# Formwork Prop Calculator

Prop count and per-prop load check - see at once if the grid is dense enough.

- Page: https://jianzhujisuan.com/en/formwork-prop
- Category: Temporary Works (https://jianzhujisuan.com/en/c/temporary-works)
- Chinese version: https://jianzhujisuan.com/zh/formwork-prop.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Slab length 10 m, Slab width 8 m, Longitudinal spacing 0.9 m, Transverse spacing 0.9 m, Design load 7 kN/m², Prop capacity 25 kN): **Number of props 108 bars**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Slab length | number | 10 | m |  |
| Slab width | number | 8 | m |  |
| Longitudinal spacing | number | 0.9 | m |  |
| Transverse spacing | number | 0.9 | m |  |
| Design load | number | 7 | kN/m² | Concrete + formwork + live load, 6-8 for a typical slab |
| Prop capacity | number | 25 | kN | Cup-lock/ring-lock props 25-40 kN, per maker |

## Formula

- **Prop count**: n = (⌊L÷sl⌋+1) × (⌊W÷sw⌋+1)
- **Tributary area**: A₁ = area ÷ count
- **Load per prop**: N = A₁ × design load
- **Check**: N ≤ prop capacity

## Worked example

A 10×8 m slab on a 0.9×0.9 m grid, 7 kN/m², 25 kN props: 12×9 = 108 props, 80/108 = 0.74 m² each, 5.2 kN per prop, 21% utilisation - OK.

## Questions

### What prop spacing is right?

0.9-1.2 m for a normal slab, 0.6-0.8 m for thick or heavily loaded slabs. Spacing follows from 'load per prop ≤ capacity', and wider is not cheaper - it raises the per-prop load and forces heavier props or tightening anyway. Under beams, at columns and mid-span the load is higher and needs local tightening, not one uniform grid.

### Why not strike the formwork too early?

Early-age concrete is weak; striking too soon cracks or collapses the slab. Keep the bottom form until the concrete reaches the required strength: 50% for spans up to 2 m, 75% for 2-8 m, 100% beyond 8 m. Strike from mid-span outward, remove non-load-bearing parts first, and leave temporary props - never drop the whole support at once.

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