Deck Railing Spindle Spacing Calculator

Enter the space between posts and the spindle width and get the fewest spindles that keep every gap under the IRC's 4-inch sphere, with exact spacing and positions.

Construction & DIYUnited States onlySource: 2021 IRC R312.1.3 guard opening limitsFree, no sign-up

Clear space between posts (in)

Inside face to inside face.

Baluster width (in)

At its narrowest point for turned spindles.

Opening limit

Number of balusters (optional)

Leave blank for the fewest that meet the limit.

Identical sections

The result updates as you type.

Baluster spacing

13 balusters, 3.643 in gaps

5.143 in on centre · under the 4 in limit · 13 for 1 section

Fewest balusters with every gap under the limit	13
Equal gap (post to baluster and between)	3.6429 in
On-centre spacing	5.1429 in

Centre of each baluster, measured from the left post

1: 4.393 in 2: 9.536 in 3: 14.679 in (1' 2-11/16") 4: 19.821 in (1' 7-13/16") 5: 24.964 in (2' 0-15/16") 6: 30.107 in (2' 6-1/8") 7: 35.25 in (2' 11-1/4") 8: 40.393 in (3' 4-3/8") 9: 45.536 in (3' 9-9/16") 10: 50.679 in (4' 2-11/16") 11: 55.821 in (4' 7-13/16") 12: 60.964 in (5' 0-15/16") 13: 66.107 in (5' 6-1/8")

What this page cannot tell you

Whether your guard is required, how tall it must be, or whether the posts and rails are strong enough — those are separate code sections. The sphere test applies at the largest opening, which for turned spindles is where they are slimmest — that is why the width asked for is the narrowest point. Local codes can differ from the IRC; check yours.

How to space deck railing spindles

Measure between posts. The clear distance from one post face to the other, in inches.
Enter the spindle width. 1.5 in for a 2×2, or the slimmest point of a turned or metal baluster.
Choose the limit. 4 in for deck and floor guards; 4-3/8 in on the open side of stairs.
Read the count and gap. The fewest spindles that keep every gap under the limit, spaced equally.
Mark the centres. Use the list of centre positions measured from the left post.

10 situations where a spindle spacing calculator helps

Passing a deck inspection

A gap even slightly over 4 in fails the sphere test; get a count that keeps every gap under it.

Ordering balusters

Multiply by the number of identical sections for a total.

Even spacing without a spacer block guess

Get the exact gap to cut a spacer to.

Stair railings

Use the 4-3/8 in limit for the open side of stairs.

Metal balusters

Enter the bar width; the narrow profile changes the count.

Turned spindles

Enter the slimmest point, where the opening is widest.

Replacing a few balusters

Check whether old spacing still meets today's limit.

Short and long sections

Each post-to-post run gets its own equal spacing.

Interior stair and balcony guards

The same 4 in rule applies to required guards indoors.

Tighter design spacing

Use a custom limit if your plans call for a smaller gap.

The rule and the arithmetic

2021 IRC Section R312.1.3, Opening limitations: '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.' Exception 2 allows 4-3/8 in on the open side of stairs, and exception 1 a 6 in sphere for the triangle formed by the riser, tread and bottom rail. Read on the ICC's public code site.

With n spindles of width w between posts a clear distance L apart there are n + 1 equal gaps, each (L − n·w) ÷ (n + 1). The page finds the smallest n that makes the gap strictly less than the limit — a gap of exactly 4 in would let a 4 in sphere through — then spaces them equally.

Limits: the page checks spacing only. Guard height, post and rail strength, the bottom-rail gap and the stair triangle are separate requirements. Local codes can amend the IRC, so check with your building department. Everything runs in your browser.

Sources

2021 International Residential Code — Chapter 3, R312.1.3 Opening limitations — 4 in sphere; 4-3/8 in on open side of stairs

Frequently asked questions

What is the 4 inch rule for deck railings?

IRC R312.1.3 says required guards must not have openings that let a 4 in sphere pass, from the walking surface up to the required guard height.

How many spindles do I need for a 6 ft section?

For 72 in between posts with 1.5 in spindles, 13 spindles give 3.75 in gaps — the fewest that stay under 4 in.

What gap should I use between spindles?

Anything under the limit. The page uses equal gaps just under it with the fewest spindles; choose more spindles for tighter gaps.

Is it different on stairs?

Yes: the IRC allows openings up to a 4-3/8 in sphere on the open side of stairs, and 6 in for the riser-tread-bottom rail triangle.

Do I measure at the spindle's widest or narrowest point?

The opening is largest where the spindle is slimmest, so enter the narrowest width.

Does my town use the IRC?

Most US jurisdictions adopt the IRC, sometimes amended. Your building department can tell you which edition and changes apply.