CONSTRUCTION · MATERIALS

Concrete Slab Calculator

Estimate the concrete volume, rebar grid layout, bar count, weight, and total cost for a reinforced concrete slab. Includes a 10% waste factor for concrete.

LAST REVIEWED · APR 24, 2026 · BY D. MARQUEZ, P.E.
You need
5.43 yd³
Est. cost
$1,526
Slab DimensionsReset
LengthSlab length
ft
WidthSlab width
ft
Depth4" typical for slabs
in
Rebar
Bar sizeDiameter
Rebar spacingCenter-to-center
in
Cost Assumptions
Concrete pricePer cubic yard
USD
Rebar pricePer linear foot
USD
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How the concrete slab calculator works

This calculator combines concrete volume estimation with a rebar grid layout. Enter your slab dimensions and rebar preferences, and it returns both the concrete needed (in cubic yards with a 10% waste factor) and the full rebar bill — bar count, total linear feet, and weight.

Choosing rebar size and spacing

For residential slabs, #4 rebar at 12-inch spacing is the most common configuration. Garage floors and driveways often use #4 at 18-inch spacing. Heavy-duty applications like shop floors may call for #5 at 12 inches.

  • #3 at 18" — light-duty patios, walkways
  • #4 at 12" — standard residential slabs, driveways
  • #5 at 12" — shop floors, heavy equipment pads

Rebar placement tips

Position rebar at mid-depth of the slab using rebar chairs. Maintain 3 inches of cover from any edge. Overlap bars by at least 24 inches at splices and tie every intersection with wire ties.

Methodology. Concrete volume = Length × Width × (Depth ÷ 12) ÷ 27 cubic yards. Rebar grid: bars lengthwise = ceil(Width ÷ Spacing) + 1; bars widthwise = ceil(Length ÷ Spacing) + 1. Weight calculated using standard rebar weight per foot (#3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502 lb/ft).

Sources

  • ACI 318 Building Code Requirements for Structural Concrete
  • CRSI Manual of Standard Practice for rebar weights
  • Average ready-mix price: $165/yd³ delivered (ENR, 2026)
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RELATED CONSTRUCTION CALCULATORS
Concrete Calculator ↗
Estimate the cubic yards, bags, and cost of concrete for slabs, footings, columns, and walls. Includes a 10% waste factor and side-by-side comparison of ready-mix versus bag pricing.
Rebar Calculator ↗
Estimate the number of rebar bars, total linear feet, weight, and cost for a reinforced concrete slab. Supports #3 through #8 bar sizes at any spacing.

Frequently asked questions

How much rebar do I need for a 20×20 slab? +
At standard #4 rebar with 12-inch spacing, a 20×20 slab needs 21 bars in each direction — 42 bars total — plus about 840 linear feet of rebar weighing roughly 561 pounds.
Can I use wire mesh instead of rebar? +
Wire mesh (6×6 W2.9/W2.9) works for light-duty slabs like patios and walkways. For driveways, garage floors, or anything supporting vehicle weight, rebar provides significantly better crack control and structural integrity.
What spacing should I use for rebar? +
12-inch spacing is the residential standard. Go to 18 inches for light-duty patios, or tighten to 8–10 inches for heavy loads. Always check local building codes — they may specify minimum requirements.
Do I need rebar in a 4-inch slab? +
For slabs over 4 feet in any dimension, yes. Without reinforcement, a 4-inch slab will develop cracks from shrinkage and thermal expansion. Rebar doesn't prevent cracks but keeps them tight and maintains structural integrity.
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