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 calculator?

Soil design is a critical aspect that varies significantly based on site conditions. It should be evaluated by a qualified professional to ensure proper load-bearing capacity.

What thickness is recommended for a residential slab?

A thickness of 5 inches is commonly recommended for residential slabs, but it may vary based on specific load requirements and local codes.

10 by 20 ft slab at 5 inches

Calculate the requirements for a residential slab measuring 10 ft by 20 ft at 5 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

3.40 yd³

2.596 m³ placed+waste

Order weight

13,750 lb

6,237 kg

80 lb bags

153

4 pallets at 42 bags

60 lb bags

204

0.45 ft³ yield each

Rebar

280.00 ft

21 × 20 ft · 28 × 6 m · 206 lb

Ready-mix vs bags

$685.19

Truck $685.19 · 80 lb bags $1,032.75

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	3.09 yd³	10%	3.40 yd³ / 2.596 m³	USD	$560.19	Source Spec Material / Tool
Short-load fee	Under 4 yd³	—	1 truck @ 10 yd³	USD	$125.00	Source Spec Material / Tool
80 lb pre-mix bags	138.9 bags	10%	153 bags	USD	$1,032.75	Source Spec Material / Tool
60 lb pre-mix bags	185.2 bags	10%	204 bags	USD	$1,101.60	Source Spec Material / Tool
#4 rebar sticks, 20 ft	21 sticks	10%	24 sticks / 28 × 6 m	USD	$336.00	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

$685.19

0.34 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,032.75

60 lb alternative $1,101.60 · delivery weight 13,750 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	2.72	2.077	123	308	$573.15	$830.25
5 in	3.40	2.596	153	308	$685.19	$1,032.75
6 in	4.07	3.115	184	308	$672.22	$1,242.00

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: 10 by 20 ft slab at 5 inches

Prepared by

Date

Verified by (PE / electrician / inspector)

License number

Signature

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 slab that measures 10 ft by 20 ft with a thickness of 5 inches. The reinforcement specified includes #4 bars placed at 18-inch intervals, ensuring compliance with IRC R506 for residential concrete slabs.

It is important to note that soil design considerations are excluded from this calculator. Proper soil assessment should be conducted to ensure the slab's performance and longevity.

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 control cracking and improve structural integrity.
Why is soil design excluded from this calculator?: Soil design is a critical aspect that varies significantly based on site conditions. It should be evaluated by a qualified professional to ensure proper load-bearing capacity.
What thickness is recommended for a residential slab?: A thickness of 5 inches is commonly recommended for residential slabs, but it may vary based on specific load requirements and local codes.