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Rebar Chair Spacing Calculator

Chair height = slab thickness − bottom cover − top cover − 2 × bottom bar − 2 × top bar. In a 250 mm slab with 40/25 mm cover, 12 mm bottom and 10 mm top mesh, chairs are 141 mm high. At 1 m spacing, a 10 × 8 m slab needs 99 chairs.

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Behind this panel: an example with the default sizes.

Inputs

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Slab
m
m
mm
mm
mm
Chairs
m
mm
mm
Rebar chair in the slab250 mmChair height 141 mm · 99 chairs@ 1 m each waymyconstructioncalculator.comNot to scale
Diagram updates as you type.

Results

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Chairs required
99chairs
Chair height141 mm
Cutting length per chair882 mm
Total chair steel77.5 kg
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Working
  1. Height = slab − covers − 2 × bottom bar − 2 × top bar = 250 − 40 − 25 − 24 − 20 = 141 mm
  2. Chairs = (⌈L ÷ s⌉ + 1) × (⌈W ÷ s⌉ + 1) = 99
  3. Cutting length = top + 2 × height + 2 × foot = 300 + 2 × 141 + 2 × 150 = 882 mm
Results are estimates for planning and material ordering only. They are not a substitute for structural design or approval by a qualified engineer. Read the full disclaimer.
Formulas


Chair height    = slab − cover_bottom − cover_top − 2·d_bottom − 2·d_top
Chairs          = (⌈L ÷ s⌉ + 1) × (⌈W ÷ s⌉ + 1)
Cutting length  = top leg + 2 × height + 2 × foot


Worked Example


A 10 × 8 m slab, 250 mm thick, 40 mm bottom and 25 mm top cover, 12 mm bottom and 10 mm top mesh, 12 mm chairs at 1 m:


    

  1. Height = 250 − 40 − 25 − 24 − 20 = 141 mm
    2. 

  2. Chairs = 11 × 9 = 99
    3. 

  3. Each chair = 300 + 282 + 300 = 882 mm → 99 × 0.882 × 0.888 = 77.5 kg
    4. 



Chair Height by Slab Thickness (40/25 mm cover, 12 mm bottom, 10 mm top)






Slab
Chair height






150 mm
41 mm




200 mm
91 mm




250 mm
141 mm




300 mm
191 mm






Frequently Asked Questions
How far apart should rebar chairs be?
Commonly 0.6–1.2 m (2–4 ft) each way. Closer spacing is needed for small top bars, heavy foot traffic or concrete placed from a pump hose on the mesh.
How do I calculate chair height?
Start from the slab thickness and take off both covers, the two layers of bottom bars and the two layers of top bars. The chair stands on the bottom mesh and supports the top mesh.
How much steel goes into chairs?
Cutting length = top leg + 2 × height + 2 × feet. A 141 mm chair with a 300 mm top and 150 mm feet uses 882 mm of bar: 0.78 kg in 12 mm.
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