How many balusters you need and how far apart

Whether it is a railing, a guard or a fence, balusters should be spread with the same gap between them and at the posts, without exceeding a maximum gap. This calculator tells you how many balusters each section needs, the exact gap to leave and the centre spacing to mark out for installation.

How the calculation works

• Number of balusters: the clear span minus one gap, divided by baluster width plus maximum gap, rounded up.
• Actual gap: the span minus the width of all balusters, divided by the number of gaps, which is the number of balusters plus one.
• Centre spacing: actual gap plus baluster width, the distance from centre to centre.

Measure the clear span, the distance between the inner faces of the posts, not between their centres.

The 4-inch (10 cm) sphere rule

Many codes for guards, balconies and stairs require that a sphere of about 10 cm cannot pass between balusters, so that a child cannot slip through. In the United States the limit is 4 inches (10.16 cm); in Germany 12 cm is often found. For a garden fence that does not guard against a fall, you can choose any gap you like; for a guard always check the local code.

Examples by section span

4 cm balusters and a 10 cm maximum gap:

At 150 cm the gap is exactly 10 cm: within the limit, but with no margin. In such cases it is worth adding a baluster, because a few millimetres of installation error are enough to exceed it.

How much baluster width matters

180 cm section, 10 cm maximum gap:

Wider balusters fill more of the span, so fewer are needed; centre spacing grows, but the gap between them stays below the maximum.

Installation tips

• Cut a wooden spacer as wide as the actual gap: it makes installation faster and more even.
• Start from one post and mark the centres across the whole section before fixing.
• Also check the gap below the bottom rail and above the floor, which often has its own limit.

Limits of the calculator

The calculation applies to identical vertical balusters in straight sections. On stairs the sloped geometry changes the gap measured horizontally. The calculator does not check the railing's strength or other guard rules, such as minimum height: your country's codes apply for those.

Frequently asked questions

What is the maximum gap between balusters?

Many guard codes require that a sphere of about 10 cm (4 inches) cannot pass between balusters. For a garden fence the gap is a matter of looks.

How many balusters for 180 cm?

With 4 cm balusters and a 10 cm maximum gap you need 13 balusters, with 9.14 cm between them.

Is the gap at the posts the same as the others?

Yes: the calculator spreads the same gap between all balusters and between balusters and posts.

What is centre spacing?

The distance from centre to centre between two balusters: the gap plus the baluster width. It is the easiest measurement to mark.

How do I measure the span?

Between the inner faces of the two posts, not between their centres.

What if the gap is right at the limit?

Add a baluster: the gap drops and small installation errors will not push it over.

Does it work for stairs?

Partly: on stairs the railing slopes and the gap must be checked horizontally. Use the result as a starting point.

What is the limit in the US or Germany?

In the United States 4 inches (10.16 cm); in Germany 12 cm is often found. Always check the local code.

Does the calculator check the railing's safety?

No: it only calculates baluster layout. Height and strength depend on your country's codes.