Put all your fancy software away and use your head. A typical rise run ration is 7.5" rise and 11" run. Two at 1.5" = 3"
11'' - 3 = 8", so 2 per tread ought to do it, so just count the number of treads and multiply x 2 ought to be something you can keep under your hat and figure at a glance.

Then if the stairs are open ended treads, then balusters end on the tread, so start with one 2" each side of the center measured back from the nose of the tread.

Although I think 3 per tread is a better look if it's your choice.

Down load the BalusterPro app on your phone. I use it all the time for spacing balusters and anything that needs equal spacing. It will even call out the measurements to you as you mark them down.

Nicko

While I appreciate the different in sights, I really need someone to break the mathematics down for me without the parentheses.

I know how to divide something evenly, but I really want to know the mathematical equation behind finding out how many spindles and what the dimension in between each of them will be without gassing it on a whim and backtracking

I can’t believe I’m replying to this one but...

88 13/16 divided by 5 1/2”(which is your max space and spindle width) gives you 16.something, telling you you need 16 spindles.

16 times your spindle width of 1 1/2” is 24”, or the space they will occupy.

88 13/16” minus the 24” of spindles equals 64 13/16” of space between the spindles.

Divide the 64 13/16” by 17(spaces needed) gets you 3 13/16” of space between spindles!

Matt, that’s EXACTLY what I wanted to know! THANK YOU! (Ex) 88-13/16 / 5.5

That’s where I was stuck.

Well,,,, Yes, but you make no mention of where the top and bottom newels are on their respective treads so you may end up with some of your balusters being centered on the drop from the nose of one tread down to the next. I still would think about finding the center of the treads from nose to nose, and see how that works out at each end relative to the newels. Having a bit less than 4" on each end isn't a problem. The location of the newels at each end is a variable you have not shared with anyone, unless I missed that.

Before you start cutting and drilling, if that is part of it, I would start by measuring over level from the top of the bottom newel to a tread to see where they fall, relative to the ones lower down. Your really going to be pulling your hair out if you find those numbers have some balusters trying to be half over the nose of a tread due to creep because it doesn't match up to the tread length. Do you understand my concern?

I do Keith. Even tho I did it the long way math wise, it all worked out in the drawing acad. i just wanted to know a faster way of determining how many spindles, based upon the required spacing and Matt’s formula was exactly what I needed

Not so fast........ If you are using something like a 5015, with 1-1/4" square base and a 7/8" taper turned top, then the calculations need to be made at the top of the spindles, using 7/8" and 4". Not 1-1/4". That 3/8" can cause a lot of misery.

And, sidebar here. The spindles do not locate off the center of a tread. The front face of the spindle needs to be aligned with the face of the riser and the outside face of the spindle base needs to be aligned with the skirt board or bracket.

The photo is a good example of what NOT to do. That first spindle is pushed out to the start of the bullnose, and looks like it is out in space instead of bearing the load directly.

View higher quality, full size image (720 X 491)

David, the treads have nothing to do with this situation, as there is a board that rides on top of the stringer that the spindles nail to on the angle.

David has got that right. However it would have been nice to know you were using a closed end treads when you ask the question.

Sorry Keith. I tried to post a picture from my iPhone, but it says it is too big and I don’t know how to resize it to allow it to let me post it. I don’t know why they have a file restrictions set to such a small file size

I could be wrong , but I think David was saying if at some point the spindle is 7/8 diameter, you need 4 inches between the narrowest part of the spindle. So if you have 4 inches at the bottom you would have an opening of 4 3/4 at the narrowest part of the spindle, which would not meet code. A child could maybe fit their head through the wider spot?

You are correct Scott. That difference is real, and can cause real problems. Given the freedom of a curbed stair, spindle placement is more critical.

I mentioned the placement since it was incorrectly stated by an earlier responder.

Ok, as pointed out, the placement of the spindles is critical if your stairs are like the pic posted, e.g. placement on each step. But the OP does not state, what if the construction is similar to this pic (sorry my nomenclature on stair parts is deficient) in which they are evenly spaced regardless of each step?

Mike, in order to change the file size of your iPhone pic, google "how to change file size of iPhone pictures". That will list numerous articles and sites to show you.

Or, simply forward it to yourself (email) and edit it likety split in Preview/change size.