The number of spindles you need depends on your:
Spindle width;
Inside railing distance; and
Maximum allowed spindle spacing.
Let's say you need to install 3-cm wide spindles between two posts 200 cm apart, and you want to space them 10 cm max.
Add the spindle width and max allowed spacing: 3 cm + 10 cm = 13 cm.
Divide the inside railing distance of 200 cm by 13 cm to get: 200 cm / 13 cm = 15.38.
Round down this result to a whole number. In this case 15 spindles.
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