Rebar calculator

For each direction of a grid, take the span the bars are spaced across, in inches, subtract the edge clearance twice, divide by the spacing, take the whole number and add one. The plus one is the bar at the far end, and forgetting it is the commonest error in a hand count. Each bar's own length is the other dimension less the clearance at both ends.

Both directions are counted and added, the total feet convert to metres at exactly 0.3048 metres to the foot, and the weight is that figure times the bar's mass per metre. Two Ontario suppliers, 2AZ Group in Mississauga and Brampton and Dass Rebar in Woodbridge, publish the same CSA G30.18 masses, which is a useful sign the figures are the standard's rather than a supplier's.

The stock-bar count is given twice on purpose. Reusing offcuts is the cheaper number and the one a fabricator would quote; cutting every run from a fresh bar is what actually happens on a small site with no bar bender, and on a slab with awkward dimensions the difference is not small.

Spacing is a design decision, not a default. The figures on this page are 2AZ Group's published Ontario guidance for ordinary residential work, and they are a sensible starting point for a shed pad or a garden wall. They are not a substitute for engineered drawings on anything that carries a building, and underpinning in Toronto requires engineer-stamped drawings before a permit is issued at all.

Three things the count leaves out: lap splices where one bar continues past another, chairs that hold the grid at the right height rather than letting it sit on the ground where it does nothing, and tie wire. The lap length in particular depends on the bar, the concrete strength and the job, so it belongs on a drawing.

For what the reinforced slab itself costs, the slab cost calculator prices mesh and rebar as separate line items, which is how they appear on a real quote.