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Rebar for a Slab Calculator

Lay out a rebar grid for a concrete slab. Enter slab size and grid spacing to get bar counts, total linear feet, and the number of 20 ft sticks.

You need approximately
190 linear ft
20 bars · 100 tie points
Bars lengthwise
10
Bars widthwise
10
20 ft sticks to buy
10

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.

Field notes

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.

How the rebar for a slab calculator works

The math behind the tool:

bars along a side = floor(side length ÷ spacing) + 1
total length = bars_x × width + bars_y × length

Sources: Portland Cement Association (concrete density and mix guidance).

Frequently asked questions

How do you calculate rebar for a slab?

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.

What spacing should rebar be in a concrete slab?

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.

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