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 excluded because it requires specific geotechnical analysis to determine the load-bearing capacity and suitability of the soil for supporting the slab.

What thickness is recommended for residential slabs?

For residential applications, a thickness of 8 inches is commonly recommended, depending on the load requirements and local building codes.

How do I ensure my slab meets IRC R506 standards?

To ensure compliance with IRC R506, follow the guidelines for material specifications, reinforcement placement, and curing methods as outlined in the code.

18 by 12 ft slab at 8 inches

Calculate the specifications for a residential slab measuring 18 ft by 12 ft at 8 in thickness with #4 bars at 18 in spacing. Ensure compliance with IRC R506 guidelines.

Formula SSC-2026.1

Formula SSC-2026.1 · 4,000 PSI

Order volume

6.63 yd³

5.067 m³ placed+waste

Order weight

26,840 lb

12,174 kg

80 lb bags

299

8 pallets at 42 bags

60 lb bags

398

0.45 ft³ yield each

Rebar

438.00 ft

28 × 20 ft · 30 × 6 m · 322 lb

Ready-mix vs bags

$1,093.48

Truck $1,093.48 · 80 lb bags $2,018.25

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	6.02 yd³	10%	6.63 yd³ / 5.067 m³	USD	$1,093.48	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	271.1 bags	10%	299 bags	USD	$2,018.25	Source Spec Material / Tool
60 lb pre-mix bags	361.5 bags	10%	398 bags	USD	$2,149.20	Source Spec Material / Tool
#4 rebar sticks, 20 ft	28 sticks	10%	31 sticks / 30 × 6 m	USD	$434.00	Source Spec Material / Tool
Rebar chairs	117 crossings	10%	129 chairs	USD	$36.12	Source Spec Material / Tool
Tie wire rolls	117 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

$1,093.48

0.66 nominal 10 yd³ loads · 1 to order

80 lb bags

$2,018.25

60 lb alternative $2,149.20 · delivery weight 26,840 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	4.45	3.406	201	482	$734.96	$1,356.75
5 in	5.00	3.821	225	482	$824.59	$1,518.75
6 in	5.54	4.236	250	482	$914.22	$1,687.50

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 12 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 requirements for a residential concrete slab measuring 18 ft by 12 ft with a thickness of 8 inches. The design incorporates #4 rebar spaced at 18 inches on center, ensuring structural integrity.

Please note that soil design considerations are excluded from this calculation. It is essential to evaluate the underlying soil conditions separately to ensure proper load-bearing capacity.

Code reference

IRC R506 - Concrete Floors on Ground

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 excluded because it requires specific geotechnical analysis to determine the load-bearing capacity and suitability of the soil for supporting the slab.
What thickness is recommended for residential slabs?: For residential applications, a thickness of 8 inches is commonly recommended, depending on the load requirements and local building codes.
How do I ensure my slab meets IRC R506 standards?: To ensure compliance with IRC R506, follow the guidelines for material specifications, reinforcement placement, and curing methods as outlined in the code.