The count formula
A run of n balusters creates n + 1 clear openings: one before the first baluster, one after the last, and n − 1 between them. Those openings are what the code limit applies to, so they are what decides the count.
Round n up, never down: rounding down widens every opening. With a 120 in clear span, 1-1/2 in balusters and a 4 in maximum opening, that gives 22 balusters, a 3 3/4" opening and 5 5/16" centers.
Counts for common railing spans
Counts are for a clear span between post inside faces, equal spacing everywhere, and a 4 in maximum clear opening. The number in parentheses is the clear opening you would actually get, rounded down.
| Clear span | 1-1/4 in | 1-1/2 in | 2-1/4 in |
|---|---|---|---|
| 4 ft (48 in) | 9 (3 5/8") | 8 (4") | 8 (3 5/16") |
| 5 ft (60 in) | 11 (3 13/16") | 11 (3 5/8") | 9 (3 15/16") |
| 6 ft (72 in) | 13 (3 15/16") | 13 (3 3/4") | 11 (3 15/16") |
| 8 ft (96 in) | 18 (3 13/16") | 17 (3 7/8") | 15 (3 7/8") |
| 10 ft (120 in) | 23 (3 3/4") | 22 (3 3/4") | 19 (3 13/16") |
| 12 ft (144 in) | 27 (3 15/16") | 26 (3 7/8") | 23 (3 13/16") |
| 14 ft (168 in) | 32 (3 7/8") | 30 (3 15/16") | 27 (3 13/16") |
| 16 ft (192 in) | 36 (3 15/16") | 35 (3 7/8") | 31 (3 13/16") |
Widths: 1-1/4 in (thin turned or metal spindle), 1-1/2 in (dressed 2x2 wood, common iron), 2-1/4 in (heavy turned or 2x3 stock). A different opening limit changes the count — the calculator will do the same for your number.
Why the same span can need one more baluster
The count jumps whenever the span crosses a threshold, and the thresholds are w + gmax apart — 5-1/2 in for a 1-1/2 in baluster at a 4 in opening. That is why a 98 in railing and a 102 in railing both want 18 balusters, while a 104 in railing wants 19. Check the number instead of assuming a spacing “looks right”.
The two places people under-order
1. Forgetting the end gaps
A layout that divides the span into equal gaps between balusters but forgets the two end gaps gets one or two balusters too many or too few. The span must hold the balusters and every opening including the ends.
2. Counting the rail instead of the span
On a stair, the sloped rail is longer than the horizontal span by 1 / cos(slope) — about 12% on a 26° stair. Dividing the rail length by a spacing gives you a count that is roughly one baluster too high per flight, and openings that are all slightly narrow.
How many spares to buy
Buy one extra for a cut that goes wrong mid-run, and a second if the balusters are turned or profiled and you want a consistent face pattern. For long runs over about 20 pieces, add 5% and round up: the cost is trivial next to a second order that does not match the first.
If your railing is split by a post, a landing or a direction change, calculate each section separately. Each section needs its own end gaps, so two 6 ft sections never equal one 12 ft section.
Related
- Baluster spacing by baluster size — the maximum center distance for every common width.
- The baluster spacing formula — the variables, assumptions and worked examples behind the count.
- The 4-inch sphere rule — where the opening limit comes from and what it does not cover.