How many balusters fit between two posts?
Measure the clear horizontal span between the inside faces of posts. Enter baluster face width and the permitted maximum clear gap. The calculator finds a whole baluster count and distributes the remaining space equally across the end and intermediate gaps.
Which railing dimensions matter for spacing?
The clear run excludes the posts themselves. Width describes the horizontal face projection of a vertical baluster. On stair rails, enter the horizontal run and rise for that same section. These dimensions describe spacing, not guard height or structural strength.
| Input | Units |
|---|---|
| Clear horizontal run between posts | ft / m |
| Baluster face width projected horizontally | in / cm |
| Maximum permitted clear horizontal gap | in / cm |
| Rise over this run | ft / m |
| Identical railing sections (optional) | as labeled |
| Spare baluster allowance (%) (optional) | as labeled |
| Price per baluster (optional) | as labeled |
What formula gives the minimum baluster count?
For N balusters, there are N + 1 clear gaps. Solving (run − N × width)/(N + 1) ≤ maximum gap gives N ≥ (run − maximum gap)/(width + maximum gap). Round that threshold up to the smallest whole count.
Minimum balusters = ceil((clear run − maximum gap)/(baluster width + maximum gap)); equal clear gap = (run − count × width)/(count + 1)
Equal clear gaps
How does a ten-foot railing example work?
A 120 in clear span with 1.5 in balusters and an entered 4 in maximum gap requires 22 balusters. The equal clear gap is (120 − 22 × 1.5)/23, or about 3.783 in. The chosen 4 in value is an example, not a universal code rule.
Where should you mark the first baluster center?
From the inside post face, mark the equal clear gap plus half the baluster width. Following centers are one clear gap plus one baluster width apart. The result provides these distances so you can check the layout before drilling or fixing.
How do stair rails change center distances?
Vertical baluster spacing is based on horizontal projection. The rail slope factor is √(run² + rise²)/run. Multiplying the horizontal center spacing by this factor gives distance measured along the sloped rail; angled and nonvertical systems need their own layout method.
Does this check the building code for your railing?
It checks only the maximum gap that you enter. Applicable guard opening limits, stair triangular openings, height, load capacity and product instructions must be verified separately. The calculator does not select a jurisdiction or certify an installation.
How are repeated sections and spare balusters counted?
Identical section count repeats the installed quantity. An optional spare percentage rounds the overall order up, without changing installed spacing. Sections with different spans or widths need separate calculations; they should not be represented as identical.
How can you compare baluster widths or layouts?
Save one calculation, then change width or the permitted gap and compare the installed count and equal gaps. A wider baluster can reduce the gap space and alter the count. Keep the code requirement and clear span fixed when comparing products.
Which tool helps with the rest of the stairs?
Use the Stair Calculator for rise and going relationships. Railing spacing is a separate check from stair dimensions, stringer geometry and guard design.
