➕Rebar Calculator
Plan a rebar grid for a slab: bars, linear feet and chairs.
Your slab
You'll need about
44 bars
780 linear ft of rebar · 96 chairs (incl. 10% waste on bars)
bars
44
linear ft
780
chairs
96
grid
20 × 20
🛒 Shopping list
- Rebar (20 ft bars)44 bars
- Rebar chairs96 chairs
- Tie wire (roll)2 rolls
A common rule of thumb for lap length is 40 bar diameters — about 20 inches for #4 bar. Check your plans or local code.
How it works
Take off the edge clearance on every side. Bars each way = (inner width ÷ spacing) + 1. If a run is longer than one bar, extra bars are added with the lap overlap. Chairs ≈ one per 4 sq ft. Waste is added to the bar count.
Worked example
A 20 × 20 ft slab, 12 in grid, 3 in clearance and 20 ft bars: 20 bars each way at 19.5 ft = 40 bars, 780 linear feet. With 10% waste buy 44 bars, plus 96 chairs.
Questions people ask
12–18 in for slabs; follow your plans.
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Last updated October 8, 2026. These results are friendly estimates to help you plan — always double-check before buying or making big decisions.