Next, find the center point on
the railing and make a pencil mark. This will be the location of your center
baluster. The baluster should be installed so that it is centered directly
under this pencil mark.
Now, continue to make pencil
marks on either side of the railing center point mark, at oncenter spacings
of 4 inches (assuming you want an oncenter baluster spacing of 4 inches).
At each mark install a baluster such that it is centered directly under the
mark.
This approach will provide even
spacing of the balusters along the length of the railing.

The only limitation with this
approach is that at the far ends of the railing the final baluster spacing
between the newel post and wall will most likely be less than 4 inches.
This approach meets code, but might be aesthetically unappealing to some.
Method Two
for Calculating Even Baluster Spacing
The second method is a little
more complicated to explain, however the solution provides even spacing of
balusters throughout the entire length of the railing. 
Again, first determine the
oncenter spacing for your balusters. As previously stated, 4 inches
oncenter spacing is a common building code requirement.
Next measure the length of
the railing and add the width of one baluster to it. Make a note of this
measurement + baluster width. I will call it (RL).
Now add the oncenter spacing
requirement and the width of one baluster and make a note of this
calculation. I will call it (BW). 
Photo by Mark Donovan 
Then divide RL by BW and round
down to the next whole number. I will call this result (RB = Required
Balusters). So for example if we had a railing length of 101” and a baluster
width of 2”, RL=103”. If the baluster oncenter spacing requirement is 4”
and the baluster width = 2” then BW = 6”.
RB would then equal the
rounded down value of RL/BW = 103/6 = 17.166 = 17. Since RB=17, this
is the number of balusters required.
Next multiply the number of
balusters required (RB) by the baluster width, and then subtract this value
from the railing length, plus one baluster width. So for example, 103 
(17x2) = 69.
Now divide this result by BR+1,
e.g. 69 / (17+1) = 3.83 inches. This value (Baluster spacing = BS)
represents the required even, oncenter, baluster spacing.
For an odd number of balusters
locate the first one at the center of the railing. The baluster should be
centered directly underneath the center of the railing. For an even number
of required balusters, you will need to locate the first two balusters from
the center railing point half the baluster spacing (BS) amount. After the
first balusters have been installed you can continue to space them out from
each other by the BS calculation (in this example 3.83 inches).
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