Spindle Spacing Calculator — Baluster Gap for Decks & Stairs
Find the exact gap between equally spaced balusters (spindles) in a deck or stair railing. Enter the total railing run, spindle width, and number of spindles — the calculator spaces them evenly and checks the result against the US IRC residential 4-inch limit.
in
in
Exceeds 4 in limit — add more spindles
- 1
Total spindle width
14 × 1.5 = 21 - 2
Total gap space
120 − 21 = 99 - 3
Gap between spindles
99 ÷ (14 + 1) = 6.600Divided by n + 1: one gap before the first spindle, one between each pair, one after the last.
How does this calculator work?
gap = (total run − n × spindle width) / (n + 1). For a 120 in run with 1.5 in balusters: 14 spindles → gap = (120 − 21) / 15 = 6.6 in (fails ≤ 4 in code); 24 spindles → (120 − 36) / 25 = 3.36 in (passes). Increase the count until gap ≤ 4 in.
Formula
How this is calculated
Equal baluster spacing distributes n spindles across a run so that the gap between adjacent spindles — and the end gaps — are identical. With total run W and each spindle of width sw, there are n + 1 gaps (one before the first spindle, one between each pair, and one after the last), so gap = (W − n × sw) / (n + 1).
US residential building code (IRC R312.1.3) requires that openings in guards cannot allow a 4-inch sphere to pass through, meaning the clear gap must be ≤ 4 inches (101.6 mm). Increase the spindle count until the gap is at or below this limit. Commercial codes and some local jurisdictions may be stricter — always verify with your local authority before construction.
The inputs use inches. For metric work, enter all measurements in the same unit (e.g. centimetres) and compare the result against your local code limit (often 100 mm). The range meter shows where the computed gap falls relative to the 4-inch code threshold.
Frequently asked questions
IRC R312.1.3 (2018 edition) requires that openings in guards cannot allow the passage of a 4-inch sphere, so the clear gap between balusters must be ≤ 4 inches (101.6 mm). Some jurisdictions adopt stricter local amendments — check with your local building department.
This calculator assumes all n + 1 gaps are equal, including the gap at each end between the post and the nearest spindle. If your design uses a half-gap at each end (the classic "centre the layout" approach), the formula changes slightly — see a framing reference for that variant.
Yes — the formula is identical for any equally spaced linear elements. Enter total run, element width, and count. The code-check reference is IRC-specific to guard balusters, but the spacing result applies universally.
Also known as
TG we-Calculate Editorial Team. (2026). Spindle Spacing Calculator — Baluster Gap for Decks & Stairs [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/spindle-spacing-calculator
TG we-Calculate Editorial Team. "Spindle Spacing Calculator — Baluster Gap for Decks & Stairs." TG we-Calculate. 2026. https://we-calculate.com/calculator/spindle-spacing-calculator.
TG we-Calculate Editorial Team, "Spindle Spacing Calculator — Baluster Gap for Decks & Stairs," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/spindle-spacing-calculator
@misc{wecalculate_spindle_spacing_calculator, title = {Spindle Spacing Calculator — Baluster Gap for Decks & Stairs}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/spindle-spacing-calculator}}, year = {2026}, note = {TG we-Calculate} }
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