Baluster Calculator
Even spacing, code-compliant gaps.
Balusters need to be spaced evenly between posts, with gaps small enough to meet building code. Get it wrong and you're either over a 4-inch sphere, ripping it out, or eyeballing it and hoping the inspector doesn't bring a level.
This calculator takes the guesswork out. Enter the inside-to-inside distance between your posts, the width of your baluster stock, and your target maximum gap. It returns the exact number of balusters to use, the actual gap dimension, and the on-center spacing for layout.
It also checks your layout against the standard IRC 4-inch sphere rule and warns you if the gap exceeds the limit. If the math produces gaps too tight to fit a tool through, it flags that too.
Common questions
How do you calculate baluster spacing?
Start with the total inside-to-inside distance between your posts. Subtract the combined width of all your balusters. The remaining space is divided among the gaps. The number of gaps is always one more than the number of balusters, since there's a gap on each side and between every baluster.
This calculator does all of that for you. You give it the distance, the baluster width, and your maximum allowed gap. It returns the smallest number of balusters that keeps every gap at or below your limit.
What is the 4-inch sphere rule?
The International Residential Code (IRC) requires that guards on stairs, decks, balconies, and landings be built so a 4-inch sphere cannot pass through any opening. That includes the gaps between balusters. In practice, this means your gap dimension must be less than 4 inches.
Your local code may have different requirements or stricter limits. Always check with your local building department before final layout. This calculator uses 4 inches as the default reference, but the value is adjustable.
What's the difference between gap and on-center spacing?
Gap is the clear space between two adjacent balusters. On-center spacing is the distance from the centerline of one baluster to the centerline of the next. On-center spacing always equals the gap plus one baluster width.
Most carpenters lay out from centerlines because it's easier to mark and easier to verify. The calculator gives you both numbers so you can use whichever measurement fits your workflow.
Why does it warn about impractical layouts?
If the math produces gaps narrower than about half an inch, you physically can't fit most tools, fingers, or hardware between the balusters. The math is correct but the layout is unworkable. The warning tells you to either widen the gap target, use narrower balusters, or use fewer of them.
Does it work for stair railings too?
Yes. The horizontal distance between posts on a stair railing follows the same math as a level railing. Just measure the horizontal inside-to-inside distance between your stair posts (not the diagonal along the rail) and enter it like any other run.
The IRC sphere rule for stairs is slightly different in some jurisdictions, often allowing a 4 3/8-inch sphere instead of 4 inches. Check your local code.
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