📏 Spindle Spacing Calculator
Work out how many spindles (balusters) fit a railing run and the exact even gap between them, staying under your maximum allowed opening.
What this calculator does
The Spindle Spacing Calculator figures out how many spindles (balusters) fit evenly across a railing run and what the exact gap between them will be, so every opening stays at or under your maximum allowed gap.
The formula
The calculator assumes spindles are spaced evenly along the run with equal gaps at both ends: Rail length = (spindles × spindle width) + ((spindles + 1) × gap). Starting from one spindle, it adds spindles one at a time and recalculates the resulting gap, Gap = (Rail length − spindles × spindle width) / (spindles + 1), stopping as soon as the gap is at or under your maximum.
Many U.S. residential building codes require railing openings to not allow a 4-inch sphere to pass through, which is why 4 inches is the default maximum gap — always confirm the exact requirement with your local code.
- A smaller maximum gap or wider spindles both increase the number of spindles needed.
- This tool is for general planning purposes; verify the final layout and required gap against your local building code before installation.