Rebar Calculator
Plan a rebar grid for a concrete slab: enter the slab size, on-center spacing, and bar size to get the bar count each way, total linear feet, and the weight of steel to order.
Example: with Slab length (ft) 20 · Slab width (ft) 10 · Grid spacing (in on center) 12 · Bar size #4 (1/2 in), 0.668 lb/ft · Edge clearance (in) 3 → Total rebar: 385 linear ft.
Computed by the calculator below using its default values. Change any input to see your own numbers.
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Check it outHow much rebar do I need for a slab?
Rebar for a slab runs in a grid, so count each direction separately: subtract the edge clearance from the span (about 3 in of concrete cover on every side keeps steel from rusting), divide by the on-center spacing, round down, and add one for the starting bar. For a 20 × 10 ft slab on a 12-in grid, that’s 10 bars running the length at 19.5 ft plus 20 bars across at 9.5 ft — 385 linear feet, or about twenty 20-ft sticks weighing roughly 257 lb in #4.
Residential slabs, patios, and driveways typically use #3 or #4 bar on a 12–18 in grid; tighter spacing and thicker bar suit heavier loads. Two adders the grid math can’t see: laps where two sticks splice (overlap 30–40 bar diameters, about 15–20 in for #4) and rebar chairs to hold the grid mid-slab — steel lying on the dirt does nothing for the concrete.
How it’s calculated
Usable span each way = slab dimension − 2 × edge clearance. Bars per direction = floor(perpendicular usable span ÷ on-center spacing) + 1, each cut to the usable span in that direction. Total linear feet = the sum across both directions; weight multiplies by the ASTM A615 nominal unit weight for the chosen size (#3 = 0.376, #4 = 0.668, #5 = 1.043 lb/ft); stick count assumes 20-ft bars, rounded up. Splice laps and chairs are not included — add them for slabs longer than one stick.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Counting gaps instead of bars — a grid needs the +1 starting bar in each direction, just like fence posts.
- Ignoring lap splices — any run longer than one stick must overlap 30–40 bar diameters at each splice, which adds steel.
- Letting the grid lie on the ground — use chairs so the bar sits in the middle third of the slab, with about 3 in of cover at edges.
Frequently asked questions
How much rebar do I need for a 10x10 slab?
On a 12-in grid with 3-in edge clearance: 10 bars each way at 9.5 ft, so 190 linear feet — about 127 lb of #4 rebar.
What rebar spacing should I use?
Most residential slabs and driveways use a 12–18 in on-center grid. Tighter spacing adds strength for vehicle loads; verify structural slabs with your local code or an engineer.
How much does rebar weigh?
Nominal weights per ASTM A615: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft, and #5 is 1.043 lb/ft. A 20-ft stick of #4 weighs about 13.4 lb.
How much should rebar overlap at a splice?
A common rule is 30–40 bar diameters: for #4 (1/2-in) bar that is roughly 15–20 in of overlap, tied in at least two places.
Is wire mesh or rebar better for a slab?
Welded wire mesh suits thin, lightly loaded slabs like sidewalks; rebar grids handle driveways and structural slabs. Some pours use both — mesh for shrinkage control, bar for strength.