Spindle Calculator
March 5, 2026 · View on GitHub
A trade tool for carpenters and joiners to calculate spindle counts and spacings for balustrades, right on site — no phone or calculator needed.
What it does
Given a length and spindle size, it calculates:
- How many spindles fit while keeping gaps within the 99 mm building regulation limit
- The exact gap between each spindle
- Centre-to-centre spacing
There are two modes:
Stair mode
Enter the diagonal (rake) length between posts and the stair angle. The app converts the diagonal to a horizontal run using horiz = diagonal × cos(angle), then calculates all spacings from that. Results include both the horizontal gap and the rake length.
The built-in accelerometer can read the stair angle automatically — just lay the Flipper flat on a tread and press OK.
Straight run mode
Enter the total horizontal length between posts. The app calculates count and spacing directly.
Controls
Angle screen
| Button | Action |
|---|---|
| Up / Down | ±0.1° |
| Left / Right | ±1.0° |
| OK | Lock angle and continue |
| Back | Return to menu |
Length input screens
| Button | Action |
|---|---|
| Up / Down (short) | ±1 mm |
| Up / Down (hold) | ±10 mm |
| Left / Right (short) | ±10 mm |
| Left / Right (long press) | ±100 mm |
| Left / Right (on spindle field) | Change spindle size |
| OK | Next field / Calculate |
| Back | Previous field / Menu |
Result screens
| Button | Action |
|---|---|
| Up / Down | Scroll results |
| Back | Return to inputs |
Building
Requires the Flipper Zero firmware SDK or uFBT.
# Using uFBT (recommended)
ufbt
# Deploy directly to connected Flipper
ufbt launch
Installing from the catalog
- Open the Flipper Mobile App or go to lab.flipper.net
- Browse to Apps → Tools → Spindle Calculator
- Tap Install
Building regulation note
This app enforces the 99 mm maximum gap between spindles as required by UK Building Regulations Approved Document K (and equivalent standards). Always verify final measurements on site.
License
MIT — see LICENSE