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

Using #4 bars provides reinforcement to the concrete slab, helping to control cracking and improve load-bearing capacity.

Why is soil design excluded from this calculation?

Soil design is a critical aspect that requires site-specific analysis and should be performed by a qualified engineer to ensure proper support for the slab.

What thickness is recommended for a residential slab?

A thickness of 6 inches is commonly recommended for residential slabs, providing adequate strength for typical loads.

How often should the rebar be spaced in the slab?

In this design, the #4 rebar is spaced at 18 inches on center, which is a standard spacing for residential concrete slabs.

16 by 20 ft slab at 6 inches

Calculate the requirements for a 16 ft by 20 ft residential slab at 6 inches thick with #4 bars at 18 inches spacing. Compliant with IRC R506. Soil design is excluded.

Formula SSC-2026.1

Formula SSC-2026.1 · 4,000 PSI

Order volume

7.90 yd³

6.043 m³ placed+waste

Order weight

32,010 lb

14,519 kg

80 lb bags

356

9 pallets at 42 bags

60 lb bags

475

0.45 ft³ yield each

Rebar

588.00 ft

33 × 20 ft · 44 × 6 m · 432 lb

Ready-mix vs bags

$1,304.11

Truck $1,304.11 · 80 lb bags $2,403.00

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	7.19 yd³	10%	7.90 yd³ / 6.043 m³	USD	$1,304.11	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	323.3 bags	10%	356 bags	USD	$2,403.00	Source Spec Material / Tool
60 lb pre-mix bags	431.1 bags	10%	475 bags	USD	$2,565.00	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

$1,304.11

0.79 nominal 10 yd³ loads · 1 to order

80 lb bags

$2,403.00

60 lb alternative $2,565.00 · delivery weight 32,010 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 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 assists in determining the specifications for a residential concrete slab measuring 16 feet by 20 feet with a thickness of 6 inches. The slab reinforcement includes #4 rebar placed at 18 inches on center, ensuring structural integrity.

Note that this design adheres to the guidelines set forth in IRC R506, which governs the construction of concrete slabs in residential settings. However, 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 provides reinforcement to the concrete slab, helping to control cracking and improve load-bearing capacity.
Why is soil design excluded from this calculation?: Soil design is a critical aspect that requires site-specific analysis and should be performed by a qualified engineer to ensure proper support for the slab.
What thickness is recommended for a residential slab?: A thickness of 6 inches is commonly recommended for residential slabs, providing adequate strength for typical loads.
How often should the rebar be spaced in the slab?: In this design, the #4 rebar is spaced at 18 inches on center, which is a standard spacing for residential concrete slabs.