In most communities there is a four inch rule for baluster spacing. What this means is that a four inch ball should not be able to pass between the balusters of your stairway or balcony. The four inch rule will dictate whether you can get by with two balusters per tread or three.

Note: when measuring the depth of the tread DO NOT include the nosing of the tread. I call this the “run” of the tread. So if you have a tread that is 12″ overall, and the nosing is 1 1/4″ the “run” would be 10 3/4″ (12″ – 1 1/4″ =10 3/4″).

So the first measurement you should take is the “run” of your tread and divide by 2.
Secondly, find the most narrow part of the balusters you will use and add that to 4″

So, for example, if your run is 10 1/2″, then 10 1/2″ divided by 2 = 5 1/4″
And the narrow part of you baluster is 3/4″ – then 3/4″ + 4″ is 4 3/4″.
Since 5 1/4″ is more than 4 3/4″ you will have to use three balusters per tread.
The spacing of 3 balusters per tread on a 10 1/2″ tread run is 3 1/2″ (10 1/2″ divided by 3 = 3 1/2″).

There is an exception to this method: If the small diameter of the baluster is immediately adjacent to a larger diameter then you can use the larger of the two diameters. Remember the rule is not being able to pass a 4 inch ball between the balusters. In some cases the four inch ball will touch the larger of the two diameters without contacting the small diameter because of their proximity to each other. In this case use the larger diameter and add this to 4″ to obtain your measurement.

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