How to space balusters evenly
Pick the largest gap you want, work out how many balusters that needs, then spread them so every gap is equal:
- Measure the clear space between the posts.
- Count: n = (space − gap) ÷ (baluster width + gap), rounded up.
- Even gap: (space − n × baluster width) ÷ (n + 1). It is always at or under your target.
- Mark centres from one post using the list in the results; a story stick with one mark saves measuring every baluster.
Baluster spacing code
Guard openings must not let a sphere of a set size pass. The limits differ by country:
| Code | Opening limit | Guard height |
|---|---|---|
| US IRC[1][2] | 4 in sphere; 4-3/8 in on the open side of a stair; 6 in triangle at the stair bottom rail | 36 in (34 in on stairs); required over 30 in drop |
| England, Approved Doc K[3] | 100 mm sphere | 900 mm at stairs and landings; 1,100 mm on external balconies |
| Canada NBC 2020[4] | 100 mm sphere; 150 mm triangle | 900 mm in dwellings and low decks; 1,070 mm otherwise |
| Australia NCC 2022[5] | 125 mm sphere | 1 m at decks and landings; 865 mm on stairs |
"Under 4 in" means the gap must be smaller than 4 in, so most builders aim near 3-1/2 to 3-3/4 in to leave room for bowed balusters.
Stair balusters
On a stair the IRC allows a 4-3/8 in sphere along the open side[1]. Balusters on a stair are spaced along the slope; two per tread is the usual way to meet the limit with 10–11 in treads. Plan the stair itself with the stair calculator.
Decks and railings
A deck guard is required where the walking surface is more than 30 in above grade within 36 in of the edge (IRC)[1]. The deck calculator adds up rail posts and balusters for a whole deck, and the deck framing calculator covers the frame the posts bolt to.
Frequently asked questions
What is the code for baluster spacing?
Under the IRC no opening in a guard may pass a 4 in sphere (4-3/8 in on the open side of a stair). The UK and Canada use 100 mm and Australia 125 mm.
How many balusters per foot?
With 1-1/2 in balusters and 3-3/4 in gaps, about 2.3 per foot, or roughly 3 balusters for every 16 in.
How do you calculate baluster spacing?
Count n = (space − gap) ÷ (baluster + gap), rounded up, then gap = (space − n × baluster) ÷ (n + 1).
How many balusters for a 6 ft railing?
About 13 1-1/2 in balusters between 4x4 posts 6 ft on centre, with gaps under 3-3/4 in.
Is a spindle spacing calculator the same?
Yes. Spindles and balusters are spaced the same way: count = (space − gap) ÷ (spindle width + gap), rounded up, then divide the leftover space evenly.
Sources
- 2021 International Residential Code, Chapter 3: Building Planning (R301.5, R311.7 stairways, R312 guards)
- 2024 International Residential Code, Chapter 3 (R318.7 stairways, R320 handrails, R321 guards)
- Approved Document K: Protection from falling, collision and impact (2013 edition, for use in England)
- National Building Code of Canada 2020, Division B, Part 9, Section 9.8 Stairs, ramps, handrails and guards
- ABCB Housing Provisions, Part 11.3 Barriers and handrails (NCC 2022)