Measure the clear opening between the inside faces of the posts, not the distance to post centres. Then enter the baluster width and your maximum desired clear gap.
The result uses a whole number of balusters and divides remaining space into equal end and intermediate gaps. A common 4-inch preset is only a convenience; verify local requirements.
Before you buy
- Measure each non-identical opening separately.
- Keep your selected product’s actual baluster width handy.
- Add a small user-chosen spare quantity if that suits your project.