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Free online rebar calculator

A rebar calculator counts the reinforcing bars needed for a concrete slab, giving you total linear footage, weight, and material cost before you order.

About this calculator

How to use

  1. Enter the slab length and width in feet.
  2. Choose bar spacing in inches (12 in is standard residential).
  3. Select the bar size (#4 is most common for slabs).
  4. Set a waste factor (10% is typical).
  5. Read bar counts, linear footage, weight, and cost.

Formulas

barsAlongLength = ceil(slabWidth × 12 / spacing) + 1
barsAlongWidth  = ceil(slabLength × 12 / spacing) + 1

linFtLength     = barsAlongLength × slabLength  [+ lap allowance if length > 20 ft]
linFtWidth      = barsAlongWidth  × slabWidth   [+ lap allowance if width > 20 ft]

totalLinearFt   = (linFtLength + linFtWidth) × (1 + waste% / 100)
totalWeightLbs  = totalLinearFt × weightPerFt
totalTons       = totalWeightLbs / 2000
estimatedCost   = totalWeightLbs × $0.75

Bar weights (CRSI):

BarDiameterlb/ft
#33/8 in0.376
#41/2 in0.668
#55/8 in1.043
#63/4 in1.502

Worked example

20 ft × 20 ft slab, 12-in spacing, #4 bar, 18-in lap, 10% waste:

barsAlongLength = ceil(20 × 12 / 12) + 1 = 20 + 1 = 21
barsAlongWidth  = ceil(20 × 12 / 12) + 1 = 21

linFtLength     = 21 × 20 = 420 ft  (slab = 20 ft, no lap needed)
linFtWidth      = 21 × 20 = 420 ft

totalLinearFt   = (420 + 420) × 1.10 = 840 × 1.10 = 924 lin ft

totalWeightLbs  = 924 × 0.668 = 617.23 lbs
totalTons       = 617.23 / 2000 = 0.31 tons
estimatedCost   = 617.23 × $0.75 ≈ $463

Notes

  • This calculator provides a material quantity estimate for ordering purposes. Rebar placement details (cover depth, support chairs, tie patterns) must follow your structural drawings or engineer’s specifications.
  • Always verify spacing and bar size with your local building official for any structural slab.

Frequently asked

How far apart should rebar be spaced in a concrete slab?
12-inch on-center spacing is standard for residential slabs (driveways, patios, floors). Structural slabs and footings often use 8-inch or 6-inch spacing as specified by a structural engineer.
What bar size should I use?
#4 bar (1/2-inch diameter) is the most common choice for residential slabs. #3 bar (3/8 in) is used for sidewalks and light-duty flatwork. #5 and #6 bars are specified for heavier structural applications.
What is a lap splice and why does the calculator add footage for it?
When a slab is longer than a standard bar length (20 ft), bars must be overlapped (spliced) to maintain continuity. IRC and ACI 318 require a minimum lap of 40 bar diameters — about 18 in for #4 bar. The calculator adds this footage automatically for slabs longer than 20 ft.
How heavy is rebar?
Weight per linear foot by bar size (CRSI standard): #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft. These values are used directly in the weight calculation.
How is the estimated cost calculated?
The calculator uses $0.75 per pound as a rough market reference. Actual prices vary by region, supplier, and steel market conditions. Get a current quote from your local supplier.
How do I share my rebar estimate?
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