Baluster count = (space between posts − maximum gap) ÷ (baluster width + maximum gap), rounded up. Between two posts 6 ft (72 in) apart, 1½ in balusters and a 3¾ in maximum gap take 13 balusters, every gap comes out 3¾ in and they sit 5¼ in center to center. The same formula covers balusters, spindles, balusters on a balustrade and the pickets of a railing: only the word changes.
How baluster spacing is calculated
Start from the widest gap you will allow. Code says a sphere must not pass through the guard (4 in in the IRC), so the gap has to be under the sphere size, and 3¾ in leaves room for a slightly out-of-plumb baluster. Each baluster plus one gap uses up (width + gap) of the rail, and there is one more gap than there are balusters, because the last gap runs to the second post. That gives the count above. The calculator then spreads the balusters so all the gaps are the same, including the two against the posts.
Count = ⌈(space − max gap) ÷ (width + max gap)⌉. Equal gap = (space − count × width) ÷ (count + 1). Center to center = gap + width.
Worked example for the default: (72 − 3.75) ÷ (1.5 + 3.75) = 13.0, so 13 balusters. Gap = (72 − 13 × 1.5) ÷ 14 = 3.75 in. The first baluster starts 3¾ in from the post face and the last one ends 3¾ in before the second post. In metric, 1.83 m between posts with 38 mm balusters and a 95 mm limit gives 14 balusters and 87 mm gaps.
Baluster spacing for common rail lengths
Space between posts, 2×2 balusters (1½ in), largest gap 3¾ in:
| Space between posts | Balusters | Equal gap | Center to center |
|---|---|---|---|
| 3 ft | 7 | 3 ³⁄₁₆ in | 4 ¹¹⁄₁₆ in |
| 4 ft | 9 | 3 ⁷⁄₁₆ in | 4 ¹⁵⁄₁₆ in |
| 5 ft | 11 | 3 ⅝ in | 5 ⅛ in |
| 6 ft | 13 | 3 ¾ in | 5 ¼ in |
| 8 ft | 18 | 3 ⅝ in | 5 ⅛ in |
| 10 ft | 23 | 3 ⁹⁄₁₆ in | 5 ¹⁄₁₆ in |
Balusters on a stair rail (rake rail)
On a stair the rail climbs but the balusters stay plumb, so the gap that counts is the horizontal one. Enter the horizontal run between posts and the rise, and the count works out exactly as on a level rail. Ten feet of run with a 3 ft rise takes 23 balusters with 3 ⁹⁄₁₆ in gaps. The rail itself is 10 ft 5 ⁵⁄₁₆ in long on the slope at 16.7°, and the center to center distance measured along it is 5 ⁵⁄₁₆ in, a little more than the 5 ¹⁄₁₆ in horizontal one. Many builders set two balusters to each tread so the pattern repeats: with 1½ in balusters and 10 in treads that gives 3½ in gaps. Set the tread depth in the calculator to check your own stair. The stair calculator gives the rise and run for the flight, and the stair stringer calculator the cuts that hold the posts.
Baluster spacing by country: 4 in, 100 mm and 125 mm
The IRC (R312.1.3) says a required guard must not let a 4 in sphere pass. The British Columbia Building Code, which follows the national code, sets 100 mm in Article 9.8.8.5, and New Zealand's clause F4 also uses a 100 mm sphere. Australia's NCC allows a 125 mm sphere (Housing Provisions 11.3.4, as summarised by the source we used). Pick the rulebook in the calculator and it sets the largest gap a small margin under the sphere. Height differs too: 36 in for a deck guard in the IRC, 34 in on stairs measured from the nosings, 900 mm or 1070 mm in British Columbia depending on the drop, and 1 m in New Zealand and Australia. Local amendments and your inspector win over all of these numbers.
Marking list and cutting the balusters
The table under the result gives each baluster's position from the face of the first post: the near edge, and the center if you mark with a story stick. Cut a spacer block to the equal gap and you can also just butt each baluster against the block. For stock, 2×2 balusters come from 2×2 lumber, which dresses to 1½ in square (NIST PS 20). Balusters run between the rails, so their length is the guard height less the rails and the gap under the bottom rail. The deck railing calculator counts the whole railing: posts, rails, balusters and hardware. Building the deck itself? Start with the deck calculator and the deck board calculator, and price the job with the deck cost calculator. More jobs for decks and stairs are on the decks and stairs hub.
Spindle spacing works exactly like baluster spacing: the spindles are the balusters, and the gap between them must stay under the code limit.