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Rebar Spacing Calculator

Free rebar spacing calculator for contractors. Enter slab/footing length, width, and spacing to calculate bar count, total linear feet, waste/overage, and stock bars needed.

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Last updated: 2026-02-13

Quick Answer

Use this rebar spacing calculator to estimate a rebar grid takeoff for a rectangular slab or footing. Enter length, width, and spacing (inches o.c.) to get bar counts, total linear feet (with optional waste), and an optional stock stick count (20 ft / 40 ft).

Inputs you’ll need

  • Slab/footing length (ft)
  • Slab/footing width (ft)
  • Bar spacing (inches on-center)
  • Waste/overage % (optional)
  • Stock bar length (ft, optional — typically 20 or 40)

Related tools: Rebar Weight Calculator and Concrete Footing Calculator.

ft

Longer dimension of the slab

ft

Shorter dimension of the slab

in

On-center spacing (common: 12", 16", 18", 24")

%

Typical: 5-10% for laps, cuts, waste

ft

Standard: 20 ft or 40 ft sticks

Assumptions

  • Bars form a uniform grid with equal spacing in both directions.
  • Each bar runs the full slab dimension (length or width) with no inset.
  • Waste % covers laps/splices, hooks, cut-off waste, and field overage.
  • Stock bar count assumes no reuse of cut-off pieces (worst case).

Rebar Spacing Results

Bars Along Length

11

Bars Along Width

21

Total Bars

32

Total Length

430.00 ft

Detailed Breakdown

Slab dimensions20.00 × 10.00 ft
Bar spacing12" o.c.
Bars along length11 bars × 20.00 ft = 220.00 ft
Bars along width21 bars × 10.00 ft = 210.00 ft
Total bars32
Total linear length430.00 ft
Total length (with waste)430.00 ft
Stock bars needed (20 ft sticks)22 bars

Tip: Use our rebar weight calculator to convert linear feet into pounds for ordering.

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Common Rebar Spacing Reference

Spacing (in o.c.)Bars per 10 ftCommon Use
6"21Heavy structural slabs, retaining walls
8"16Structural slabs, grade beams
12"11Standard residential slabs, footings
16"8Light-duty slabs, sidewalks
18"7Driveways, patios
24"6Temperature/shrinkage steel, light pads

Rebar Spacing & Placement Guide

Standard spacing requirements, bar sizes, and cover requirements for concrete reinforcement.

Standard Rebar Spacing for Concrete

Residential concrete slabs typically use #3 or #4 rebar on 12"–18" centers, while structural footings and walls use tighter spacing based on engineering calculations.

  • Residential slab (4"): #3 rebar at 18" OC both ways, or wire mesh 6×6-W1.4×W1.4
  • Driveway slab (5–6"): #4 rebar at 12"–16" OC both ways
  • Foundation walls: #4 or #5 rebar at 12" OC vertical, 16"–24" OC horizontal
  • Footings: 2 runs of #4 rebar continuous, 3" from bottom and edges

Maximum rebar spacing is generally 3× the slab thickness or 18 inches, whichever is less (ACI 318 guidelines).

Key Takeaways

  • Residential slab: #3 or #4 at 18" OC
  • Driveway: #4 at 12"–16" OC both ways
  • Max spacing: 3× slab thickness or 18" (whichever less)

Rebar Sizes & When to Use Each

Rebar is designated by bar number, where each number represents 1/8" of diameter (e.g., #4 = 4/8" = 1/2" diameter).

  • #3 (3/8"): Light-duty slabs, patio slabs, thin walls. Weight: 0.376 lb/ft
  • #4 (1/2"): Most common residential — slabs, footings, walls. Weight: 0.668 lb/ft
  • #5 (5/8"): Foundation walls, structural elements. Weight: 1.043 lb/ft
  • #6 (3/4"): Heavy structural, commercial foundations. Weight: 1.502 lb/ft
  • #7-#8: Commercial and industrial applications only

Key Takeaways

  • Bar # = diameter in 1/8 inches (#4 = 1/2")
  • #4 rebar most common for residential
  • #5 and #6 for structural/commercial

Rebar Placement & Cover Requirements

Minimum concrete cover over rebar is 1.5"–3" depending on the application and exposure conditions.

  • Interior slabs: 1.5" minimum cover (use rebar chairs/supports)
  • Exterior slabs: 2" minimum cover
  • Footings (cast against earth): 3" minimum cover
  • Foundation walls: 2" minimum cover on both faces

Position rebar in the lower third of slabs (tension zone) for flat slabs. For cantilevers, place rebar in the top. Use plastic rebar chairs ($0.10–$0.50 each) to maintain proper position during the pour.

