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

The #4 bars provide reinforcement to the concrete slab, helping to prevent cracking and improve structural integrity.

Why is soil design excluded from this calculation?

Soil design is excluded because it requires specific geotechnical analysis that varies by location and is essential for ensuring the slab's stability.

What does IRC R506 cover?

IRC R506 provides guidelines for the design and construction of concrete slabs, including reinforcement and thickness requirements.

10 by 20 ft slab at 8 inches

Calculate the requirements for a residential slab measuring 10 ft by 20 ft at 8 in thickness with #4 bars at 18 in spacing. Note: Soil design is excluded.

Formula SSC-2026.1

Formula SSC-2026.1 · 4,000 PSI

Order volume

6.19 yd³

4.735 m³ placed+waste

Order weight

25,080 lb

11,376 kg

80 lb bags

279

7 pallets at 42 bags

60 lb bags

372

0.45 ft³ yield each

Rebar

400.00 ft

27 × 20 ft · 36 × 6 m · 294 lb

Ready-mix vs bags

$1,021.78

Truck $1,021.78 · 80 lb bags $1,883.25

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	5.63 yd³	10%	6.19 yd³ / 4.735 m³	USD	$1,021.78	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	253.3 bags	10%	279 bags	USD	$1,883.25	Source Spec Material / Tool
60 lb pre-mix bags	337.8 bags	10%	372 bags	USD	$2,008.80	Source Spec Material / Tool
#4 rebar sticks, 20 ft	27 sticks	10%	30 sticks / 36 × 6 m	USD	$420.00	Source Spec Material / Tool
Rebar chairs	98 crossings	10%	108 chairs	USD	$30.24	Source Spec Material / Tool
Tie wire rolls	98 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

$1,021.78

0.62 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,883.25

60 lb alternative $2,008.80 · delivery weight 25,080 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	4.24	3.239	191	440	$699.11	$1,289.25
5 in	4.73	3.613	213	440	$779.78	$1,437.75
6 in	5.21	3.987	235	440	$860.44	$1,586.25

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: 10 by 20 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 specifications for a residential concrete slab measuring 10 ft by 20 ft with a thickness of 8 inches. The design incorporates #4 rebar bars spaced at 18 inches apart, following the guidelines set forth in the IRC R506.

Please note that soil design considerations are excluded from this calculation. It is crucial to ensure that the underlying soil is adequately assessed to support the slab's load-bearing requirements.

Code reference

IRC R506

Field questions

What is the purpose of using #4 bars in the slab?: The #4 bars provide reinforcement to the concrete slab, helping to prevent cracking and improve structural integrity.
Why is soil design excluded from this calculation?: Soil design is excluded because it requires specific geotechnical analysis that varies by location and is essential for ensuring the slab's stability.
What does IRC R506 cover?: IRC R506 provides guidelines for the design and construction of concrete slabs, including reinforcement and thickness requirements.