How the count is worked out
Spindles sit between two posts, so n spindles make n plus one gaps. The run is the spindle widths plus the gaps: run = n times the width, plus n plus one times the gap.
Setting the gap to the maximum allowed and solving for n gives the smallest number of spindles that keeps every gap legal. Rounding up is what guarantees it.
The gap you actually get is then the leftover space divided by the number of gaps, which is why it comes out a little under the limit rather than exactly on it.
Worked example
A 96 inch run with 1.5 inch spindles needs 17 balusters, which leaves 18 gaps of 3.917 inches each. Spacing centre to centre is 5.417 inches.
Adding one more spindle shortens every gap slightly, which is usually what you want if the run is fixed and the cut is awkward.
Why the gap is capped, not the spacing
The IRC requires a guard of at least 36 inches, and the gaps between balusters must not let a 4 inch sphere pass. The sphere is about the size of a small child's head, which is what the rule is for.
Spacing centre to centre is not the regulated figure, because a wide spindle with a legal centre spacing can still leave a gap that a child fits through. Check the clear gap, not the centres.
A decorative or turned spindle is measured at its narrowest point, not its widest, because that is where the gap is.
Limits of this calculator
It calculates spacing on a straight, level run. A raked balustrade on a stair has the same gap rule but the spindles are cut at an angle, and a curved run needs each bay calculated separately.
It does not check the guard height against the drop, the fixing of the posts, or whether the balustrade has to resist a loading. Those are separate requirements.
It assumes the posts are fixed and the run is the clear distance between them. Measure the clear run, not the overall length.
For the stair the balustrade sits on, use the US stair calculator.
Inputs for the US spindle calculator
Length between posts
Enter the clear run between posts. If the balustrade turns a corner, treat each straight run separately.
Spindle width
Face width of one spindle, 1.5 in for a standard square spindle.
Guard height
Height above the floor or nosing. The IRC minimum is 36 in.
CALCULATOR QUESTIONS
US spindle calculator questions
What is the maximum gap between spindles?
100 mm in the UK and 4 inches in the US, measured so that a sphere of that size cannot pass.
Is it the gap or the spacing?
The gap. Centre to centre spacing depends on the spindle width, so a wide spindle can be legal on centres and still leave a gap that fails.
How tall should the guard be?
900 mm on a UK stair and 1,100 mm on a landing, or 36 inches under the IRC.
Does this work for a stair?
The gap rule is the same, but the spindles are cut at an angle on a rake, so check the vertical gap after cutting.