 6 m · 309 lb

Ready-mix vs bags

$806.67

Truck $806.67 · 80 lb bags $1,485.00

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	4.44 yd³	10%	4.89 yd³ / 3.738 m³	USD	$806.67	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	200.0 bags	10%	220 bags	USD	$1,485.00	Source Spec Material / Tool
60 lb pre-mix bags	266.7 bags	10%	294 bags	USD	$1,587.60	Source Spec Material / Tool
#4 rebar sticks, 20 ft	35 sticks	10%	39 sticks / 35 × 6 m	USD	$546.00	Source Spec Material / Tool
Rebar chairs	153 crossings	10%	169 chairs	USD	$47.32	Source Spec Material / Tool
Tie wire rolls	153 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

$806.67

0.49 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,485.00

60 lb alternative $1,587.60 · delivery weight 19,800 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	3.91	2.990	176	462	$770.33	$1,188.00
5 in	4.89	3.738	220	462	$806.67	$1,485.00
6 in	5.87	4.485	264	462	$968.00	$1,782.00

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: 12 by 24 ft slab at 5 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 for residential slab design, specifically for a slab measuring 12 feet by 24 feet with a thickness of 5 inches. Reinforcement is provided with #4 bars spaced at 18 inches on center. Please note that soil design considerations are excluded from this calculation.

For compliance with the International Residential Code (IRC) Section R506, ensure that all local building codes and regulations are followed during construction.

Code reference

IRC R506

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 improve overall structural integrity.
Why is soil design excluded from this calculator?: Soil design is a complex process that depends on various factors such as soil type, load-bearing capacity, and moisture conditions, which require site-specific analysis.
What thickness is recommended for a residential slab?: A thickness of 5 inches is commonly recommended for residential slabs, but this can vary based on load requirements and local building codes.