What are the dimensions of the slab being calculated?

The slab is 16 ft by 20 ft with a thickness of 5 inches.

What type of reinforcement is used in the slab?

The slab uses #4 rebar spaced at 18 inches on center.

16 by 20 ft slab at 5 inches

Calculate the requirements for a 16 ft by 20 ft residential slab with #4 bars at 18 in spacing. Ensure compliance with IRC R506 while excluding soil design considerations.

Formula SSC-2026.1

Formula SSC-2026.1 · 4,000 PSI

Order volume

5.43 yd³

4.153 m³ placed+waste

Order weight

22,000 lb

9,979 kg

80 lb bags

245

6 pallets at 42 bags

60 lb bags

326

0.45 ft³ yield each

Rebar

444.00 ft

25 × 20 ft · 36 × 6 m · 326 lb

Ready-mix vs bags

$896.30

Truck $896.30 · 80 lb bags $1,653.75

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	4.94 yd³	10%	5.43 yd³ / 4.153 m³	USD	$896.30	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	222.2 bags	10%	245 bags	USD	$1,653.75	Source Spec Material / Tool
60 lb pre-mix bags	296.3 bags	10%	326 bags	USD	$1,760.40	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

$896.30

0.54 nominal 10 yd³ loads · 1 to order

80 lb bags

$1,653.75

60 lb alternative $1,760.40 · delivery weight 22,000 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 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 concrete slab measuring 16 ft by 20 ft and 5 inches thick, utilizing #4 rebar spaced at 18 inches on center. Please note that soil design considerations are excluded from this calculation.

For compliance, refer to the International Residential Code (IRC) R506, which outlines the requirements for concrete slabs in residential construction.

Code reference

IRC R506

Field questions

What are the dimensions of the slab being calculated?: The slab is 16 ft by 20 ft with a thickness of 5 inches.
What type of reinforcement is used in the slab?: The slab uses #4 rebar spaced at 18 inches on center.
Is soil design included in this calculation?: No, soil design considerations are excluded from this calculation.
Which code should I refer to for compliance?: You should refer to the International Residential Code (IRC) R506 for compliance.