Rebar Chair / Bar-Support Count
Bar supports (chairs and bolsters) that hold rebar or mesh at the design cover.
Example
You enter
- Slab / mat area (ft²) 1000
- Chair spacing each way (ft) 4
- Waste allowance (%) 5
You get
- Chairs 66
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Scopechairs = 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 tie wire and the bars themselves.
Formula or tablechairs = ceil(slab_area_sf / support_spacing_ft^2 x (1 + waste_pct/100)).
Source and editionBar-support count identity by name (chairs on a square grid over the slab area, plus waste); first-principles count arithmetic.
Free accessThe count arithmetic is public first-principles; the support spacing comes from the spec and CRSI practice.
What governsEstimate. AHJ and licensed professional govern.
API field namesslab_area_sf, support_spacing_ft, waste_pct, chairs
- Support spacing ~3 to 4 ft each way (default 4 ft); tighter under heavy bars per CRSICRSI / project spec
- Grid square grid: chairs per unit area = 1 / spacing^2first principles