Baluster Spacing Calculator

Common Questions

How many balusters do I need for an 8-foot deck rail?

For a 96″ rail with 1.5″ balusters and a 4″ max gap (US code): 22 balusters with an exact ~3.66″ gap between each. Use the calculator below for any rail length.

What is the maximum gap between balusters by code?

In the US and Canada the IRC limits gaps to less than 4″ (101 mm) — small enough that a 4″ sphere cannot pass through. UK and most of Europe set the limit at 100 mm. The calculator defaults to these limits per unit system.

How do I calculate even baluster spacing?

Two-step formula: (1) count = ceil((rail length − max gap) / (baluster width + max gap)), (2) exact gap = (rail − count × width) / (count + 1). The +1 in the denominator accounts for the gap on either side of the run.

What is a standard baluster width?

Most common is 1-1/2″ (38 mm) — the actual width of a 2×2 baluster. Decorative metal balusters are often 1/2″ to 3/4″. Wider posts (5-1/2″, e.g., 4×4 newels) sit at the corners and are not counted as balusters in this calculator.

Do balusters go on the outside or inside of the posts?

Either works structurally — what matters for code is the gap between adjacent balusters being below the limit (typically 4″). Most modern decks mount balusters on the outside (face-mount) so the rail looks clean from the deck side. This calculator measures the rail length between posts; the mount style does not affect spacing.