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Formwork Prop Calculator

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

Quick answer

With the default inputs: Slab length 10 m · Slab width 8 m · Longitudinal spacing 0.9 m etc.Number of props 108 bars

Use your own dimensions ↓

Inputs

kN/m²
Concrete + formwork + live load, 6-8 for a typical slab
kN
Cup-lock/ring-lock props 25-40 kN, per maker

Result

Number of props
108bars
Longitudinal rows12bars
Transverse rows9bars
Tributary area per prop0.74m²
Load per prop5.2kN
Prop capacity25kN
Utilisation21%
CheckOK
How this is worked out

Props form a grid, count = (⌊length ÷ longitudinal⌋ + 1) × (⌊width ÷ transverse⌋ + 1) with a row at each edge. Load per prop = design load × tributary area, checked against capacity. This is the average split; under beams, at edges and near pour strips loads concentrate and need local tightening. The design load already includes concrete, formwork and live load - 6-8 kN/m² for a typical slab. Tall or heavy slabs need a dedicated scheme, not this rough check.

Next up: Formwork Turnover Calculator · Formwork Lateral Pressure Calculator

Formula

Prop countn = (⌊L÷sl⌋+1) × (⌊W÷sw⌋+1)
Tributary areaA₁ = area ÷ count
Load per propN = A₁ × design load
CheckN ≤ prop capacity
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.

How to work it out

  1. Props = ⌈slab length ÷ longitudinal spacing⌉ × ⌈slab width ÷ transverse spacing⌉; spacing is usually 0.9-1.2 m.
  2. Loaded area per prop = spacing × spacing; prop load = slab pressure × loaded area.
  3. Check prop load ≤ allowable capacity; if exceeded, tighten the grid.
  4. Horizontal ledgers run continuously at each lift with the prescribed storey height and step - never omit them.
  5. Vertical and horizontal bracing must be provided; this is the crux of overall stability.

Where it goes wrong

Material and cost

Falsework is priced by volume or developed area and rented by tonne·day. The extra material for tighter props and bracing is far less than the compensation and downtime of one collapse - safety margin is not where you save.

On site

Basis

FAQ

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.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Formwork Prop Calculator. 2026-10-25. https://jianzhujisuan.com/en/formwork-prop

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.