CONSTRUCTION · MATERIALS

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.

LAST REVIEWED · APR 24, 2026 · BY D. MARQUEZ, P.E.
You need
42 bars
Est. cost
$630
Slab DimensionsReset
LengthSlab length
ft
WidthSlab width
ft
Rebar Specification
Bar sizeDiameter
SpacingCenter-to-center
in
Pricing
Price per footRetail average
USD
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How the rebar calculator works

The calculator creates a grid layout for your slab dimensions. It calculates bars running in both directions at your specified spacing, then sums the total linear footage and multiplies by the weight per foot for your chosen bar size.

Rebar size guide

  • #3 (⅜") — 0.376 lb/ft — light residential, temperature reinforcement
  • #4 (½") — 0.668 lb/ft — standard residential slabs, driveways
  • #5 (⅝") — 1.043 lb/ft — structural slabs, retaining walls
  • #6 (¾") — 1.502 lb/ft — commercial foundations
  • #7 (⅞") — 2.044 lb/ft — heavy structural applications
  • #8 (1") — 2.670 lb/ft — commercial and industrial

Ordering tips

Rebar is sold in 20-foot lengths at most suppliers. Plan your cuts to minimize waste — a 10-foot slab uses half a bar with no waste, while an 11-foot slab wastes 9 feet per bar unless you plan lap splices.

Methodology. Bars lengthwise = ceil(Width ÷ Spacing) + 1. Bars widthwise = ceil(Length ÷ Spacing) + 1. Total length = lengthwise bars × Length + widthwise bars × Width. Weight per foot by size: #3 (0.376), #4 (0.668), #5 (1.043), #6 (1.502), #7 (2.044), #8 (2.670) lb/ft.

Sources

  • CRSI Manual of Standard Practice (Concrete Reinforcing Steel Institute)
  • ASTM A615 Standard Specification for Deformed Steel Bars
  • Average rebar price: $0.50–$1.00/linear foot (2026)
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Frequently asked questions

How much rebar for a 20×20 slab? +
At #4 rebar with 12" spacing: 21 bars in each direction = 42 bars total, 840 linear feet, weighing about 561 lbs. That's roughly 42 standard 20-foot bars.
How much does rebar cost per foot? +
In 2026, #4 rebar runs $0.60–$0.90 per linear foot at retail. Bulk pricing from steel suppliers drops to $0.40–$0.60/ft. For a 20×20 slab needing 840 ft, expect $500–$750 in rebar costs.
What size rebar for a 4-inch slab? +
#4 rebar at 12–18" spacing is standard for 4-inch residential slabs. Some engineers specify #3 at 12" for lighter-duty applications like patios. Always follow your structural engineer's specifications if available.
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