Baluster and Spindle Spacing Calculator
Enter the clear length of your rail and the width of your balusters and get the count, the gap between them, the on-centre spacing and every mark to measure from, with the gap compared with the quoted model-code sphere.
Estimates for planning, not professional advice. Read the full notice.
Example: spacing mark strip
Enter the numbers above to see your layout.
Summary
- Items
- 17
- Gap
- 3 15/16 in
- On centre
- 5 7/16 in
- First item, left edge
- 3 15/16 in
- First item, centre
- 4 11/16 in
Estimate for planning, from what you entered and the product data shown. Check product labels and your supplier before you buy.
Spacing mark strip
2021 IRC comparison
-
Gap between items: 3 15/16 in within the 2021 IRC model-code value
The 2021 IRC, section R312.1.3, says “Required guards shall not have openings from the walking surface to the required guard height that allow passage of a sphere 4 inches (102 mm) in diameter.” 4 in[1]
Local codes and amendments differ; your building department decides.
Marks from the start
| Item | Left edge | Centre |
|---|---|---|
| Item 1 | 3 15/16 in | 4 11/16 in |
| Item 2 | 9 5/16 in | 10 1/16 in |
| Item 3 | 14 3/4 in | 15 1/2 in |
| Item 4 | 20 3/16 in | 20 15/16 in |
| Item 5 | 25 9/16 in | 26 5/16 in |
| Item 6 | 31 in | 31 3/4 in |
| Item 7 | 36 7/16 in | 37 3/16 in |
| Item 8 | 41 13/16 in | 42 9/16 in |
| Item 9 | 47 1/4 in | 48 in |
| Item 10 | 52 11/16 in | 53 7/16 in |
| Item 11 | 58 1/16 in | 58 13/16 in |
| Item 12 | 63 1/2 in | 64 1/4 in |
| Item 13 | 68 15/16 in | 69 11/16 in |
| Item 14 | 74 5/16 in | 75 1/16 in |
| Item 15 | 79 3/4 in | 80 1/2 in |
| Item 16 | 85 3/16 in | 85 15/16 in |
| Item 17 | 90 9/16 in | 91 5/16 in |
Drawing
A strip of 17 items. The mark table lists every mark.
Scale 1:16 (1 in = 16 in) when printed at 100 % (Actual size).
Worked example: a rail with balusters
Show the working
- Balusters: the fewest that leave every gap less than the largest gap, rounded up once; a gap equal to the limit is not accepted, so an exact fit takes one more 8 ft ÷ 4 in[1] = 17 balusters
- Gap: the clear length less the width of every baluster, shared between the gaps (one more gap than balusters) 8 ft = 3 15/16 in
- On-centre spacing: the gap plus one baluster width 3 15/16 in + 1 1/2 in = 5 7/16 in
- First baluster, left edge: one gap from the post 3 15/16 in = 3 15/16 in
- First baluster, centre: the left edge plus half a baluster width 3 15/16 in = 4 11/16 in
Assumptions
The figures behind the example. Sourced figures link to their source below; a default is a starting value to check against your label or supplier.
| Assumption | Value | Details and changes |
|---|---|---|
| Largest gap between balusters | 4 in[1] | |
| Default Prefilled with the 2021 IRC sphere size for guard openings; a tighter target gives a narrower gap. | ||
| Largest gap between balusters on a stair | 4.375 in[1] | |
| Default Prefilled with the 2021 IRC sphere size for guard openings on the open side of stairs. | ||
Baluster marks for this example
Where each baluster sits, measured from the start of the clear length: left edge, centre and right edge
| Baluster | Left edge | Centre | Right edge |
|---|---|---|---|
| Baluster 1 | 3 15/16 in | 4 11/16 in | 5 7/16 in |
| Baluster 2 | 9 5/16 in | 10 1/16 in | 10 13/16 in |
| Baluster 3 | 1 ft 2 3/4 in | 1 ft 3 1/2 in | 1 ft 4 1/4 in |
| Baluster 4 | 1 ft 8 3/16 in | 1 ft 8 15/16 in | 1 ft 9 11/16 in |
