Why size sets the spacing
The limit is on the clear opening, so the baluster width comes straight out of the space available for gaps. For a given baluster width w and a required maximum opening gmax, the widest center distance that still satisfies the limit is exactly w + gmax. That single line is the whole table:
The lookup table
A 4 in maximum clear opening, equal spacing everywhere, and a 10 ft (120 in) clear span between post inside faces. “Max centers” is the largest center-to-center distance that keeps the opening at or under 4 in — it is a ceiling, not a target.
| Baluster width | Max centers | Count (10 ft) | Clear opening | Center to center |
|---|---|---|---|---|
| 1/2 in round metal baluster | 4 1/2" | 26 | 3 15/16" | 4 7/16" |
| 1-1/4 in 1x2 nominal, thin turned | 5 1/4" | 23 | 3 3/4" | 5 1/16" |
| 1-1/2 in 2x2 nominal, most common | 5 1/2" | 22 | 3 3/4" | 5 5/16" |
| 1-3/4 in 1-3/4 square stock | 5 3/4" | 21 | 3 3/4" | 5 9/16" |
| 2-1/4 in 2x3 nominal, heavy turned | 6 1/4" | 19 | 3 13/16" | 6 1/8" |
| 3 in heavy square profile | 7" | 17 | 3 13/16" | 6 13/16" |
Read the last row carefully: a 3 in baluster on 7 in centers leaves exactly a 4 in opening, which is at the limit with no margin for a bowed rail or a wavy tread line. The planner will hold that margin for you if you enter a lower maximum, such as 3-3/4 in.
Nominal size is not the width that matters
Measure the baluster, do not read the invoice. Dressed lumber loses material: a nominal 2x2 is 1-1/2 in square, a nominal 1x2 is 3/4 in by 1-1/2 in, and a nominal 2x3 is 1-1/2 in by 2-1/2 in. For a rectangular profile, the width that governs the clear opening is the dimension measured along the run, which is not always the wider face.
| Nominal | Dressed | Typical use |
|---|---|---|
| 1x2 | 3/4 x 1-1/2 in | Light spindle, painted infill |
| 2x2 | 1-1/2 x 1-1/2 in | The standard wood baluster |
| 2x3 | 1-1/2 x 2-1/2 in | Heavier profile, short runs |
| 3x3 | 2-1/2 x 2-1/2 in | Newel posts, not balusters |
Turned, tapered and round balusters
A turned baluster is narrower at the belly than at its blocks, and a round metal baluster has no flat face at all. The opening is set by the widest part of the profile at the height a sphere would try to pass, which for a guard is the whole span between the rail and the tread. Use the widest point, then check that the narrow sections do not create a second, wider opening you have not measured.
For round balusters the arithmetic does not change: a vertical cylinder blocks exactly its diameter, so a 1/2 in round metal baluster behaves like a 1/2 in wide square one for spacing purposes. Only the drawing looks different, which is why the planner treats shape as a visual setting.
“Spindle spacing calculator” is the same calculation
If you searched for a stair spindle spacing calculator, you are in the right place. Spindle, picket and baluster name the same vertical infill piece, and the spacing math is identical because all three are governed by their width and the clear opening. Only the profile changes.
Related
- How many balusters do I need — counts for spans from 4 to 16 ft at three common widths.
- The baluster spacing formula — where n + 1 openings come from.
- The 4-inch sphere rule — what the limit actually measures, and when it is not 4 in.