Railing Post and Gap Calculator
Post count plus clear-gap compliance check.
Inputs
Result
Formula
| Bays = ceil(length ÷ spacing); posts = bays + 1 | |
| Gap = (span − bar width × (bars+2)) ÷ (bars+1) | end posts count as obstruction |
| Gap ≤ 110 mm | child safety |
| Height ≥ 1050 / 1100 / 1200 mm | by 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.
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.
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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.