| Baluster 5 | 2 ft 1 9/16 in | 2 ft 2 5/16 in | 2 ft 3 1/16 in |
| Baluster 6 | 2 ft 7 in | 2 ft 7 3/4 in | 2 ft 8 1/2 in |
| Baluster 7 | 3 ft 7/16 in | 3 ft 1 3/16 in | 3 ft 1 15/16 in |
| Baluster 8 | 3 ft 5 13/16 in | 3 ft 6 9/16 in | 3 ft 7 5/16 in |
| Baluster 9 | 3 ft 11 1/4 in | 4 ft | 4 ft 3/4 in |
| Baluster 10 | 4 ft 4 11/16 in | 4 ft 5 7/16 in | 4 ft 6 3/16 in |
| Baluster 11 | 4 ft 10 1/16 in | 4 ft 10 13/16 in | 4 ft 11 9/16 in |
| Baluster 12 | 5 ft 3 1/2 in | 5 ft 4 1/4 in | 5 ft 5 in |
| Baluster 13 | 5 ft 8 15/16 in | 5 ft 9 11/16 in | 5 ft 10 7/16 in |
| Baluster 14 | 6 ft 2 5/16 in | 6 ft 3 1/16 in | 6 ft 3 13/16 in |
| Baluster 15 | 6 ft 7 3/4 in | 6 ft 8 1/2 in | 6 ft 9 1/4 in |
| Baluster 16 | 7 ft 1 3/16 in | 7 ft 1 15/16 in | 7 ft 2 11/16 in |
| Baluster 17 | 7 ft 6 9/16 in | 7 ft 7 5/16 in | 7 ft 8 1/16 in |
Each mark is worked out from its own place in the run, then written to the nearest sixteenth of an inch. Marks are never added up from the rounded spacing, which would drift.
This example's gap and the quoted level sphere
The gap between balusters of the level example beside the quoted sphere
| Item | This example | Model-code value |
|---|---|---|
| Gap, level guard: within the 2021 IRC value | 3 15/16 in | 4 inModel-code (2021 IRC R312.1.3) diameter of the sphere that guard openings may not pass; local adopted codes differ. |
Source: 2021 International Residential Code, Chapter 3 Building Planning, International Code Council, 2021 IRC, checked Nov 12, 2026.
The row reads within, outside or at the quoted value. Balusters use a strict rule: a gap equal to the quoted value is not accepted. These are model-code values; the code your area has adopted may differ.
Balusters per section
Balusters, gap and on-centre spacing for common clear lengths and two baluster widths, at the level and stair defaults
| Clear length, baluster width, guard | Balusters | Gap | On-centre |
|---|---|---|---|
| 4 ft clear, 1.5 in balusters, level guard | 9 balusters | 3 7/16 in | 4 15/16 in |
| 4 ft clear, 1.5 in balusters, stair guard | 8 balusters | 4 in | 5 1/2 in |
| 4 ft clear, 0.75 in balusters, level guard | 10 balusters | 3 11/16 in | 4 7/16 in |
| 4 ft clear, 0.75 in balusters, stair guard | 9 balusters | 4 1/8 in | 4 7/8 in |
| 6 ft clear, 1.5 in balusters, level guard | 13 balusters | 3 3/4 in | 5 1/4 in |
| 6 ft clear, 1.5 in balusters, stair guard | 12 balusters | 4 1/8 in | 5 5/8 in |
| 6 ft clear, 0.75 in balusters, level guard | 15 balusters | 3 13/16 in | 4 9/16 in |
| 6 ft clear, 0.75 in balusters, stair guard | 14 balusters | 4 1/8 in | 4 7/8 in |
| 8 ft clear, 1.5 in balusters, level guard | 17 balusters | 3 15/16 in | 5 7/16 in |
| 8 ft clear, 1.5 in balusters, stair guard | 16 balusters | 4 1/4 in | 5 3/4 in |
| 8 ft clear, 0.75 in balusters, level guard | 20 balusters | 3 7/8 in | 4 5/8 in |
| 8 ft clear, 0.75 in balusters, stair guard | 18 balusters | 4 5/16 in | 5 1/16 in |
| 7 ft 8 in clear, 1.5 in balusters, level guard | 17 balusters | 3 11/16 in | 5 3/16 in |
| 7 ft 8 in clear, 1.5 in balusters, stair guard | 15 balusters | 4.34 in | 5 7/8 in |
| 7 ft 8 in clear, 0.75 in balusters, level guard | 19 balusters | 3 7/8 in | 4 5/8 in |
| 7 ft 8 in clear, 0.75 in balusters, stair guard | 18 balusters | 4 1/8 in | 4 7/8 in |
Every row leaves a gap at each end and uses the strict rule: a gap equal to the largest gap is not accepted, so a section that would land on it takes one more baluster.
