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There are two methods that I have used for calculating how to evenly space out balusters along a railing. I summarize them below.

Method One for Calculating Even Baluster Spacing

First, determine the on-center spacing for your balusters. Many local building codes require a maximum on-center baluster spacing of 4 inches.

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 on-center spacings of 4 inches (assuming you want an on-center 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 on-center spacing for your balusters. As previously stated, 4 inches on-center 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 on-center 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 on-center 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, on-center, 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).