What is the maximum gap between railing infill bars?

Under Building Regulations Part K (Protection from falling, collision and impact), the maximum gap between infill bars must not exceed 100mm. 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.

What height should metal railings be?

For domestic properties, railings protecting a drop of 600mm or more must be at least 900mm high on stairs and 1100mm on landings, balconies, and flat areas. For commercial buildings, the minimum is typically 1100mm throughout. Always check Building Regulations Part K for your specific application.

What is the recommended post spacing for metal railings?

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.

Should I use base plates or cast-in sockets for railing posts?

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.

How many bolts per base plate for metal railing posts?

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).

TradeCalculator

Metal Railing Calculator

Calculate posts, handrail lengths, infill bars and fixings for metal railings. Compliant with Building Regulations Part K (100mm max gap).

Total Railing Length

Total run of railing required

Railing Height

Min 1100mm for balconies (Part K)

Post Spacing (centres)

Typically 1.2-1.5m centres

Railing Style

Vertical bar is most common for Part K

Post Fixing Method

Base plate for retrofit, cast-in for new builds

Price Per Metre of Railing (optional)

Your supplier's price per metre

Price Per Post (optional)

Your supplier's price per post

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 ÷ (Bar diameter + Max gap)) - 1
Total infill bars = Bars per bay × Number of bays

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 at 100mm gaps, each 1.5m bay needs ~12 infill bars, totalling 84 bars.

Part K compliance

Maximum 100mm gap between any infill bars (100mm sphere test)
Minimum 1100mm height for balconies and level areas
Railings must withstand horizontal loading of 0.36 kN/m for domestic use
No climbable features below 600mm height on the inner face

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.