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

Using #4 bars helps to reinforce the concrete slab, providing additional tensile strength and preventing cracking.

Why is soil design excluded from this calculation?

Soil design is a separate consideration that requires specific analysis based on soil type and conditions, which is not covered in this slab design.

What thickness is recommended for a residential slab?

A thickness of 6 inches is commonly recommended for residential slabs to ensure adequate support and durability.

12 by 20 ft slab at 6 inches

Calculate the requirements for a 12 ft x 20 ft residential slab with #4 bars at 18 in spacing. Ensure compliance with IRC R506 standards. Soil design is excluded.

Formula SSC-2026.1

Formula SSC-2026.1 · 4,000 PSI

Order volume

6.11 yd³

4.672 m³ placed+waste

Order weight

24,750 lb

11,226 kg

80 lb bags

275

7 pallets at 42 bags

60 lb bags

367

0.45 ft³ yield each

Rebar

476.00 ft

31 × 20 ft · 40 × 6 m · 350 lb

Ready-mix vs bags

$1,008.33

Truck $1,008.33 · 80 lb bags $1,856.25

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	5.56 yd³	10%	6.11 yd³ / 4.672 m³	USD	$1,008.33	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	250.0 bags	10%	275 bags	USD	$1,856.25	Source Spec Material / Tool
60 lb pre-mix bags	333.3 bags	10%	367 bags	USD	$1,981.80	Source Spec Material / Tool
#4 rebar sticks, 20 ft	31 sticks	10%	35 sticks / 40 × 6 m	USD	$490.00	Source Spec Material / Tool
Rebar chairs	126 crossings	10%	139 chairs	USD	$38.92	Source Spec Material / Tool
Tie wire rolls	126 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

$1,008.33

0.61 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,856.25

60 lb alternative $1,981.80 · delivery weight 24,750 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	4.89	3.738	220	524	$806.67	$1,485.00
5 in	5.50	4.205	248	524	$907.50	$1,674.00
6 in	6.11	4.672	275	524	$1,008.33	$1,856.25

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: 12 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

When designing a residential slab of dimensions 12 ft by 20 ft and a thickness of 6 inches, it is essential to incorporate #4 rebar at 18-inch spacing for structural integrity. This design adheres to the guidelines set forth in IRC R506, which outlines the requirements for concrete slabs in residential construction.

Please note that soil design considerations are excluded from this calculation and should be addressed separately by a qualified professional.

Code reference

IRC R506

Field questions

What is the purpose of using #4 bars in the slab?: Using #4 bars helps to reinforce the concrete slab, providing additional tensile strength and preventing cracking.
Why is soil design excluded from this calculation?: Soil design is a separate consideration that requires specific analysis based on soil type and conditions, which is not covered in this slab design.
What thickness is recommended for a residential slab?: A thickness of 6 inches is commonly recommended for residential slabs to ensure adequate support and durability.