Rebar Calculator
Calculate rebar quantity, weight, and materials for concrete slabs. Get bar counts, tie wire totals, and chair support estimates for any grid pattern.
Results
Bars Along Length
20
spaced 12" o.c. across width
Bars Along Width
20
spaced 12" o.c. across length
Total Linear Feet
780.0
of #4 rebar
Total Weight
521.0
lbs (0.668 lb/ft)
20ft Bars Needed
80
with 24" overlap
Tie Wires
400
1 per intersection
Chairs / Supports
100
1 per 4 sq ft (400 sq ft slab)
Grid Pattern Summary
Slab: 20 ft x 20 ft (400 sq ft)
Rebar: #4 (1/2" diameter)
Grid: 12" x 12" on center (both directions)
Edge setback: 3" from forms on all sides
Effective rebar area: 19.50 ft x 19.50 ft
Layout: 20 bars running 19.5 ft (length) + 20 bars running 19.5 ft (width)
Overlap: 24" lap splices (2.0 ft), effective bar length = 18.0 ft per 20ft bar
How to Use This Calculator
Enter the slab dimensions in feet, choose a rebar size, and set the grid spacing. You can use the same spacing in both directions or set them independently. Adjust the overlap length (24 inches / 40 bar diameters is standard) and the edge setback from forms (3 inches is typical). The calculator gives you total bars, linear footage, weight, tie wires, and chair supports needed.
Formula Used
Bars in each direction = floor(effective span / spacing) + 1. Linear feet = bars x bar length. 20ft bars needed = bars x ceil(bar length / (20 - overlap)). Tie wires = bars in length dir x bars in width dir. Chairs = ceil(slab area / 4).
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Disclaimer
This calculator is for estimation purposes only. Results may vary based on local conditions, materials, and building codes. Always consult a licensed professional before making decisions based on these calculations. MyContractorTools is not responsible for any errors or omissions.