MEASURE. ESTIMATE. BUILD.
Lay out a rebar grid for a concrete slab.
Change any measurement to update your estimate.
Enter slab size and grid spacing to get bar counts, total linear feet, and the number of 20 ft sticks.
A slab grid needs about 3 inches of cover from every edge or the bar rusts and spalls the concrete, which is why this view trims the grid inward. Residential slabs commonly run #3 or #4 bar on a 12–18 inch grid.
The edge clearance matters more than people expect — rebar needs about 3 inches of concrete cover from any face, or it rusts and spalls the slab from the inside. That's why the calculator trims the grid in from the edges. Buy your bar in 20 ft sticks and cut on site rather than ordering odd lengths; a rebar bender and cutter rents for less than the premium on pre-cut bar.
The math behind the tool:
bars along a side = floor(side length ÷ spacing) + 1
total length = bars_x × width + bars_y × lengthSources: Portland Cement Association (concrete density and mix guidance).
Divide each slab dimension by the bar spacing and add one to get the bar count in that direction, then multiply by the opposite dimension for total length. A 10 × 10 ft slab at 12-inch spacing uses about 22 bars and 220 linear feet.
Residential slabs commonly use #3 or #4 rebar on a 12-to-18-inch grid; check your local code and engineer for load-bearing slabs.