What is the significance of IRC R506?

IRC R506 provides guidelines for the design and construction of concrete slabs in residential buildings, ensuring safety and structural integrity.

Why is soil design excluded?

Soil design is excluded because it requires site-specific analysis that varies based on soil type, load-bearing capacity, and other local conditions.

What does #4 rebar mean?

#4 rebar refers to a steel reinforcing bar that is 1/2 inch in diameter, commonly used for providing tensile strength in concrete structures.

How do I ensure proper spacing of rebar?

Rebar should be placed according to the specified spacing, in this case, 18 inches on center, to ensure adequate reinforcement throughout the slab.

8 by 20 ft slab at 6 inches

Calculate the requirements for an 8 ft by 20 ft residential slab 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

4.32 yd³

3.302 m³ placed+waste

Order weight

17,490 lb

7,933 kg

80 lb bags

195

5 pallets at 42 bags

60 lb bags

260

0.45 ft³ yield each

Rebar

344.00 ft

26 × 20 ft · 32 × 6 m · 253 lb

Ready-mix vs bags

$712.56

Truck $712.56 · 80 lb bags $1,316.25

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	3.93 yd³	10%	4.32 yd³ / 3.302 m³	USD	$712.56	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	176.7 bags	10%	195 bags	USD	$1,316.25	Source Spec Material / Tool
60 lb pre-mix bags	235.6 bags	10%	260 bags	USD	$1,404.00	Source Spec Material / Tool
#4 rebar sticks, 20 ft	26 sticks	10%	29 sticks / 32 × 6 m	USD	$406.00	Source Spec Material / Tool
Rebar chairs	84 crossings	10%	93 chairs	USD	$26.04	Source Spec Material / Tool
Tie wire rolls	84 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

$712.56

0.43 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,316.25

60 lb alternative $1,404.00 · delivery weight 17,490 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	3.59	2.741	162	378	$716.56	$1,093.50
5 in	3.95	3.021	178	378	$777.06	$1,201.50
6 in	4.32	3.302	195	378	$712.56	$1,316.25

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: 8 by 20 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 for a residential slab measuring 8 ft by 20 ft with a thickness of 6 inches. The use of #4 rebar at 18-inch spacing is specified for reinforcement. Please note that soil design considerations are excluded from this calculation.

For compliance, refer to IRC R506, which outlines the requirements for concrete slabs in residential construction.

Code reference

IRC R506

Field questions

What is the significance of IRC R506?: IRC R506 provides guidelines for the design and construction of concrete slabs in residential buildings, ensuring safety and structural integrity.
Why is soil design excluded?: Soil design is excluded because it requires site-specific analysis that varies based on soil type, load-bearing capacity, and other local conditions.
What does #4 rebar mean?: #4 rebar refers to a steel reinforcing bar that is 1/2 inch in diameter, commonly used for providing tensile strength in concrete structures.
How do I ensure proper spacing of rebar?: Rebar should be placed according to the specified spacing, in this case, 18 inches on center, to ensure adequate reinforcement throughout the slab.