Consult building regulations to determine if there is a maximum spacing allowed for stair spindles. Your local building department or home supply retailer will be able to offer you guidance with these regulations.
Measure the horizontal distance between the base of two consecutive support posts -- that is, the two end posts that border the area where the spindles will be installed.
Measure the width of your spindles.
Add the width of your spindles to the spacing width you prefer. The spacing width might be the regulation maximum or an amount chosen for aesthetics. Divide this into the distance between posts to calculate the number of spindles required. As an example, if you had a distance of 120 inches with spacing of 4 inches and spindle width of 2 inches, you would divide 120 by 6 to calculate 20 spindles.
Multiply the spindle width by the number of spindles. Subtract this figure from the distance between posts. This will give you the cumulative amount of spacing. In the example, you would multiply 2 inches per spindle times 20 spindles to calculate 40 inches. You would then subtract that from 120 to give you 80 inches of total spacing.
Add one to the number of spindles to calculate the number of spaces required. Divide this figure into the total amount of spacing to calculate horizontal spacing between spindles or posts. In the example, 80 inches of spacing divided by 21 gives you a horizontal spacing of 3.81 inches.