How the Balustrade Calculator Works

Enter length of your balustrade run, width of your spindles and number of newel posts. The calculator subtracts posts to find effective length, then works out the minimum number of spindles that keeps every gap at 99mm or less.

The maths behind it: minimum spindles = (effective length - 99) ÷ (spindle width + 99), rounded up. The number of gaps is always one more than number of spindles because there is a gap at each end as well as between every pair. The calculator then divides remaining space evenly so the finished run looks balanced with equal gaps at start and end.

The 100mm Sphere Rule Explained

Approved Document K of Building Regulations requires that a 100mm sphere cannot pass through any opening in guarding where children under five may use the building. The rule exists to stop a young child slipping through or trapping their head and building control officers genuinely carry a 100mm ball to test it.

Designing to a 99mm maximum gap is standard good practice and gives a safety margin. The rule applies to every opening: between spindles, between glass panels, under the base rail and between the bottom rail and a deck surface.

One detail that catches people out is turned or decorative spindles. The gap must pass sphere test at the widest opening, which on a turned spindle is at its narrowest point. Always measure decorative spindles at their thinnest section, not the square ends.

Balustrade Height Requirements in UK

Height requirements sit alongside spacing rule:

• Internal domestic stairs and landings: minimum 900mm from pitch line or floor
• External domestic balconies, decks and raised areas: minimum 1100mm
• Commercial and public buildings: typically 1100mm at edges with stair handrails between 900mm and 1000mm

Guarding is required wherever people could fall 600mm or more in a domestic setting. A low deck a couple of steps off the garden may not legally need a balustrade, though many people fit one anyway.

Raked Balustrades on Staircases

On a staircase balustrade follows pitch of stairs, usually between 38 and 42 degrees. The 99mm maximum still applies because spindles sit vertically, so opening between them is narrowest horizontally and that is the direction a sphere would pass through. The rake changes your centre to centre setting out along handrail but never relaxes gap limit. Measure horizontal going between newel posts rather than the sloped rail length when calculating spindle numbers for a flight.

Getting the Structure Right

Spindles fill gaps but newel posts do structural work. Posts on a deck should be bolted through to joists with coach bolts rather than fixed to deck boards, because a surface fixing has nowhere near the lateral strength to stop someone falling against the rail. On long landing runs allow a newel post roughly every 2.4 metres for rigidity.

Balustrades must also resist the horizontal loads set out in BS 6180, which is standard Approved Document K points to for barrier design. For a standard domestic job traditional build up of newels, handrail, base rail and spindles handles this comfortably when fixed properly.

Worked Example

A 2400mm landing run with 41mm spindles between two 90mm newel posts:

• Effective length: 2400 - 180 = 2220mm
• Minimum spindles: (2220 - 99) ÷ (41 + 99) = 15.2, rounded up to 16
• Gaps: 17
• Actual gap: (2220 - 16 x 41) ÷ 17 = 92mm

Every gap comes in at 92mm, comfortably inside the limit and evenly spaced for a clean professional finish.