How to calculate equal baluster spacing

Choose how the balusters meet the posts

Equal end gaps continue the spacing rhythm all the way to each post and often make a bay feel balanced. Balusters tight to the posts can create a more framed-in appearance or suit a particular edge detail, with the remaining spaces distributed only between balusters.

That choice changes the count and the first centre mark. Tight ends do not remove the need to check fastening access, brackets and any required material clearances. For either arrangement, choose an opening target that leaves room below the permitted maximum for real stock and installation variation.

Measure the clear opening

Measure between the finished inside faces of the posts or boundaries that contain the balusters. Do not use the outside-to-outside rail length. Check the opening at the actual baluster line because posts and rails are not always perfectly parallel.

Find a count that respects the maximum gap

For a chosen number of balusters, subtract their combined width from the clear opening. Divide what remains by the number of spaces, which is one more than the baluster count when equal end gaps are used.

Equal gap = (opening − baluster count × baluster width) ÷ (baluster count + 1)

Choose equal end gaps or balusters tight to posts

Equal end gaps leaves a gap at both posts and between every baluster. Balusters tight to posts places the two outside balusters against the post faces and distributes the gaps between balusters only. Enter the original clear opening; do not subtract the two end balusters yourself.

For a 72-inch level opening with 1.5-inch balusters and a 4-inch target, tight-to-posts mode uses 14 balusters and 13 gaps: (72 − 14 × 1.5) ÷ 13 = about 3.923 inches. The first centre is 0.75 inches from the post. Choose the end condition before transferring the marks.

Tight-to-posts gap = (opening − count × baluster width) ÷ (count − 1)
First centre = ½ baluster width

Mark centres for the selected end condition

Add the equal gap to one baluster width for centre-to-centre spacing. With equal end gaps, the first centre is one gap plus half a baluster width from the post. With tight-to-posts ends, it is half a baluster width. The calculator updates its numbered marks for the chosen condition.

On-centre spacing = equal gap + baluster width
First centre, equal ends = equal gap + ½ baluster width
First centre, tight ends = ½ baluster width

Worked level opening

For a 72-inch opening, 1.5-inch balusters and a 4-inch maximum, 13 balusters leave 14 gaps. Each gap is (72 − 13 × 1.5) ÷ 14 = 3.75 inches. Centres are 5.25 inches apart and the first centre is 4.5 inches from the post.

Choose a target below the permitted maximum to leave room for measuring, stock-width and installation variation. The calculator target is a user input, not a code determination.