Rebar Calculator
Rebar pieces and linear feet for a slab grid at your spacing.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Rebar for a slab is laid as a grid, so the estimate is two sets of parallel bars — one running each direction — converted into 20 ft sticks you can actually buy. This calculator counts both directions and totals the linear feet.
How it works
Bars running each direction = the crossing dimension ÷ spacing, plus one. Total linear feet is rows × length + columns × width, bought as 20 ft sticks.
Bars running the slab's length are spaced across its width: width ÷ spacing, plus one to close the far edge. Bars running the width are spaced along the length the same way. Multiply each count by the bar's run and add the two for total linear feet.
Real grids need splice overlap wherever a run exceeds the 20 ft stock length, and every bar stops 2–3 inches shy of the slab edge for concrete cover. The stick count rounds up, which absorbs modest overlap; add a bar or two on big slabs with many splices.
Bars each way = crossing dimension (in) ÷ spacing + 1. Linear feet = (bars running length × length) + (bars running width × width); sticks = linear feet ÷ 20, rounded up.
Worked examples
A 20 × 10 ft slab on a 12 in grid. → 430 linear feet — 22 twenty-foot bars.
Across the 10 ft width: 11 bars × 20 ft = 220 ft. Along the 20 ft length: 21 bars × 10 ft = 210 ft. Total 430 ft ÷ 20 = 21.5, rounded up to 22.
A 12 × 12 ft shed slab on a 16 in grid. → 240 linear feet — 12 bars.
Each direction: 144 ÷ 16 = 9, plus 1 = 10 bars × 12 ft = 120 ft; both directions total 240 ft.
Tips & gotchas
- Support the grid on chairs or dobies so it sits in the middle of the slab thickness — rebar lying on the ground does nothing.
- Tie intersections with wire every 2–3 crossings; the ties just hold position for the pour, they add no strength.
- For light-duty slabs, 6×6 welded wire mesh or fiber mix can replace a #3 grid — price both before buying sticks.
FAQ
What size rebar for a slab?
#3 or #4 bar on a 12–18 in grid covers most patios and driveways. Footings and structural work follow the engineer's spec, not a calculator.
How much should bars overlap at a splice?
The rule of thumb is 30–40 bar diameters — 15–20 inches for #4 (½ in) bar. Stagger splices so they do not line up in one row.
Rebar or wire mesh for a driveway?
Driveways carrying vehicles want #4 rebar on a 12–18 in grid. Mesh suits patios and walks. Either way, the concrete thickness and base matter more than the steel.
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