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 site-specific and depends on various factors such as soil type, moisture content, and load-bearing capacity, which are not addressed in this calculator.

What does IRC R506 cover?

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

16 by 20 ft slab at 8 inches

Calculate the requirements for a residential concrete slab measuring 16 ft by 20 ft at 8 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

9.61 yd³

7.351 m³ placed+waste

Order weight

38,940 lb

17,663 kg

80 lb bags

433

11 pallets at 42 bags

60 lb bags

577

0.45 ft³ yield each

Rebar

588.00 ft

33 × 20 ft · 44 × 6 m · 432 lb

Ready-mix vs bags

$1,586.44

Truck $1,586.44 · 80 lb bags $2,922.75

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	8.74 yd³	10%	9.61 yd³ / 7.351 m³	USD	$1,586.44	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	393.3 bags	10%	433 bags	USD	$2,922.75	Source Spec Material / Tool
60 lb pre-mix bags	524.4 bags	10%	577 bags	USD	$3,115.80	Source Spec Material / Tool
#4 rebar sticks, 20 ft	33 sticks	10%	37 sticks / 44 × 6 m	USD	$518.00	Source Spec Material / Tool
Rebar chairs	154 crossings	10%	170 chairs	USD	$47.60	Source Spec Material / Tool
Tie wire rolls	154 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

$1,586.44

0.96 nominal 10 yd³ loads · 1 to order

80 lb bags

$2,922.75

60 lb alternative $3,115.80 · delivery weight 38,940 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	6.19	4.735	279	647	$1,021.78	$1,883.25
5 in	7.05	5.389	318	647	$1,162.94	$2,146.50
6 in	7.90	6.043	356	647	$1,304.11	$2,403.00

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: 16 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 is designed to assist in the preliminary design of a residential concrete slab measuring 16 ft by 20 ft with a thickness of 8 inches. The slab will utilize #4 rebar spaced at 18 inches on center. Please note that soil design considerations are excluded from this calculation.

For compliance with the International Residential Code (IRC) R506, ensure that all local building codes and regulations are followed during construction.

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 site-specific and depends on various factors such as soil type, moisture content, and load-bearing capacity, which are not addressed in this calculator.
What does IRC R506 cover?: IRC R506 provides guidelines for the design and construction of concrete slabs on grade, including reinforcement, thickness, and curing requirements.