What this calculator does

You enter the inside distance between posts, the spindle width, and the maximum allowable gap. The tool instantly returns a count of spindles and two spacing patterns:

• Evenly-spaced spindles — every gap including both ends is equal.
• Centered spindles — the gap between spindles equals your code limit while the two end gaps share the leftover distance.

Building a stair rail? The calculator also gives slanted spacing along the shoe rail once you enter the stair pitch. You mark those values directly on the sloped base and your layout will still satisfy the horizontal gap rule.

How spindle spacing works

Most residential codes set a simple test. No opening in a guard may allow a 4-inch sphere to pass through. The International Residential Code states this directly and most jurisdictions follow it R312.1.3 “Opening limitations”.

Stairs introduce small exceptions. The triangular gap near the tread can allow a 6-inch sphere and the gaps along the sloping rail can allow a 4⅜-inch sphere in many adoptions industry summary. Always check your local amendments before you order materials.

Formulas used by the calculator

The math is straightforward once you define three inputs:

• L — inside railing distance measured horizontally.
• w — spindle width measured across the face.
• s — maximum allowable clear gap. In many regions you set s = 4 in or 102 mm for decks and 4⅜ in for stair runs when adopted.

How many spindles do you need?

First estimate how many full “spindle plus gap” modules fit after you reserve one end gap.

n = ceil( (L − s) / (w + s) )

Evenly-spaced pattern

You want every gap including both ends to match. Solve the real gap g from the remaining length.

g = (L − n·w) / (n + 1)

The calculator caps nothing here because the formula already yields a gap at or under s when you use the same n from the first step.

Centered pattern

Many crews prefer to keep each interior gap right at the code limit then split the leftovers on the ends.

interior gap = s end gap e = (L − n·w − (n − 1)·s) / 2

Stair rails and “slanted” spacing

Codes measure openings horizontally not along the slope. If your stair pitch equals θ then the distance along the sloped rail grows by 1 / cos θ.

slanted spacing = horizontal spacing / cos(θ)

The calculator auto-computes θ from rise and run when you enter both.

Step-by-step: how to use it

1. Choose “Deck” or “Stairs.” Stairs unlock slanted spacing fields and a pitch input.
2. Enter the inside distance. Measure from post face to post face. Keep the tape in a level line for stairs.
3. Type your spindle width. Measure the actual piece you plan to install since milled dimensions can vary.
4. Confirm the maximum gap. Leave the default 10 cm or 4 in if your local jurisdiction uses the 4-inch sphere rule. Check your local code before you change it.
5. For stairs enter rise and run for one tread then let the tool find the pitch. You can also type the pitch directly if you have it.
6. Pick a spacing style. Use evenly-spaced for a balanced look or centered for a fast code-check layout.
7. Mark the rail. Transfer the horizontal gaps to your work then use the slanted values on stair shoe rails if needed.

Worked examples

Example A — deck rail, metric

Inside distance L = 3.0 m. Spindle width w = 35 mm. Max gap s = 100 mm.

• Raw modules: (L − s)/(w + s) = (3000 − 100)/(35 + 100) = 2900/135 = 21.481…
• n = 22 spindles.
• Even gap: g = (3000 − 22·35)/(22 + 1) = 2230/23 = 96.957 mm.
• Centered pattern interior gap = 100 mm. End gaps: e = (3000 − 770 − 21·100)/2 = 65 mm.

The evenly-spaced option squeezes every gap to roughly 96.96 mm so the rail looks uniform. The centered option keeps code-max gaps between spindles and shows 65 mm on each end.

Example B — stair rail, imperial

Inside distance L = 8 ft. Spindle width w = 1.5 in. Max gap s = 4 in. Rise 7.5 in. Run 10 in. The pitch is atan(7.5/10) ≈ 36.87°. The cosine equals 0.7986.

• Convert L to inches: 8 × 12 = 96 in.
• Estimated modules: (96 − 4)/(1.5 + 4) = 92/5.5 = 16.727…. So n = 17.
• Even horizontal gap: g = (96 − 17·1.5)/(17 + 1) = (96 − 25.5)/18 = 70.5/18 = 3.9167 in.
• Even slanted gap along the rail: 3.9167 / 0.7986 = 4.904 in.
• Centered interior gap = 4 in. End gaps horizontally: e = (96 − 25.5 − 16·4)/2 = 60/2 = 3 in.
• Centered slanted end gap: 3 / 0.7986 = 3.756 in.

You now have a staircase layout that meets the 4-inch horizontal rule. Your marks on the sloped rail use the larger slanted numbers so the installed openings still pass code.

Quick cosine helper table

The table below gives common stair pitches and cosines. Use it for a fast mental check.

Special notes for stair railings

Stair rails add two extra checks.

