Chair Bar Calculator
Enter slab area, thickness and chair spacing to get the count, length per chair, total rebar length and weight.
Inputs
Result
Formula
| Chair count | N = ceil(area ÷ spacing²) |
| Leg height | h = thickness − cover (top + bottom) |
| Length per chair | L = top flat + 2 × h × √2 |
| Total weight | W = N × L × 0.00617 × d² |
100 m² slab, 120 mm thick, chairs at 1 m, Φ8, 0.2 m top flat, 30 mm total cover: 100 chairs, 0.09 m legs, 0.4545 m each, 45.5 m total, 17.9 kg (about 0.18 kg/m²).
FAQ
Do chair bars count in the rebar total?
Yes. They carry no structural load but consume real material and labour. Convention is to list them as a separate 'temporary/positioning bars' line, or add 0.5-1% to the total. The length from this tool goes straight into the bar schedule.
What spacing should I use?
0.8-1.2 m both ways is normal. Thicker slabs and heavier top mats need tighter spacing; concentrate loads, cantilevers and construction-joint edges need 0.6-0.8 m. Too wide and the top mat sags, thinning the cover, which fails inspection.
Plastic chairs or steel chairs?
Plastic is cheap, non-corroding and fast, fine for 100-150 mm slabs. Steel chairs carry more load and can be tied or welded, suitable for thick slabs, long spans, vibration or heat. Plastic chairs come in fixed heights, so a thickness change means a model change.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.