How this calculator works
Vertical spindles on stairs occupy a horizontal opening equal to the sloping rail length times the cosine of stair pitch. The minimum whole count is found from n spindle widths and n + 1 gaps. Equal layout spreads the gaps uniformly. Centred layout uses the requested interior gap when it fits and splits remaining space between the ends. Gap limits must come from the project requirements.
Formula
Pitch = atan(rise/run); horizontal opening = rail length x cos(pitch); n = max(0, ceil((opening - maximum gap)/(spindle width + maximum gap))); equal gap = (opening - n x width)/(n + 1); sloped gap = horizontal gap / cos(pitch).
Worked example
A 1 m horizontal opening with 40 mm spindles and a 100 mm maximum gap needs seven spindles. Equal gaps are 90 mm. Using 100 mm interior gaps leaves two 60 mm end gaps.
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Frequently asked questions
Do I measure stair openings along the rail?
Yes, enter the sloped inside rail length. Rise and run project it onto the horizontal plane for vertical spindle spacing.
Is the default gap a code ruling?
No. Enter the maximum clear opening that applies to your project. Stair geometry and guard rules vary by jurisdiction and assembly.
How do I mark spindle centres?
For equal layout, start at end gap plus half a spindle width. Each next centre is one spindle width plus the interior gap farther along horizontally.