Balusters on a stair
The same clear length and baluster width at the stair guard default: balusters, gap and on-centre spacing
| Item | This example |
|---|---|
| Balusters | 16 balusters |
| Gap between balusters | 4 1/4 in |
| On-centre spacing | 5 3/4 in |
On the open side of a stair the largest gap is the stair guard default, which differs from the level default. The triangle formed by the riser, tread and bottom rail has its own quoted value.
The stair example's gap and the quoted stair sphere
The gap between balusters of the stair example beside the quoted sphere for the open side of a stair
| Item | This example | Model-code value |
|---|---|---|
| Gap, stair guard: within the 2021 IRC value | 4 1/4 in | 4.375 inModel-code (2021 IRC R312.1.3) diameter of the sphere guard openings on the open side of stairs may not pass; local adopted codes differ. |
Source: 2021 International Residential Code, Chapter 3 Building Planning, International Code Council, 2021 IRC, checked Nov 12, 2026.
The row reads within, outside or at the quoted value.
Guard openings, as quoted
The sphere that guard openings may not admit, for a level guard, the open side of a stair and the triangle beside a stair guard
| Provision | Model-code value |
|---|---|
| Section R312.1.3, guard opening | 4 inModel-code (2021 IRC R312.1.3) diameter of the sphere that guard openings may not pass; local adopted codes differ. |
| Section R312.1.3, open side of a stair | 4.375 inModel-code (2021 IRC R312.1.3) diameter of the sphere guard openings on the open side of stairs may not pass; local adopted codes differ. |
| Section R312.1.3, triangle beside a stair guard | 6 inModel-code (2021 IRC R312.1.3) diameter of the sphere the triangle at a stair guard may not pass; local adopted codes differ. |
Source: [1]
These are model-code values. The code your area has adopted may differ.
- The 2021 IRC, section R312.1.3, says “Required guards shall not have openings from the walking surface to the required guard height that allow passage of a sphere 4 inches (102 mm) in diameter.”
- The 2021 IRC, section R312.1.3, says “Guards on the open side of stairs shall not have openings that allow passage of a sphere 4 3/8 inches (111 mm) in diameter.”
- The 2021 IRC, section R312.1.3, says “The triangular openings at the open side of stair, formed by the riser, tread and bottom rail of a guard, shall not allow passage of a sphere 6 inches (153 mm) in diameter.”
In the 2024 IRC these provisions are in Sections R318.7 (stairways) and R321 (guards). The 2024 chapter.
Questions
How do I space balusters evenly?
Find the fewest balusters that leave every gap smaller than the largest gap you allow, then share the clear length less their total width between the gaps. The gap is the same everywhere, and the on-centre spacing is the gap plus one baluster width.
How many balusters do I need for an 8 ft section?
It depends on the baluster width and on whether the section is a level guard or the open side of a stair. The table of sections above gives the count for common lengths at two widths for each, and the calculator gives it for your own sizes.
How are balusters spaced on stairs?
The open side of a stair has its own quoted sphere for the gaps, and the triangle formed by the riser, tread and bottom rail has another. The calculator above works a level section; the stair-gap values are in the section on the page, not in the calculator. The triangle depends on the stair itself, so it is quoted rather than worked out here.
How do I measure the gap between balusters?
The code limits the size of sphere that an opening may admit, so the gap to check is the open space between neighbouring balusters at their closest. This page names no direction of measurement that the quoted section does not give. Measure the finished opening and compare it with the quoted value.
Can I use a tighter target than the code value?
Yes. The largest gap is a field, so you type your own target into it. A smaller target adds balusters and narrows the gap, and the level strip's comparison with the quoted value still reads within, outside or at.
Local codes and amendments differ; your building department decides.
Sources (1)
-
2021 International Residential Code, Chapter 3 Building Planning (opens in new tab), International Code Council, 2021 IRC.
Code or government.
“Required guards shall not have openings from the walking surface to the required guard height that allow passage of a sphere 4 inches (102 mm) in diameter.”
“Guards on the open side of stairs shall not have openings that allow passage of a sphere 4 3/8 inches (111 mm) in diameter.”
“The triangular openings at the open side of stair, formed by the riser, tread and bottom rail of a guard, shall not allow passage of a sphere 6 inches (153 mm) in diameter.”
Checked November 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 1)
These are summaries of what the sources say, not professional advice. Codes and product data change. Check with your building department and the product label about your own project.
Sources
- [1] International Code Council, 2021 International Residential Code, Chapter 3 Building Planning. Checked November 12, 2026. https://codes.iccsafe.org/content/IRC2021P2/chapter-3-building-planning