Rebar Calculator

How to Calculate Rebar for a Concrete Slab or Wall

Rebar (reinforcing bar) dramatically increases the tensile strength of concrete. Calculating the right amount means counting bars in both directions, accounting for overlap at splices, and knowing the weight for ordering and transport.

The Rebar Grid Formula

Bars in each direction = (Dimension in inches ÷ Spacing) + 1. The total linear footage is: (Bars along length × Width) + (Bars along width × Length). Add lap splice length at each joint where bars overlap.

Example: A 20×20 ft slab with #4 rebar at 12" OC = 21 bars each way × 20 ft = 840 total linear feet.

Rebar Sizes & Weights

• #3 (⅜" dia) — 0.376 lb/ft — Temperature/shrinkage reinforcement, light slabs
• #4 (½" dia) — 0.668 lb/ft — Most common residential bar, slabs, footings
• #5 (⅝" dia) — 1.043 lb/ft — Driveways, heavier residential, light commercial
• #6 (¾" dia) — 1.502 lb/ft — Commercial slabs, retaining walls, foundations
• Standard 20 ft lengths are most common; #3 and #4 also available in 10 ft lengths

When to Use Each Size

• #3 at 18"–24" OC — Minimum temperature steel for slabs that don't carry structural loads
• #4 at 12" OC — Standard residential slab reinforcement (patios, garages, driveways)
• #4 at 16" OC — Standard wall reinforcement, both horizontal and vertical
• #5 at 12" OC — Heavy-duty residential, commercial slabs, elevated decks
• #6 at 12" OC — Commercial and industrial applications, retaining walls

ACI Requirements & Best Practices

• ACI 318 requires minimum ¾" clear cover for concrete cast against soil, 1½" for formed surfaces
• Lap splice length: minimum 40× bar diameter (#4 = 20", #5 = 25") per ACI 318
• Use rebar chairs or dobies to maintain proper cover — never lay rebar directly on grade
• Tie wire every intersection is ideal; at minimum, tie alternating intersections in a checkerboard pattern
• One 3.5 lb roll of tie wire covers approximately 200 intersections