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Railing Post and Gap Calculator

Post count plus clear-gap compliance check.

Quick answer

With the default inputs: Total length 30 · Post spacing 1.1 · Vertical bars per bay 9 pcs etc.Posts required 29 bars

Edit the inputs below to recalculate.
Spacing sRun L
Post pitch is back-calculated from the clear gap: not over 110 mm in housing, and thinner bars demand closer posts. Every corner needs a post.

Inputs

pcs

Result

Posts required
29bars
Bays28bays
Post spacing1.07m
Clear gap between bars94mm
Gap within 110 mmYes
Minimum height1050mm
Height adequateYes
Horizontal load to check1kN/m

Two hard requirements: clear gaps between vertical bars must not exceed 0.11 m (to stop a child passing through or climbing), and heights are at least 1.05 m for residential balconies, 1.10 m on public building edges and 1.20 m above 24 m - local rules may be stricter. The top rail is checked for about 1.0 kN/m horizontal load in housing and 1.5-2.0 kN/m in schools and malls, which requires a structural check of posts and anchors. Geometry only here.

Formula

Bays = ceil(length ÷ spacing); posts = bays + 1
Gap = (span − bar width × (bars+2)) ÷ (bars+1)end posts count as obstruction
Gap ≤ 110 mmchild safety
Height ≥ 1050 / 1100 / 1200 mmby location
30 m of railing with 1.1 m posts and 9 Φ12 bars per bay: 28 bays and 29 posts; clear gap = (1100 − 12×11) ÷ 10 = 96.8 mm, within the 110 mm limit; the 1050 mm residential requirement is met by 1100 mm.

How to work it out

  1. Check the height requirement by location first: guardrails are at least 1.05 m where the drop is under 24 m and 1.10 m at 24 m and above; places used by children also need a non-climbable detail.
  2. Posts = total length ÷ post spacing, rounded up plus one — both ends need a post.
  3. Infill per bay = bay length ÷ design clear gap, rounded up.
  4. Verify the clear gap: bay length ÷ infill count − bar diameter must not exceed 0.11 m. This is the acceptance item and a fail means rework.
  5. Check the horizontal load for the occupancy: 1.0 kN/m for dwellings and dormitories, 1.2 kN/m for schools and retail; posts and anchors must be verified against it.

Where it goes wrong

Material and cost

Stainless tube 200-400 CNY/m, powder-coated galvanised steel 120-250, laminated glass balustrade 400-800, solid timber 300-600. Installation 40-80 CNY/m, more for work at height.

On site

Basis

FAQ

Where does 110 mm come from?
Child anthropometrics: a toddler head is roughly 90-100 mm wide while the body passes through a smaller gap, so 110 mm is the threshold where the head cannot pass but the body still could. Below 110 mm a child cannot get their head through to become trapped or climb out. It is more critical than the height - many falls happen with adequate height but oversized gaps.
How do glass and cable railings work?
A continuous laminated glass balustrade has no gap issue but must satisfy glass strength and keep the gap to the slab under 50 mm. Cable railings must hold spacing below 110 mm, and since cables spread under load, start at 80-90 mm and re-tension periodically or the gap opens up. Both need proper anchorage design; the bar formula here does not apply.
Is 1.0 kN/m enough?
1.0 kN/m is the code minimum for housing and offices; schools, malls, theatres and stadiums need 1.5-2.0 kN/m because crowds push and loads accumulate. Anchorage matters more: railing collapses almost always come from failed base anchors - weak concrete, shallow embedment, insufficient edge distance - not from undersized posts.

Cite this page

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

Building Calculator. Railing Post and Gap Calculator. 2026-10-25. https://jianzhujisuan.com/en/railing-post

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.