Formula used
Equal gap = (available span − total spindle width) ÷ (spindle count + 1)
End-spindle gap = (available span − total spindle width) ÷ (spindle count − 1)
Center spacing = clear internal gap + spindle width
The quantity mode tests whole spindle counts. It stops at the first compliant arrangement. Every end and internal opening is checked against your selected limit.
How to use this calculator
Select whether you need quantity, spacing, or a marking schedule. Choose one measurement unit for every entered dimension. Then enter the usable railing span and spindle width.
Choose the layout style matching your installation method. Enter a maximum opening based on local requirements. Submit the form to review spacing, marks, warnings, and compliance status.
For stairs, provide the angle or rise-and-run values. Select whether the entered span is sloped or horizontal. Also select the direction used for evaluating spacing.
Worked example
Assume a clear opening measuring 72 inches. Each square spindle measures 1.5 inches wide. The selected maximum clear opening is four inches.
Quantity mode increases spindle count until all openings comply. Equal end gaps are included in that compliance check. The final schedule provides every center marking position.
| Input | Example value | Purpose |
|---|---|---|
| Clear span | 72 in | Space available between posts |
| Spindle width | 1.5 in | Actual installed member width |
| Maximum opening | 4 in | Selected project limit |
| Layout | Equal gaps | Balances end and internal spaces |
Horizontal and stair spacing
Horizontal railings usually use the clear distance between posts. Stair railings can use sloped rail length or horizontal projection. Those two distances produce different spacing values.
The calculator converts between both directions using the stair angle. Rise and run can also determine that angle. Confirm which measurement method your authority expects.
Common spacing mistakes
Do not confuse clear spacing with center spacing. Center spacing includes one spindle width. Installation marks are commonly placed on spindle centers.
Measure actual spindle width before creating marks. Decorative products may differ from listed nominal dimensions. Also subtract brackets or post deductions when necessary.
A rounded marking value can change the final end gap. Always verify the last position before drilling. Recheck completed openings with a physical gauge.
Frequently asked questions
What does clear spindle spacing mean?
It is the open distance between adjacent spindle surfaces. It does not include spindle width.
What is center-to-center spacing?
It measures from one spindle center to the next center. It equals clear spacing plus spindle width.
Can the calculator include equal end gaps?
Yes. Select the equal end and internal gap method. Both end spaces will match every internal opening.
Can spindles start at both ends?
Yes. The end-spindle layout places the first and last spindle against each endpoint.
Does it support custom margins?
Yes. Enter separate left and right margins. The remaining internal gaps are distributed equally.
Can I enter fractional inches?
Yes. Values such as 1 1/2 or 3/8 are supported. Feet-and-inches notation is also accepted.
How are stair spans converted?
The calculator uses the stair angle and cosine relationship. It converts sloped lengths and horizontal projections.
Why might rounded spacing fail?
Rounding changes accumulated positions across many spindles. Always verify the final opening after practical rounding.
Does the pass result guarantee legal compliance?
No. The result only checks your entered limit. Local requirements, inspection methods, and exceptions can vary.
Can I export the marking schedule?
Yes. Use CSV for spreadsheets. Use PDF or print for field installation notes.
Should I measure nominal or actual spindle width?
Use the actual installed width. Small dimensional differences can noticeably change every calculated gap.
Important project notice
This tool supports planning and estimating. It does not replace engineering review, manufacturer instructions, permits, or inspections. Always confirm current local railing and guard requirements.