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 reducing the risk of cracking.

Why is soil design excluded from this calculation?

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

What thickness is recommended for residential slabs?

For most residential applications, a thickness of 4 to 8 inches is common, depending on the load requirements and local building codes.

How often should the rebar be spaced in the slab?

In this design, the #4 bars are spaced 18 inches apart, which is a common spacing for residential slabs to ensure adequate support.

16 by 18 ft slab at 8 inches

Calculate the requirements for a residential slab measuring 16 ft by 18 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

8.69 yd³

6.645 m³ placed+waste

Order weight

35,200 lb

15,966 kg

80 lb bags

392

10 pallets at 42 bags

60 lb bags

522

0.45 ft³ yield each

Rebar

542.00 ft

32 × 20 ft · 32 × 6 m · 398 lb

Ready-mix vs bags

$1,434.07

Truck $1,434.07 · 80 lb bags $2,646.00

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	7.90 yd³	10%	8.69 yd³ / 6.645 m³	USD	$1,434.07	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	355.6 bags	10%	392 bags	USD	$2,646.00	Source Spec Material / Tool
60 lb pre-mix bags	474.1 bags	10%	522 bags	USD	$2,818.80	Source Spec Material / Tool
#4 rebar sticks, 20 ft	32 sticks	10%	36 sticks / 32 × 6 m	USD	$504.00	Source Spec Material / Tool
Rebar chairs	143 crossings	10%	158 chairs	USD	$44.24	Source Spec Material / Tool
Tie wire rolls	143 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

$1,434.07

0.87 nominal 10 yd³ loads · 1 to order

80 lb bags

$2,646.00

60 lb alternative $2,818.80 · delivery weight 35,200 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	5.65	4.319	255	596	$932.15	$1,721.25
5 in	6.41	4.901	289	596	$1,057.63	$1,950.75
6 in	7.17	5.482	323	596	$1,183.11	$2,180.25

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: 16 by 18 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 for a residential slab measuring 16 ft by 18 ft with a thickness of 8 inches. The reinforcement consists of #4 bars placed at 18 inches on center. Please note that soil design considerations are excluded from this calculation.

Ensure that your design complies with the International Residential Code (IRC) R506 for concrete slabs, which provides guidelines for proper construction practices.

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 reducing the risk of cracking.
Why is soil design excluded from this calculation?: Soil design is excluded because it requires a separate analysis based on soil type, load-bearing capacity, and other site-specific conditions.
What thickness is recommended for residential slabs?: For most residential applications, a thickness of 4 to 8 inches is common, depending on the load requirements and local building codes.
How often should the rebar be spaced in the slab?: In this design, the #4 bars are spaced 18 inches apart, which is a common spacing for residential slabs to ensure adequate support.