Free online rebar 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):

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.