123 123Calculator

Rebar Calculator

Use the rebar calculator to find out how many rebars are needed for a concrete slab and how much it will cost.

Codes commonly want 30–40 diameters for a tension lap.
Leave at zero to skip the costing.
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Total rebar length414 ft126.187 m in a 12 in grid
Bars running lengthwise11each 19.5 ft long
Bars running crosswise21each 9.5 ft long
Intersections to tie231allow a tie wire at each, or every other one
Weight125.4 kg276.5 lb of #4 bar
Bars to buy2120 ft lengths, including laps
Bar diameter12.7 mm0.5 in, #4
Grid covers185.25 sq ftinside 3 in of cover
Steel ratio0.8468%by area, assuming a 4 in slab

The method

bars each way = side ÷ spacing + 1; total length = both directions

Cover is what makes it last

Rebar works because the concrete around it keeps water and air away from the steel. Too close to the surface and it rusts, and rusting steel expands enough to spall the concrete off — which then exposes more steel. Three inches from earth, two inches from weather, ¾ inch indoors are the usual minimums.

Laps matter as much as cover. Bars have to overlap far enough for the load to transfer through the concrete between them, typically 30 to 40 bar diameters — about 20 inches for #4 bar. A lap that is merely “a good overlap” is not a design.

Position the steel where the tension is: in the bottom third of a slab spanning between supports, and near the top over a support. Rebar lying on the subgrade does nothing at all.