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 thorough analysis by a qualified engineer to ensure proper load-bearing capacity and stability.

What does IRC R506 cover?

IRC R506 provides guidelines for the design and construction of concrete floors on the ground, including reinforcement requirements and thickness specifications.

12 by 30 ft slab at 6 inches

Calculate the specifications for a residential slab measuring 12 ft by 30 ft at 6 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

8.96 yd³

6.853 m³ placed+waste

Order weight

36,300 lb

16,465 kg

80 lb bags

404

10 pallets at 42 bags

60 lb bags

538

0.45 ft³ yield each

Rebar

690.00 ft

49 × 20 ft · 49 × 6 m · 507 lb

Ready-mix vs bags

$1,478.89

Truck $1,478.89 · 80 lb bags $2,727.00

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	8.15 yd³	10%	8.96 yd³ / 6.853 m³	USD	$1,478.89	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	366.7 bags	10%	404 bags	USD	$2,727.00	Source Spec Material / Tool
60 lb pre-mix bags	488.9 bags	10%	538 bags	USD	$2,905.20	Source Spec Material / Tool
#4 rebar sticks, 20 ft	49 sticks	10%	54 sticks / 49 × 6 m	USD	$756.00	Source Spec Material / Tool
Rebar chairs	189 crossings	10%	208 chairs	USD	$58.24	Source Spec Material / Tool
Tie wire rolls	189 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

$1,478.89

0.90 nominal 10 yd³ loads · 1 to order

80 lb bags

$2,727.00

60 lb alternative $2,905.20 · delivery weight 36,300 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	7.06	5.399	318	759	$1,165.19	$2,146.50
5 in	8.01	6.126	361	759	$1,322.04	$2,436.75
6 in	8.96	6.853	404	759	$1,478.89	$2,727.00

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 30 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 determining the specifications for a residential concrete slab measuring 12 ft by 30 ft with a thickness of 6 inches. The design incorporates #4 rebar spaced at 18 inches, adhering to the guidelines set forth in IRC R506.

Please note that soil design considerations are excluded from this calculation. It is essential to consult with a qualified engineer for soil analysis and site-specific requirements.

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 site-specific and requires a thorough analysis by a qualified engineer to ensure proper load-bearing capacity and stability.
What does IRC R506 cover?: IRC R506 provides guidelines for the design and construction of concrete floors on the ground, including reinforcement requirements and thickness specifications.