Key Takeaways

  • Interior slab cover: 1.5" minimum
  • Footings against earth: 3" minimum cover
  • Use rebar chairs to maintain position during pour

Worked examples (verify the math)

These examples are designed to be checkable by hand so you can confirm the calculator output.

Example Inputs Expected result
1 20 ft × 10 ft slab, 12" o.c., 0% waste, 20 ft sticks Bars along length = floor(10×12/12)+1 = floor(10)+1 = 11
Bars along width = floor(20×12/12)+1 = floor(20)+1 = 21
Total bars = 11 + 21 = 32
Total length = (11×20) + (21×10) = 220 + 210 = 430 ft
Stock bars (20 ft) = ceil(430/20) = ceil(21.5) = 22
2 12 ft × 12 ft slab, 8" o.c., 10% waste, 20 ft sticks Bars along length = floor(12×12/8)+1 = floor(18)+1 = 19
Bars along width = same = 19
Total bars = 38
Total length = (19×12) + (19×12) = 456 ft
Length w/ waste = 456×1.10 = 501.6 ft
Stock bars (20 ft) = ceil(501.6/20) = ceil(25.08) = 26

Common mistakes (avoid these)

  • Forgetting to convert feet ↔ inches when doing spacing math.
  • Not adding waste for lap splices, hooks, and cut pieces.
  • Assuming stock sticks will nest perfectly (offcuts usually aren’t reusable in the field).
  • Mixing up “bars along length” vs “bars along width.”

How to Use This Calculator

Measure the slab (or footing) dimensions

Enter the overall length and width in feet. Use the actual rebar coverage dimensions if you have edge offsets.

Enter your on-center spacing

Type the bar spacing in inches (o.c.). Common values are 6", 8", 12", 16", 18", and 24".

Add waste/overage (optional)

Add a waste percentage to cover lap splices, hooks, cut-offs, and field waste. 5–10% is common.

Estimate stock bars (optional)

Enter a stock bar length (usually 20 ft or 40 ft) to estimate how many sticks to order. This assumes worst-case cuts (no reuse of offcuts).

Rebar Spacing Formulas (grid takeoff)

barsAlongLength = floor((widthFt × 12) / spacingIn) + 1
barsAlongWidth  = floor((lengthFt × 12) / spacingIn) + 1
totalBars       = barsAlongLength + barsAlongWidth
totalLengthFt   = (barsAlongLength × lengthFt) + (barsAlongWidth × widthFt)
totalWithWaste  = totalLengthFt × (1 + wastePct/100)
sticksNeeded    = ceil(totalWithWaste / stockBarLengthFt)

Where:

lengthFt / widthFt
= Overall slab/footing dimensions in feet
spacingIn
= On-center spacing in inches (must be > 0)
wastePct
= Optional overage percentage (0–50% clamp in the UI)
stockBarLengthFt
= Optional stick length (commonly 20 ft or 40 ft)

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Frequently Asked Questions

How do you calculate rebar count from spacing?
Convert the slab dimension to inches, divide by spacing, then add 1 bar for the starting edge: Bars = floor((dimension in inches) / spacing) + 1. You do this once for each direction of the grid.
Why do you add 1 bar?
Because spacing is the distance between bars. If you have N spaces across the slab, you have N + 1 bars (one at each end).
Does this include edge cover (offset from the form)?
No. This tool assumes bars run the full slab length/width with no inset. If your plans call for edge offsets, you can reduce the entered slab dimensions to match the rebar coverage area.
What waste percentage should I use for rebar?
For simple mats with long runs, 3–5% can work. For heavy splicing, hooks, congestion, and lots of cut pieces, 7–10% is more typical. Always follow the engineer’s details for lap splice lengths and hooks.
How do I convert rebar linear feet into weight (lb)?
Use our Rebar Weight Calculator. Weight depends on bar size (#3–#11), but the flow is: Total weight (lb) = Total length (ft) × weight (lb/ft).
Does this calculator work for footings and stem walls?
It works for any rectangular “mat” area. For continuous footings/stem walls with specific bar runs, it’s often better to do a linear takeoff. If you’re estimating concrete too, use our Concrete Footing Calculator.
Can I use different spacing each direction?
This version assumes the same spacing both ways. If your plans specify different spacing, run the calculator twice (swap length/width) or adjust spacing and take notes.
Is stock bar count accurate?
It’s a worst-case estimate. It divides total required length (with waste) by stick length and rounds up, assuming no reuse of offcuts. Your actual stick count may be lower with smart cutting.

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