Stair balusters and straight-run balusters are two different jobs with two different rules. On a 10 inch stair tread, the spacing between the three balusters never changes, only where the group starts on the tread does. On a straight run between posts, the spacing itself has to be worked out for that exact span, and that is the part most guides shortchange. Pick the mode you are working below. Both are checked against the 4 inch sphere rule, and neither one replaces checking your own layout on site before you drill.
Baluster Spacing Calculator
Three balusters per tread, spaced 3-3/8 inch center to center. That spacing is fixed and does not change. What can change is where the group of three starts on the tread, since stair caps or a bigger nosing can push the whole group in or out.
Your marks
The riser sits at the left edge of the strip above. The three gold marks are your baluster centers, measured out from that riser face.
This is the harder half of the job, because the right spacing depends on the exact span between your posts, not a number you can memorize. Enter the clear opening and this works out how many balusters, how they space out, and whether that spacing passes the 4 inch sphere rule.
Your numbers
Balusters by style
Every gold mark lands at the same spacing, post to post. That is what "symmetrical" means here, the run reads the same from either end.
How we build these: on stairs, three balusters land on every tread at the fixed 3-3/8 inch spacing above, the whole group just slides in or out from the riser when a stair cap or a bigger nosing calls for it. On a straight run, we aim for about 3-1/2 inch center to center on purpose, tighter than the 4-1/2 inch sphere-rule ceiling allows, so a straight section next to a staircase does not read looser than the rake beside it. Whatever the math gives you, lay it out on the actual posts or treads and check the gaps with a real 4 inch ball before you drill. The full method, post layout, and how a rake run and a straight run tie together at a landing are in How to Build a Handrail for Metal Balusters Like a Pro.
This calculator covers straight-run and 10 inch stair-rake spacing for balusters. For rail height, the post connection, and how a rake run and a straight run tie together at a landing, read How to Build a Handrail for Metal Balusters Like a Pro start to finish before you drill anything.