Railing Calculator — ft, balusters, posts, cost
How to use this calculator
Enter your total railing length, choose railing height (36" for deck, 42" for stairs or elevated decks), select baluster size and spacing, and configure post spacing. The calculator counts balusters, posts, rail lengths, and hardware.
IRC R312.1 — maximum 4" sphere gap between balusters. The calculator enforces this.
Sections — choose straight, L-corner, or stair (angled) layouts.
Post spacing — typically 6–8 ft on center for structural integrity.
Saved Calculations
| Time | Length | Height | Posts | Balusters | Cost |
|---|
How to Calculate Railing Materials
This railing calculator estimates posts, balusters, top and bottom rails, post caps, and hardware for deck, porch, and stair railings. It complies with IRC R312.1 which requires a maximum 4" sphere gap between balusters and minimum 36" railing height for decks (42" for stairs and elevated areas).
Formulas Used
Posts: ⌈total length / post spacing⌉ + 1 (+ corner posts for L-layouts)
Balusters per section: ⌈section length / (baluster width + gap)⌉ + 1, then subtract 1 for each adjacent post
Rail length: total railing length + 10% waste, divided into 8 ft pieces
Actual gap: (section length − n × baluster width) / (n + 1)
Building Code Reference
IRC R312.1.1: Guards required where walking surface is > 30" above grade. Min height 36". Max 4" sphere opening. IRC R312.1.2: Stair guards min 34" measured from stair nosing.
FAQ
What is the maximum baluster spacing? Per IRC R312.1.3, a 4" sphere cannot pass through any opening. Using 3.5" gap provides a safe margin for wood movement.
How far apart should railing posts be? Typically 6–8 ft on center. Closer spacing provides more strength. Posts must resist 200 lb lateral load per IRC R301.5.
