Rebar Calculator
Enter your slab dimensions, grid spacing, and bar size to get real linear feet and weight of rebar needed.
12" on-center in both directions is the common standard for residential slabs.
#4 (1/2") at 12" spacing is the common standard for a 4-6" residential slab.
Covers overlap splices and cut offcuts -- 10% is standard.
Total rebar weight
316.0 lb
Total linear feet (before waste)
430.0 ft
This assumes a standard two-way grid -- bars running both directions across the slab, spaced evenly on-center. Structural or load-bearing work should follow an engineer's spec, not a general estimate.
Keep going
- Rebar only matters once you know the pour it's reinforcing; the concrete calculator sizes the cubic yards or bags for that slab or footing. Concrete Calculator
Frequently Asked Questions
Why does spacing change the bar count so much?
Tighter spacing means more bars in both directions -- going from 16" to 12" spacing adds roughly a third more bars, and the effect compounds because it applies in both the length and width directions at once.
What size rebar do I need for a driveway or patio slab?
#4 rebar (1/2" diameter) at 12" on-center in both directions is the common standard for a 4-6" residential slab. Thicker structural slabs, footings, or anything load-bearing should follow an engineer's spec, not a general estimate.
Why add extra for waste and laps?
Rebar comes in fixed stock lengths (commonly 20ft), so runs longer than one stick need an overlapping splice, and cut offcuts add up across a whole grid. 10% is the standard trade allowance.