What is the purpose of using rebar in a concrete slab?

Rebar provides tensile strength to the concrete, helping to prevent cracking and improve the overall durability of the slab.

Why is soil design excluded from this calculator?

Soil design is a critical factor that varies greatly depending on location and conditions. It should be assessed by a qualified professional to ensure proper load-bearing capacity.

What does IRC R506 cover?

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

16 by 20 ft slab at 4 inches

Calculate the requirements for a residential slab measuring 16 ft by 20 ft at 4 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

4.35 yd³

3.323 m³ placed+waste

Order weight

17,600 lb

7,983 kg

80 lb bags

196

5 pallets at 42 bags

60 lb bags

261

0.45 ft³ yield each

Rebar

444.00 ft

25 × 20 ft · 36 × 6 m · 326 lb

Ready-mix vs bags

$717.04

Truck $717.04 · 80 lb bags $1,323.00

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	3.95 yd³	10%	4.35 yd³ / 3.323 m³	USD	$717.04	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	177.8 bags	10%	196 bags	USD	$1,323.00	Source Spec Material / Tool
60 lb pre-mix bags	237.0 bags	10%	261 bags	USD	$1,409.40	Source Spec Material / Tool
#4 rebar sticks, 20 ft	25 sticks	10%	28 sticks / 36 × 6 m	USD	$392.00	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

$717.04

0.43 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,323.00

60 lb alternative $1,409.40 · delivery weight 17,600 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	4.35	3.323	196	488	$717.04	$1,323.00
5 in	5.43	4.153	245	488	$896.30	$1,653.75
6 in	6.52	4.984	294	488	$1,075.56	$1,984.50

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 4 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 design a residential concrete slab measuring 16 ft by 20 ft with a thickness of 4 inches. The design includes #4 rebar spaced at 18 inches on center, which is essential for structural integrity.

Please note that soil design considerations are excluded from this calculation. For accurate results, consult a professional engineer regarding soil conditions and load-bearing capacity.

Code reference

IRC R506

Field questions

What is the purpose of using rebar in a concrete slab?: Rebar provides tensile strength to the concrete, helping to prevent cracking and improve the overall durability of the slab.
Why is soil design excluded from this calculator?: Soil design is a critical factor that varies greatly depending on location and conditions. It should be assessed by a qualified professional to ensure proper load-bearing capacity.
What does IRC R506 cover?: IRC R506 provides guidelines for the design and construction of concrete slabs, including requirements for thickness, reinforcement, and curing.