Rebar Chair / Bar-Support Count
Bar supports (chairs and bolsters) that hold rebar or mesh at the design cover: chairs = ceil(slab area / support spacing^2 x (1 + waste)). A 1,000 sf mat on a 4 ft support grid is 66 chairs with waste; a tighter 3 ft grid under heavier bars is 117 (the spacing enters squared, so a foot tighter nearly doubles the count). The support spacing (~3 to 4 ft each way) comes from the spec and CRSI practice. Distinct from the ties (rebar-tie-wire) and the bars (rebar-weight-takeoff).
Formula and source
chairs = ceil(slab_area_sf / support_spacing_ft^2 x (1 + waste_pct/100)).
Bar-support count identity by name (chairs on a square grid over the slab area, plus waste); first-principles count arithmetic.
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