How this calculator works
Edge offsets reduce the reinforcement rectangle in both directions. The number of spaces is rounded up so the maximum spacing is not exceeded; adding one gives runs in each direction. Nominal masses follow Gerdau Canadian metric rebar data. Dividing total length by stock length gives a theoretical minimum purchase count, assuming usable offcuts and unrestricted splicing.
Formula
Lgrid = L - 2c; Wgrid = W - 2c; rows = ceil(Wgrid / spacing) + 1; columns = ceil(Lgrid / spacing) + 1; total length = rows x Lgrid + columns x Wgrid; stock bars >= ceil(total length / stock length).
Worked example
A 1 m by 1 m grid at 200 mm spacing with zero edge offset has six runs each way: 12 m total. At 0.785 kg/m, 10M bar weighs 9.42 kg; two 6 m stock bars are the theoretical minimum.
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Frequently asked questions
Is the edge offset concrete cover?
The input is to the bar centreline. If a drawing gives clear cover to the steel surface, add half the bar diameter to obtain this offset.
Why might I need more stock bars?
Cutting waste, lap splices, hooks, bends and unusable offcuts can increase the purchase count. This is a straight-length lower bound, not a fabrication schedule.
Does this select bar size or spacing?
No. Those values must come from the slab design. A material takeoff does not check reinforcement capacity or code compliance.