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POST BALUSTER CLEAR GAP {{ visualGapLimit }}
Start with the closest rail bay, then replace its finished measurements.
The layout run always follows the rail centerline between inside post faces.
Use the finished inside dimension, not outside-to-outside post spacing.
{{ formatNumber(rake_angle, 1) }}°
Use the measured stair pitch; do not estimate from appearance.
Nominal lumber and finished metal sizes can differ from the measured width.
Clear spacing excludes this solid width; on-center spacing includes it.
The selected value is a planning limit, not a compliance certificate.
Every post-side and between-baluster opening is compared with this value.
Use fixed ends only when brackets, connectors, or reveals require them.
The fixed end gap is still checked against the selected maximum.
Use one when the post-to-post openings are not identical.
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Use a small allowance for field variation; verify the installed openings directly.
Match the increment you can repeat reliably on the job site.
This review cue does not change horizontal baluster spacing.
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The buy quantity always rounds up to a whole baluster.
Currency is fixed to USD and no exchange-rate conversion is performed.
USD
Choose one or two decimal places for metric marks and two or three for imperial marks.
BalusterLeft edgeCenter markRight edgeAdjacent gapsCopy
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Field decision brief

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This is a dimensional planning check. Confirm the adopted code, product instructions, guard height, attachment, structural loads, and every installed opening with the authority having jurisdiction.

Gap chart unavailable. The same values remain in the field brief.

Gap margin comparison

Compare the exact and rounded-field worst gaps with the design target and selected maximum.

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A railing can look evenly spaced and still contain an opening that is too wide. The dimension that usually controls a baluster layout is the clear space between finished parts, including the spaces beside the posts. On-center spacing includes the baluster itself, so copying an attractive centerline pattern without subtracting the actual baluster width is a common and consequential mistake.

Clear opening
The finished inside distance between post faces, measured along the rail.
Baluster width
The actual face width or diameter that occupies part of the opening.
Clear gap
The unobstructed space between a post and baluster or between adjacent balusters.

Many residential guard provisions use a sphere test. The 2024 International Residential Code gives a general 4-inch (102 mm) opening limit and a 4 3/8-inch (111 mm) exception for guards on the open side of stairs. Those are reference values, not a universal approval. The adopted code, local amendments, occupancy, product listing, and exact stair condition decide which limit applies.

  • Equal gaps divide both end openings and every interior opening evenly.
  • Fixed end gaps preserve the post-side reveals and divide only the interior openings.
  • Stair layouts need the distance along the sloped rail; a horizontal measurement must be converted with the rake angle.
  • Field rounding can move center marks enough to make one installed gap wider than the exact layout.

Measure the finished bay and the actual baluster profile. Nominal lumber sizes, powder coating, connectors, post sleeves, and bowed parts can make catalog dimensions differ from the installed geometry. A small design margin below the legal or product limit gives ordinary measurement and placement error somewhere to go.

Spacing is only one guard requirement. Height, structural loads, post attachment, stair geometry, glass or cable provisions, product approvals, and local inspection remain outside this calculation.

How to Use This Tool:

Measure one finished rail section, select the applicable opening rule, and make the virtual field marks match the way the work will be laid out.

  1. Choose a Setup preset as a starting point, then enter the finished opening between inside post faces and its unit.
  2. Select how the Opening measured. For a horizontal stair measurement, enter the measured Stair rake angle so the layout run follows the rail.
  3. Enter the Actual baluster width and choose the Gap reference. Replace the preset maximum when local code or product instructions use another value.
  4. Choose equal gaps or fixed ends. For fixed ends, enter the required post-side gap; the remaining interior spaces are divided evenly.
  5. Use the advanced safety margin and center-mark rounding to reflect field practice. Correct invalid geometry or any rounded-gap review before transferring the marks.

Interpreting Results:

The baluster count and Rounded field gap are the practical pair to check first. The exact layout can pass while rounded center marks create a wider opening. A pass includes equality: a worst gap exactly equal to the selected maximum passes, although a layout with no spare margin is less forgiving during installation.

Clear gap margin is the selected maximum minus the widest rounded gap. Positive margin leaves room below the limit; zero sits exactly on it; a negative value needs review. The bottom rail opening is checked separately only when a value above zero is entered.

Confirm the installed work with direct clear-gap measurements. The count, marks, and pass status do not certify the whole guard or override the authority having jurisdiction.

Technical Details:

All dimensions are converted to millimeters. One inch is exactly 25.4 mm. When a stair opening is measured horizontally, the run along the rail is longer and is obtained from the rake angle.

Formula Core

The along-rail run and tightened design limit establish the geometry before a baluster count is chosen.

R=Hcos(θ) D=M−S

R is the layout run, H is a horizontal stair opening, θ is rake angle, M is the selected maximum clear gap, and S is the safety margin. A measurement already taken along the rail uses its converted length directly as R.

For equal end and interior gaps, the smallest whole baluster count that meets the design limit is selected, then the leftover open space is divided into one more gap than the number of balusters.

n=max(0,⌈R−Dw+D⌉) g=R−nwn+1

n is balusters per section, w is actual baluster width, and g is every clear gap. For a 500 mm opening, 40 mm balusters, and a 100 mm design limit, the count is 3 and all four gaps are 95 mm.

Fixed-end layouts reserve two end gaps e, require at least two balusters, and distribute the remaining opening among the interior spaces. A valid run must be longer than two end gaps plus two baluster widths so that an interior gap can exist.

n=max(2,⌈R−2e+Dw+D⌉) ginterior=R−2e−nwn−1

Rule Core

Exact center marks are rounded independently to the selected increment from 0 to 25 mm. Each rounded center is expanded by half the baluster width to recover its left and right edges, and every post-side and between-baluster gap is recalculated. The exact layout passes when its worst gap is at or below the maximum; the design check uses the tighter design limit; the rounded layout passes when its recomputed worst gap is at or below the maximum.

For multiple identical sections, installed quantity is the per-section count times the section count. The buy quantity applies the spare percentage and rounds upward to a whole baluster; estimated cost multiplies that buy quantity by the entered unit price.

Limitations:

The selected code references are planning defaults. They do not identify the governing jurisdiction, occupancy, adopted edition, amendment, or product-specific approval.

  • Verify the opening limit and measurement method with current local requirements before construction.
  • Measure each non-identical bay separately; repeating one count across different openings can create an oversized gap.
  • Allow for actual part tolerance, finish thickness, rail bow, connector geometry, and installation movement.
  • Inspect the completed guard for structural, height, attachment, and all-opening requirements beyond baluster spacing.

References: