What is the purpose of using #4 bars in the slab?

Using #4 bars provides reinforcement to the concrete slab, helping to control cracking and improve the overall strength and durability of the slab.

Why is soil design excluded from this calculation?

Soil design is excluded because it requires specific geotechnical analysis based on the site's soil conditions, which can vary significantly and impact the slab's performance.

What thickness is recommended for a residential slab?

A thickness of 6 inches is commonly recommended for residential slabs, providing adequate support for typical loads.

12 by 12 ft slab at 6 inches

Calculate the specifications for a 12 ft by 12 ft residential slab at 6 inches thick, reinforced with #4 bars at 18 inches apart. Ensure compliance with IRC R506 for slab foundations.

Formula SSC-2026.1

Formula SSC-2026.1 · 4,000 PSI

Order volume

3.83 yd³

2.928 m³ placed+waste

Order weight

15,510 lb

7,035 kg

80 lb bags

173

5 pallets at 42 bags

60 lb bags

230

0.45 ft³ yield each

Rebar

312.00 ft

24 × 20 ft · 24 × 6 m · 229 lb

Ready-mix vs bags

$756.89

Truck $756.89 · 80 lb bags $1,167.75

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	3.48 yd³	10%	3.83 yd³ / 2.928 m³	USD	$631.89	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	156.7 bags	10%	173 bags	USD	$1,167.75	Source Spec Material / Tool
60 lb pre-mix bags	208.9 bags	10%	230 bags	USD	$1,242.00	Source Spec Material / Tool
#4 rebar sticks, 20 ft	24 sticks	10%	27 sticks / 24 × 6 m	USD	$378.00	Source Spec Material / Tool
Rebar chairs	81 crossings	10%	90 chairs	USD	$25.20	Source Spec Material / Tool
Tie wire rolls	81 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

$756.89

0.38 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,167.75

60 lb alternative $1,242.00 · delivery weight 15,510 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	3.15	2.409	142	343	$644.85	$958.50
5 in	3.49	2.668	158	343	$700.87	$1,066.50
6 in	3.83	2.928	173	343	$756.89	$1,167.75

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 12 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 is designed to assist in the design of a residential slab measuring 12 ft by 12 ft and 6 inches in thickness, reinforced with #4 rebar spaced 18 inches apart. Please note that soil design considerations are excluded from this calculation.

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

Code reference

IRC R506 - Concrete Floors

Field questions

What is the purpose of using #4 bars in the slab?: Using #4 bars provides reinforcement to the concrete slab, helping to control cracking and improve the overall strength and durability of the slab.
Why is soil design excluded from this calculation?: Soil design is excluded because it requires specific geotechnical analysis based on the site's soil conditions, which can vary significantly and impact the slab's performance.
What thickness is recommended for a residential slab?: A thickness of 6 inches is commonly recommended for residential slabs, providing adequate support for typical loads.