What it works out
Enter the clear length between the posts, the width of one baluster and the largest gap you may leave. You get the number of balusters, the equal gap between them, the centre-to-centre distance for marking and where the first baluster sits from the post. The count is rounded up so no gap is over the limit. Check your local rules for the gap you may use.
You enter
- Length
- Width
- Gap
The formula
- Gap
- (run − count × baluster width) ÷ (count + 1)
- Count
- (run − gap) ÷ (width + gap), rounded up
In words: Gap = (run minus count times baluster width) divided by (count plus 1); count = (run minus gap) divided by (width plus gap), rounded up
Worked example
A 156 in run with 1 1/2 in balusters and a 4 in largest gap
- Length between posts 156 in
- Baluster width 1.5 in
- Largest gap 4 in
- Count = (156 − 4) ÷ (1.5 + 4) = 27.64, rounded up to 28
- Gap = (156 − 28 × 1.5) ÷ (28 + 1) = 3.931 in
- Centre to centre = 5.431 in
The count rounds up so that no gap exceeds your limit.
Good to know
- Check your local rules for the largest gap you may leave.
- Posts are assumed square and straight.
- A calculator, not a building-code check: confirm local rules with your building authority.
Questions
Does this check the 4 in rule?
It uses the largest gap you enter. Which gap applies is your local rule, not something the app checks.
Why are gaps equal?
The leftover space is split evenly so the rail looks even.