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

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

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 covered in this general calculation.

What thickness should my slab be?

For residential applications, an 8-inch thickness is commonly used, but it is important to verify this with local building codes and site conditions.

12 by 24 ft slab at 8 inches

Calculate the requirements for a 12 ft by 24 ft residential slab at 8 inches thick with #4 bars spaced 18 inches apart. Ensure compliance with IRC R506.

Formula SSC-2026.1

Formula SSC-2026.1 · 4,000 PSI

Order volume

8.75 yd³

6.687 m³ placed+waste

Order weight

35,420 lb

16,066 kg

80 lb bags

394

10 pallets at 42 bags

60 lb bags

525

0.45 ft³ yield each

Rebar

564.00 ft

43 × 20 ft · 43 × 6 m · 414 lb

Ready-mix vs bags

$1,443.04

Truck $1,443.04 · 80 lb bags $2,659.50

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	7.95 yd³	10%	8.75 yd³ / 6.687 m³	USD	$1,443.04	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	357.8 bags	10%	394 bags	USD	$2,659.50	Source Spec Material / Tool
60 lb pre-mix bags	477.0 bags	10%	525 bags	USD	$2,835.00	Source Spec Material / Tool
#4 rebar sticks, 20 ft	43 sticks	10%	48 sticks / 43 × 6 m	USD	$672.00	Source Spec Material / Tool
Rebar chairs	153 crossings	10%	169 chairs	USD	$47.32	Source Spec Material / Tool
Tie wire rolls	153 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

$1,443.04

0.87 nominal 10 yd³ loads · 1 to order

80 lb bags

$2,659.50

60 lb alternative $2,835.00 · delivery weight 35,420 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	5.76	4.402	260	620	$950.07	$1,755.00
5 in	6.50	4.973	293	620	$1,073.31	$1,977.75
6 in	7.25	5.544	327	620	$1,196.56	$2,207.25

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 24 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 helps you determine the specifications for a residential slab measuring 12 ft by 24 ft, with a thickness of 8 inches and #4 reinforcement bars spaced 18 inches apart.

Please note that soil design considerations are excluded from this calculation, and it is essential to consult local building codes and a licensed engineer for site-specific requirements.

Code reference

IRC R506 - Concrete Floors on Ground

Field questions

What is the purpose of using #4 bars in the slab?: The #4 bars provide reinforcement to the concrete slab, helping to control cracking and improve structural integrity.
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 covered in this general calculation.
What thickness should my slab be?: For residential applications, an 8-inch thickness is commonly used, but it is important to verify this with local building codes and site conditions.