# Chair Bar Calculator

Enter slab area, thickness and chair spacing to get the count, length per chair, total rebar length and weight.

- Page: https://jianzhujisuan.com/en/chair-bar
- Category: Rebar (https://jianzhujisuan.com/en/c/rebar)
- Chinese version: https://jianzhujisuan.com/zh/chair-bar.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Slab area 100 m², Slab thickness 0.12 m, Chair spacing (both ways) 1 m, Chair bar diameter Φ8, Top flat length 0.2 m, Cover (top + bottom) 30 mm): **Total chair bar weight 17.9 kg**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Slab area | number | 100 | m² |  |
| Slab thickness | number | 0.12 | m |  |
| Chair spacing (both ways) | number | 1 | m | Typically 0.8-1.2 m; denser for thin slabs, wider for thick ones |
| Chair bar diameter | select | Φ8 | — |  |
| Top flat length | number | 0.2 | m | The flat part carrying the top mat, usually 0.15-0.3 m |
| Cover (top + bottom) | number | 30 | mm |  |

## 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

## Worked example

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).

## Questions

### 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.

## Related tools

- [Rebar Weight Calculator](https://jianzhujisuan.com/en/rebar-weight): Pick a bar diameter, enter length and quantity to get weight per metre, per bar and total tonnage.
- [Rebar Anchorage Length Calculator](https://jianzhujisuan.com/en/rebar-anchorage): Basic and seismic anchorage length from bar grade, diameter and concrete class.
- [Rebar Lap Length Calculator](https://jianzhujisuan.com/en/rebar-lap): Lap length ll / llE by splice percentage, plus the permitted number of splices.
- [Rebar Cut Length Calculator](https://jianzhujisuan.com/en/rebar-cut-length): Cut length and bar weight with standard hook allowances.
- [Rebar Spacing and Bar Count Calculator](https://jianzhujisuan.com/en/rebar-spacing): Bars and actual spacing from run length and design spacing, with total weight.
- [Stirrup Length and Count Calculator](https://jianzhujisuan.com/en/rebar-stirrup): Stirrup length, count and weight with the 11.9d seismic hook allowance.

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