• Horizontal rule still applies. Your gaps are measured horizontally even though you install spindles on a slope. The calculator handles the conversion.
• Small stair exceptions exist. Many adoptions allow 4⅜-inch gaps along the slope and a 6-inch triangle near the tread. Confirm what your building department enforces before you finalize a layout summary example adoption PDF.

OSHA rules address workplace stairs with different thresholds and clearances, which you may see on commercial projects 1910.25 1926.1052. Residential homes usually fall under the IRC not OSHA.

How to measure your railing

Inside distance

Hook your tape on the inside face of one post then pull straight to the inside face of the other. Keep the tape level for stairs and decks. Write down the distance to the nearest millimeter or sixteenth.

Spindle width

Use calipers when you can. If you only have a tape measure then read across the narrowest part of the spindle face. Many metal balusters are nominal sizes so confirm the true width.

Stair pitch

Measure the rise of one step and the horizontal run between the same points on the next step. The calculator finds the angle from those two values. You can also place a digital level on the rail if you already framed it.

Pick a spacing style

Even spacing gives a gallery look. Centered spacing gives a fast pass or fail check against code. Both produce compliant layout when you follow your local gap limit.

Material tips that affect spacing

Wood

Wood swells with humidity then shrinks as it dries. A rail that passes the 4-inch rule in spring can creep wider by late summer. Cut with a small safety buffer if your climate swings hard.

Powder-coated steel or aluminum

Factory members hold tight tolerances. You can push closer to the limit since the pieces stay stable. Follow the brand’s install sheet if they publish on-center numbers for their profiles.

Composite systems

Composite rails often ship as kits with preset brackets. Use the calculator to size the end gaps between the bracket centerlines so the field looks even.

Glass infill

Glass doesn’t use spindles yet you still check opening sizes at the edges. Many jurisdictions keep the 4-inch sphere rule between panels and at the ends.

Frequently asked questions

What’s the legal spindle spacing?

Many regions use the International Residential Code which says no guard opening may let a 4-inch sphere pass through R312.1.3. Stairs have small exceptions that you must verify with your inspector.

Do I measure stair length along the handrail or level?

Measure level. Codes judge openings on the horizontal projection. The calculator still shows slanted gaps so your marks on the shoe rail make sense.

How close should I cut to the limit?

Leave a little cushion. Lumber moves and installers are human. A 3 ⅞-inch target for a 4-inch rule keeps you safe without changing the look.

Why do end gaps change in the centered option?

Because the interior gaps lock at the code limit and the leftover distance must go somewhere. The ends split that distance so the field stays symmetrical.

Does OSHA set spacing for home decks?

No. OSHA governs workplaces. Residential decks usually follow the IRC or a local code based on it. Commercial work may follow OSHA and the International Building Code with different details 1910.29.

What about post spacing?

The calculator works between two posts so it doesn’t enforce post spacing. Many manufacturers publish their own structural limits and some states add rules. Cal-OSHA for example caps post spacing at eight feet for certain guards.

Can I use on-center layout instead?

You can when the manufacturer tells you the on-center number that produces a legal gap. This method is common with turned spindles and some metal profiles example. The calculator uses clear-gap math which stays consistent across profiles.

Why this calculator improves your build

You save time on layout. You also get consistent results across different rail lengths and profiles. The math never forgets the end gaps and it respects your code limit by design.

• Clarity — inputs on one side and read-only results on the other so no mix ups.
• Mobile friendly — large tap targets and clean labels that read well in bright light.
• Flexible units — switch between metric and imperial without re-typing numbers.
• Stair aware — shows slanted gaps that match what you mark on a sloped rail.

Common mistakes and how to avoid them

• Measuring along the sloped rail — measure horizontally for the math then use slanted values to mark the shoe rail.
• Using nominal sizes — confirm the real spindle width. Small errors repeat across a long run and drift your end gap.
• Ignoring movement — lumber moves with weather. Leave a small cushion under the gap limit.
• Overlooking mounting hardware — shoe clips and collars can add a few millimeters to the face. Account for them before you cut.

Troubleshooting odd results

If you see a huge end gap then nudge the max gap down a hair and recompute. If the tool returns one spindle when you expect many then double-check units and conversions. A common mix up is typing inches while the unit shows centimeters.

For tight inside distances the formula can return zero spindles. That’s normal. A short guard between two newel posts might look better with a solid infill panel or a thick glass lite.

Glossary

Baluster / spindle

The vertical infill member between the top and bottom rails.

Clear gap

The open air distance measured between two solid members. Codes judge this distance.

End gap

The clear gap between a post and the first spindle.

Pitch

The angle of the stair from the horizontal. The calculator computes it from rise and run when both are set.

Final take

A railing that looks good also needs to pass inspection. This Spindle Spacing Calculator gives you both. You get a layout that hits the gap limit and you get a balanced pattern that pleases the eye. Grab your inside measurement and spindle width then let the math handle the rest.