About this calculator
This calculator works out how many balusters (spindles) a railing run needs and how far apart to set them. Enter the run length over the end posts, whether it is level (a landing or balcony) or raked (on a stair), the maximum post spacing, and the post and baluster widths. The calculator places the posts evenly, then puts the fewest balusters in each bay that keep every clear gap at or below your maximum — and tells you the actual gap.
The safety rule behind the default is the sphere test: a ball of a given size must not pass through any opening in the guarding, so small children cannot slip through or get their head stuck. The default 100 mm is the sphere in England's Approved Document K for stairs that young children may use.
Formula
bays = ceil((run − post) / max post spacing)
posts = bays + 1
post centres = (run − post) / bays
clear bay width c = post centres − post
balusters per bay n = smallest n with (c − n × baluster) / (n + 1) ≤ max gap
= ceil((c − max gap) / (baluster + max gap)) (0 if c ≤ max gap)
actual gap = (c − n × baluster) / (n + 1)
baluster centres = actual gap + baluster
handrail = run / cos(pitch) (level: run)
Worked example
A 3 m level run, 90 mm posts at most 1500 mm apart, 40 mm balusters, 100 mm maximum gap:
bays = ceil((3000 − 90) / 1500) = ceil(1.94) = 2 → 3 posts
post centres = 2910 / 2 = 1455 mm
clear bay = 1455 − 90 = 1365 mm
per bay = ceil((1365 − 100) / 140) = ceil(9.04) = 10 → 20 balusters
gap = (1365 − 10 × 40) / 11 = 87.7 mm
centres = 87.7 + 40 = 127.7 mm
Nine balusters per bay would leave 100.5 mm gaps — just over the limit — so the calculator uses ten.
FAQ
What maximum gap applies in Lithuania? We could not confirm it from the official text. A reproduction of STR 2.02.01:2004 "Gyvenamieji pastatai" quotes a 120 mm limit for the vertical bars of guards in residential buildings, and one of STR 2.02.09:2005 says gaps are not regulated for single- and two-family houses; neither was checked against the official consolidated text, and STR 2.02.01:2004 is listed as ceasing to apply on 1 January 2027. The 100 mm default is stricter than 120 mm either way. Check the current rules for your building, or ask your designer.
Is the gap measured horizontally or along the stair? Horizontally, between vertical balusters — the direction a child's head would pass through. The calculator lays out a raked run horizontally and cuts the balusters to the pitch.
Why are the gaps between the posts and the first baluster the same as the others? The balusters are spread evenly in each bay, so all the gaps in a bay are equal. That is also what keeps the largest gap as small as possible.
How far apart should the posts be? That is a structural matter — the railing system's instructions or a designer gives it. The calculator only keeps to the maximum you enter.
Does it check the railing height or strength? No. Height is used for the drawing only, and the loads on a railing are outside this calculator.
Assumptions and limits
Cited: Approved Document K (England) 2013, paragraph 1.39 — the 100 mm sphere for stairs likely to be used by children under five. It is the source of the default maximum gap, not a claim that it applies to your building.
Defaults to replace: 3 m run, 90 mm posts, 1500 mm maximum post spacing, 40 mm balusters, 900 mm height, 35° pitch.
Geometry: one straight run with posts at both ends; corner posts shared between runs, newels, turns and the gap under the bottom rail or between treads are not modelled. Balusters are vertical with square or round sections of the given width. The handrail length is from the outer face of the first post to that of the last, along the pitch; joints, returns and fixings are not included.