Deck Baluster Calculator
Deck baluster count from rail length and spacing.
What it does
Balusters fill rails at code gaps: run length divided by on center spacing, rounded up. Centers bundle stock width with the legal gap, so 2x2 balusters at 4 in gaps land on 5.5 in centers. This page counts both unit sets.
The default is a 12 ft rail at 5.5 in centers. Inches total 144, division gives 26.18, and the buy is 27 balusters with gaps near 3.8 in. Metric converts the rail to 3.66 m at 14 cm and lands on the same 27.
Gap law decides legality. Most US codes pass a 4 in sphere nowhere, which the 5.5 in centers above satisfy with 1.5 in stock. Stairs rake the same rule along the slope and need their own layout.
Ends absorb fractions. Start full spacing from one post and let the last bay run short against the far post rather than stretching every gap past code.
How to use
- Pick imperial or metric first.
- Enter the rail run length.
- Enter on center spacing.
- Read count first, then spacing.
- Lay out ends and corners on site.
Formula
- rail
- Rail run length in the active units.
- spacing
- Center to center baluster spacing.
On center spacing adds baluster width to the max gap; 1.5 in stock with 4 in gaps gives 5.5 in centers (14 cm metric). Counts round up and end posts absorb the remainder. Local code sets the legal gap so confirm before buying.
Worked example
Example: 12 ft rail
12 ft rail at 5.5 in on center (metric 3.66 m at 14 cm).
- Inches: 12 x 12 = 144 in.
- Count: 144 / 5.5 = 26.18, up to 27 balusters.
- Metric: 3.66 x 100 / 14 = 26.14, up to 27.
- Gap check: 144 / 27 gives about 3.8 in gaps.
- Tool shows 27 balusters both modes.
Reference table
| Rail | Math | Buy |
|---|---|---|
| 8 ft | 17.45 | 18 |
| 10 ft | 21.82 | 22 |
| 12 ft | 26.18 | 27 |
| 16 ft | 34.91 | 35 |
| 20 ft | 43.64 | 44 |
Limits
- Count only. No posts or rails.
- One spacing per pass. Mixed runs split.
- Code varies. Confirm the 4 in rule locally.
- Stair rakes need separate layout.
Common mistakes
- Spacing edge to edge instead of centers.
- Rounding counts down past code.
- Mixing feet with inches mid chain.
- Forgetting end posts in layout.
- Assuming one spacing fits all stock.
Try it yourself
Quick checks. Answers sit tucked under each question.
8 ft rail at 5.5 in?
18 balusters (96 / 5.5 = 17.45, up to 18).
16 ft rail at 5.5 in?
35 balusters (192 / 5.5 = 34.91, up to 35).
Questions
How many balusters?
Rail over spacing, rounded up. Here 27.
Spacing input?
On center spacing entered. Here 5.5 in.
Gap rule?
Under 4 in (10 cm) between balusters per common code.
What is that in metric?
27 at 14 cm on center for 3.66 m.
Why 5.5 in?
2x2 stock at 1.5 in wide with 4 in gaps sets 5.5 in centers.
Posts included?
No. End posts and corners need layout on site.