Deck Railing & Baluster Calculator

Laying out balusters so the gaps come out even

Never start at one post and step across at a fixed spacing. You end up with an odd gap at the far post. Work out the clear distance between posts, then find the fewest balusters that keep every gap under your limit. With n balusters of width b in a clear space C, every gap is (C − n × b) ÷ (n + 1).

Mark the first baluster one gap in from the post, then mark each next one at gap + b on center. A spacer block cut to the gap width makes installation fast and consistent.

Guard rules worth knowing

• Height: 36″ minimum on decks under the IRC, measured from the decking to the top of the rail. Some jurisdictions, and commercial buildings, require 42″.
• Openings: a 4″ sphere must not pass anywhere. On stairs, the triangle between the tread, riser and bottom rail may pass a 6″ sphere, and the rest of the stair guard a 4⅜″ sphere.
• Strength: guards must resist a 200 lb concentrated load at the top. Notched 4x4 posts lag-screwed to the rim usually fail that test. Use through-bolts with blocking or tested post brackets.