Rebar Calculator
How much bar to buy for a rectangular mat — not a structural design. Click any i for detail.
Slab grid
Length (ft)
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Length
Slab length. |
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|---|---|
Width (ft)
i
Width
Slab width. |
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Spacing (in)
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Spacing
Center-to-center. Residential slabs often 12–18 in. |
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Waste / laps (%)
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Waste
Include lap splices here if you are not detailing them. 10% is a starting point. |
How to use this calculator
- Enter slab length and width.
- Set grid spacing in inches.
- Add waste for laps and cuts.
- Convert feet to 20-ft sticks when ordering.
Results explained
Bars one way = floor(other dimension ÷ spacing) + 1. Total feet = that count × length, plus the crossing set.
Quick reference: rebar
A lap is often ~40 × bar diameter unless the engineer says otherwise.
| Item | Detail |
|---|---|
| #4 bar diameter | 0.5 in |
| Typical slab spacing | 12–18 in o.c. |
| 20-ft stick | Standard stock length |
| Mesh alternative | Different takeoff — not this page |
Chairs, cover, and edge distance are placement details, not length.
How the estimate is built
nL = floor(W / s) + 1; nW = floor(L / s) + 1; feet = nL×L + nW×W.
Example scenario
20×12 ft at 18 in → 9 bars × 20 ft + 14 bars × 12 ft = 348 ft, plus 10% ≈ 383 ft.
FAQ
Bar size / weight?
Length is independent of #3 vs #5. Weight is lb/ft × feet.
Two mats?
Run twice or double the result.
Concrete yards too?
See the concrete calculator.