These are very common questions, and are often the starting point for those looking to buy spindles/balusters for their porch. Simply put, spindles are spaced according to a very easy rule:
3 1/2″ wide: 6″ on-center
2 1/2″ wide: 5″ on-center
1 3/8″ wide: 4″ on-center
3 1/2″ wide: 7 1/2″ on-center
2 1/2″ wide: 6 1/2″ on-center
1 3/8″ wide: 5 3/8″ on-center
And the number of spindles/balusters required is simply the rail length in inches divided by the on-center spacing.
How did we come up with this? For those that are curious about the logic behind these guidelines, read on…
Most building codes require that the gap between the spindles, at the narrowest portion, be no more than 4″. So spindle spacing and quantity depends upon the diameter of the spindle at it’s narrowest portion.
To get the spacing, take the gap required by code (4″) and add the diameter of the spindle at it’s smallest portion. That will give you the “on-center” measurement. You can then mark that spacing on your rail and put the spindles over the mark.
To come up with the quantity, take the length of rail (in inches) and divide by the on-center spacing. Here are some examples of Western Spindle’s more common spindle sizes:
4×4 Spindles: our best seller. They measure 3 1/2″, and the narrowest part of the turned portion is about 2″ in diameter. So 4″ code + 2″ small dia. = 6″. This means you should space your spindles every 6″ on-center. Quantity: If you have a 120″ rail (10′), divide 120″ by 6″, and you come up with 20 spindles needed to fill the space.
3×3 Spindles: 2 1/2″ wide, and 1 3/8″ at the narrowest portion. 4″ + 1 3/8″ = 5 3/8″. Theoretically you could space the spindles every 5 3/8″, but commonly they are spaced every 5″. This is for both simplicity and because it looks authentic. Quantity: Divide the rail length in inches by 5. Example: The same 120″ rail section would require 24 spindles (120 / 5 = 24)
2×2 Spindles: 1 3/8″ wide, and 5/8″ at the narrowest portion. 4″ + 5/8″ = 4 5/8″. Again, you could space them every 4 5/8″, but they are commonly spaced every 4″ on-center much for the same reasons. Quantity: Divide the rail length in inches by 4. Example: 120″ rail section would require 30 spindles.