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Metal Railing Calculator
Calculate posts, handrail lengths, infill bars and fixings for metal railings. Compliant with Building Regulations Part K (100mm max gap).
Total run of railing required
Typically 1.2-1.5m centres
Your supplier's price per metre
Your supplier's price per post
Standard: 12mm or 16mm round bar
Part K max: 100mm
Standard: 4 per base plate
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How We Calculate This
This calculator determines the number of posts, handrail length, infill bars and fixings needed for your metal railing installation. It ensures compliance with Building Regulations Part K for maximum gap spacing.
The formula
Posts = (Total length ÷ Post spacing) + 1 (rounded up)
Bars per bay = (Bay width − Max gap) ÷ (Bar diameter + Max gap), rounded up
Total infill bars = Bars per bay × Number of bays
The bar-count formula accounts for the fact that n bars in a bay create n+1clear gaps — including the two end gaps between the outer bars and the posts. Sizing only for the in-between gaps (the older “pitch − 1” rule) leaves those end gaps wider than intended and can let a 100mm sphere through, so we size for all gaps to keep every opening below the Part K limit.
For example, a 10m railing at 1.5m post centres: 10 ÷ 1.5 = 6.67, rounded up to 7 bays needing 8 posts. With 12mm bars and a 99mm maximum gap, each 1.5m bay needs 13 infill bars (giving a 96mm clear gap, comfortably inside the 100mm limit), totalling 91 bars.
Part K compliance
- Maximum 100mm gap between any infill bars (100mm sphere test)
- Minimum 900mm for domestic internal areas, 1100mm for external balconies
- Railings must withstand horizontal loading of 0.36 kN/m for domestic use
- No climbable features below 600mm height on the inner face
Bar counts assume every bay is the full post-spacing width and use the post centre spacing as the bay width. Where a run isn’t an exact multiple of the spacing the final bay is narrower, so the figures err slightly on the high side — a safe over-estimate that never leaves a gap wider than intended. Always confirm the exact bar spacing on site against the 100mm sphere test before fabrication.
Frequently Asked Questions
Under Building Regulations Part K (Protection from falling, collision and impact), the maximum gap between infill bars must be less than 100mm (in practice, 99mm maximum clear gap). This prevents small children from passing through or getting trapped. The 100mm sphere test is applied — a 100mm sphere should not be able to pass through any opening in the railing.
For domestic properties, railings protecting a drop of 600mm or more must be at least 900mm high on stairs, internal landings and edges of internal floors. On external balconies and roof edges, the minimum is 1100mm. For commercial buildings, the minimum is typically 1100mm throughout. Always check Building Regulations Part K for your specific application.
Standard post spacing for metal railings is between 1.2m and 1.5m centres, depending on the section size and height of the railing. Wider spacing may require heavier sections to maintain rigidity. For exposed or high-wind locations, closer spacing of 1.2m is recommended.
Base plates (bolt-down) are the most common fixing for metal railings as they allow adjustment and can be fitted to existing concrete or masonry. Cast-in sockets are stronger and give a cleaner finish but must be installed during the concrete pour. Core-drilled fixings suit retrofit installations into existing hard surfaces.
Standard base plates use 4 bolts (M10 or M12 expansion anchors) for domestic railings. Heavy-duty or commercial installations may require 6 bolts. The bolts should be stainless steel or hot-dip galvanised to prevent corrosion. Always use mechanical anchors rated for the substrate (concrete, stone, or steel).
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.